Philip Metzger » Uncategorized

How Big Is a Planet?

Phil Metzger — Thu, 25 Jun 2015 14:10:01 +0000

The scientific method. Credit: Wikimedia CC BY-SA 4.0

I’m sure this is not interesting to most people, but I wanted to get it onto the Internet for those who become interested in Pluto’s planethood during the flyby and want more details.

Determining the definition of a planet is really a scientific question. The first step in science is not writing down a hypothesis but rather observing nature, which means describing the things we see, naming them, and breaking them into a classification system. This clarifies our thinking and enhances discussion about unsolved problems. If we have muddled thinking then we will do muddled science.

In defining “planets”, we are faced with a continuum of bodies from dust specks up to giant stars. We have decided that planets and stars are essentially different categories because stars emit light, but drawing the boundary isn’t so simple. Even planets make some of the light that they emit (mostly in the infrared) because of their warmth. That warmth comes from many sources including gravitational collapse and radioactive decay. We have decided that a star is an object that has nuclear fusion as the source of its warmth. Planets are bodies that do not have nuclear fusion. Still, this is not an adequate boundary because there are objects that formerly supported nuclear fusion but don’t any more, and there are objects that will eventually support nuclear fusion but haven’t started yet. Are they planets? A more precise boundary is this: a planet is a body that never has supported nuclear fusion and never will. Those that do not meet this requirement are either protostars (on their way to nuclear fusion), brown dwarfs (which supported fusion of deuterium for a brief period), stars, or stellar remnants (like black holes or neutron stars).

Comparison of planet and star sizes. The boundary between planets and stars (the left edge of figure 3) is defined by nuclear fusion. Credit: Wikimedia CC BY-SA 4.0

We started with a general feeling that stars and planets are different, then we found a boundary in nature that translates this feeling into something specific. To define the lower end of the planet size-scale we must do the same thing: we have a general feeling that meteoroids (mere rocks) are different than planets, but how do we translate this feeling into something specific? The continuum of sizes includes dust specks, meteoroids, asteroids, and planets. Is there a natural break in the continuum that captures our intuition in a scientifically useful way?

Gibral Basri and Mike Brown, in their paper,”Planetesimals to Brown Dwarfs: What is a Planet?”, discuss several natural boundaries we might choose. They ask, is it a planet when it is large enough so that:

gravity holds it together as a rubble pile?
gravity overcomes material strength and pulls it into a round shape?
internal convection begins in its mantle?
gravitational compression is enough to drive chemical reactions in the interior?
gravity causes significant volumetric compression?
gravity captures and holds enough volatiles that it become an ice giant or gas giant?
free electron degeneracy pressure becomes important, so the planetary radius begin to decrease with added mass rather than increase?
fusion begins? (but that makes it a star)

This list is in order of increasing size, so item 1 happens with smaller bodies, but item 8 happens only with larger bodies. Where in this list do we draw the line? Clearly it must be something smaller than item 6, because Earth and Mars don’t meet item 6 and yet they are planets. Clearly, also, it must be something larger than item 1, because a mere rubble pile — a sandbar in space — doesn’t meet our intuition of a planet, which is a single body rather than a loose agglomeration of bodies.

We might also consider these boundaries: is it a planet if it is large enough for

differentiation to occur, so heavier materials sink to form a core, mantle, and crust?
gravity to hold an atmosphere?

We can rule those out. As for the first, differentiation can occur for even very small bodies (small asteroids) if they got hot enough through impacts or tidal forces or radioactive decay, and it depends on too many factors so it isn’t measurable for planets in other solar systems. As for the second, planets like Mercury don’t hold an atmosphere and yet our intuition says Mercury is a planet.

So that leaves us with the numbered list between items 2 and 5. Items 3, 4, and 5 don’t seem related to our intuition about the nature of planets. They occur in the interior of bodies and are therefore “invisible” to whatever formed our intuition about it. This leaves only item 2: a planet must be gravitationally rounded.

The IAU Definition of a planet from the 2006 General Assembly

The International Astronomical Union (IAU) included this as part of definition of a planet. According to many scientists (myself included), this gravitational roundness and the lack of nuclear fusion should be the only requirements in the definition of a planet. Ironically, the gravitational rounding requirement is mere window dressing in the IAU definition, because there are no bodies orbiting the sun that cleared their orbital neighborhoods and are too small to be round.

Eventually we will do the right thing and eliminate the dynamical requirements (a) and (c) from the IAU definition and add the requirement that a planet never has fusion so it applies to other solar systems. The dynamical requirements produce absurd results when we try to compare planets around other stars. This frustrates the comparison of planets, and thus it does the exact opposite of what a scientific classification system is supposed to do. Gravitational rounding and lack of nuclear fusion really are the right requirements for defining a planet.

Even saying a planet must be gravitationally rounded is not sufficiently precise. Gravity changes the shapes of some bodies very slowly, and it might take a billion years to overcome material strength and pull a body into a rounded shape. Stern and Levison, in their paper, “Regarding the criteria for planethood and proposed planetary classification schemes,” recommend a specific timescale in nature to make it even more precise. They suggest that we use the Hubble Time, which is the inverse of the expansion rate of the universe, or about 14.4 billion years. That’s approximately the age of the universe, so basically it says that if a body is big enough to ever round itself out by gravity then it is a planet. This is a slightly more inclusive boundary than saying the body must already be round. Vesta for example used to be round, but it was knocked slightly out of roundness by a giant collision, yet it is still large enough that over a Hubble time it will relax to roundness again according to some estimates. Therefore, Vesta would still be a planet according to Stern and Levison’s refined definition. This detail, like all scientific questions, is open for debate and should be continually reassessed as we learn more. Classification systems are frequently refined in other branches of science, and it should be the same in planetary science, too.

Vesta is out of hydrostatic equilibrium (gravitational roundness), although we can see that is used to be rounded before a major impact blew out a large chunk. Smaller asteroids, by comparison, are far from round.

So how many bodies in our solar system meet the size requirements to be a planet? Several hundred that we know of, so far! This includes the eight classical planets, a few of the asteroids (depending on the precise roundness requirement), over a dozen of the larger planets’ moons, and hundreds of bodies beyond Neptune. We live in an exciting time when the Kuiper Belt is overthrowing and rewriting our understanding of the solar system and the nature of planets. Observing nature is cool, because it does that for us.

Author information

Phil Metzger

Phil Metzger is a physicist/planetary scientist who works on technologies for mining the Moon, Mars, and asteroids; for developing extraterrestrial spaceports; and starting for robotic industry in space. He recently took early retirement from NASA, where he co-founded the KSC Swamp Works. He is now with the planetary science faculty at the University of Central Florida. Subscribe to the email list to get notified of updates to this blog!

The post How Big Is a Planet? appeared first on Philip Metzger.

Icy Worlds and Stars with Long Hair

Phil Metzger — Thu, 04 Jun 2015 10:24:05 +0000

When people say Pluto isn’t a planet, they often say it is an icy body and therefore it must be a comet, not a planet. But are icy worlds simply comets? Or can real planets be made of ice? And what is the difference between ice and rock, anyway? Let’s take a look!

Halley’s Comet as seen in 1910. Image: The Yerkes Observatory

Stars with Long Hair

From antiquity, long before we knew what comets are made of, we knew them as stunning phenomena in the sky. The full, original name “star comet” comes from the Greek meaning “star with long hair.” By definition, the essence of a comet is the “long hair”, the tail. If a solar system body doesn’t display this “hair” each time it goes round the sun, it’s not a comet. (That’s why we had to invent the separate name asteroid for bodies that don’t have the “hair”.) We later figured out what comets are made of. They are clumps of ice mixed with dust, flying so near the sun that they are evaporating. The ice can be a mixture of different compositions: frozen water, ammonia, and methane, for example. The tail is the evaporating ice and dust. Since they do evaporate, they must be relatively short-lived bodies. There must be a source for new ones, a reservoir of icy bodies far from the sun, or else they would be all gone by now.

The original meaning of “comet” is “star with long hair”. On the left, Chris Hemsworth is classified as a comet. On the right, he is not classified as a comet. You can discover more comets and non-comets by clicking on the picture.

Some of those bodies far from the sun have been discovered, and they are not classified as comets. They need something to kick them out of their cold orbits so they can move closer to the sun, start evaporating, and grow tails before they can become comets. (And if they are too large, like Pluto, then they still wouldn’t grow tails because their gravity will retain the gas and dust as an atmosphere.) These faraway, icy bodies are classified as Trans Neptunian Objects (TNOs). There are various types of TNOs: Kuiper Belt Objects, Scattered Disc Objects, Extended Disc Objects, and Oort Cloud bodies. Some of these TNOs are called dwarf planets and some are called small solar system bodies, but none of them are called comets because they don’t have the “long hair”.

Possible internal structure of Ganymede, an icy moon of Jupiter. Image credit: NASA/JPL-Caltech

Some bodies that were once TNOs have been kicked into orbits that come closer to the sun, crossing the orbits of one or more giant planets. but they still don’t come close enough to the sun to form a healthy tail. They form only lesser tails at times. They seemed to be half asteroid, half comet, so they were classified as centaurs. (Centaurs in Greek mythology were half human, half horse.) The centaurs of our solar system are icy bodies but they are not comets.

Some of the moons of giant planets are icy worlds and they are supremely interesting: Europa, Ganymede, Titan, Tritan, and more. Like the TNOs and the centaurs, they may be icy but they are not comets.

To be sure, some icy bodies are comets, but not all of them are comets. Icy bodies can be many things, including centaurs, moons, and even planets if they are big enough. Being icy doesn’t imply that Pluto is a comet.

Planets can be any material or state

Model of the internal structure of Uranus. Note that the icy mantle is believed to be a supercritical fluid, not a solid. Source: Wikimedia

Planets can be made out of any material. They can be rock, metal, gas, ice, or peanut butter. If space aliens have polluted the Milky Way, there might be a planet formed entirely of plastic bottles and K-cups that floated into a clump off the shoulder of Orion. Someday we will recycle that planet. Closer to home, planets like Mercury, Venus, Earth and Mars are made out of rock. Others, like Jupiter and Saturn, are made out of gas. Still other planets are made out of volatiles that have higher melting temperatures, such as methane, ammonia, and water. Planetary scientists call these volatiles ices even when they are in a gas or liquid state, as they are in Neptune and Uranus. So yes, Neptune and Uranus are made out of ices and thus they are classified as ice giants, not gas giants. And yes, they are planets. Bodies made of ices can be planets.

Planets can also be in any physical state. Some planets can be in the solid state, like Mercury. Others can have surfaces that are in the liquid state, like Earth’s oceanic surface. Still others can be in the gaseous state, like much of Jupiter. It doesn’t matter which state the matter is in.

The geophysical definition of a planet is simply this: to be a planet, a body must have enough mass to pull itself into a round shape by its own gravity, and it must not have so much mass that it initiates nuclear fusion (for that would render it a star). Composition and physical state do not matter. Icy worlds as large as Pluto easily meet this geophysical definition of a planet.

Ice and rock are not fundamentally different

In fact, the distinction between ice and rock is rather arbitrary. It isn’t a fundamental boundary in physics. When we defined some materials as ices and others as rocks, we drew the line by picking a melting temperature rather close to our body temperature. Why did we do this? We were influenced by the mental conception of ice that we developed here on Earth, where water gets hard and feels cold to our bodies in the winter, then it gets fluid and feels warmer to our bodies in the summer. We started calling the cold version of this water ice. Compounds that stayed solid all year round, on the other hand, we called rock. Thus, the distinction between ice and rock originated from our human experience. There is nothing fundamental about that.

Jeffrey Kargel wrote,

By the geologic definitions of mineral and rock, ice can be both, as can the Solar System’s more volatile condensed solids, such as ammonia dehydrate and nitrogen ice. These minerals melt, respond to stress by fracture and flow, react chemically with one another, weather when exposed at satellite surfaces, and generally do many of the things that Earth’s silicate rocks do, although they do so in unique ways. Some of the basic geologic processes of icy satellites, such as volcanism and tectonism, differ little in their fundamental physics compared to the processes that affect Earth’s silicate rocks… [1]

Also, rock can evaporate like ice and form a tail when a rocky body is close enough to the sun.

If you want to say that rocky worlds can be planets but icy worlds cannot, then to justify it you need to invent some property of ice that makes it fundamentally different from rock. But there is no such property. These materials all exist on a continuum and we drew an arbitrary, human-centered line between them. Planets can exist on both sides of that arbitrary line.

Pluto is not primarily an icy body, anyway

Possible internal structure of Pluto. Source: NASA / Pat Rawlings via Wikimedia

It is perfectly acceptable for a planet to be made out of ice, either fluid ice like Uranus and Neptune or solid ice like the outer layers of Pluto. Nevertheless, this discussion is basically moot. Although Pluto does have an icy exterior, most of its mass is rock. Even if the ice were completely removed from the rock, the amount of remaining rock would be enough mass to qualify Pluto as a planet according to the geophysical definition. Pluto’s ice is just icing on the cake. Adding icing to a cake doesn’t take away the cake. Adding ice to a planet doesn’t take away the planet. It just makes it a bigger, more interesting planet.

Summary

There’s no reason to say icy worlds are comets simply because they are icy. There’s no reason to say icy worlds cannot be planets. Icy worlds are beckoning, and we will probably find that many of them are everything we could want a planet to be. They can be complex with dynamic atmospheres, subsurface oceans, cryovolcanism, even liquid lakes on the surface. I can’t wait for the first close-up views of an icy, Kuiper Belt planet: Pluto, along with its five known moons this July. It will be awesome!

Reference: Jeffrey S. Kargel (1998), “Physical chemistry of ices in the outer solar system.” In B. Schmitt, C. de Bergh, and M. Festou (Eds.), Solar System Ices, Vol. 1. (pp. 3-4). Dordrecht: Springer.

Author information

Phil Metzger

The post Icy Worlds and Stars with Long Hair appeared first on Philip Metzger.

Space: the Ultimate Sandbox

Phil Metzger — Wed, 01 Apr 2015 04:54:56 +0000

I just got back from the Granular Matter in Low Gravity conference in Erlangen, Germany hosted by the Friedrich-Alexander Universität. It was a fun week and it reminded me why I love working with granular materials in space.

Why Granular Matter is Important in Space

Poster from the Granular Matter in Low Gravity Conference, showing Buzz Aldrin’s soil mechanics experiment with a boot print, the very first experiment of granular matter in low gravity.

Granular matter is simply any material that is a collection of small bits, like beach sand or corn flakes. Physicists love it because it can act like a solid (example: you want walk on beach sand without sinking into it), or like a liquid (you can pour it through a funnel), or even like a granular gas. Surprisingly, we can’t predict how sand will behave from “first principles”, meaning that we can’t write fundamental equations of physics that will give us the right answers about sand. It’s one of the last areas of unsolved, everyday physics.

Now that’s quite a situation, because it turns out just about anywhere we can land a spacecraft will be covered with granular matter. That’s a result of the way planets form: a solar system has lots of leftover planet stuff in the form of asteroids, which constantly rain down, busting up a planet’s surface, turning its bedrock into the loose layers of broken-up material we call “regolith”. Regolith means “rock blanket”. It includes boulders, rocks, gravel, sand, and dust.

So regolith is a granular material, and it’s everywhere in space, and we have no fundamental equations to predict how it will behave. Nice.

It’s also (arguably) one of the two materials that humanity has the most experience working with here on the Earth. The other is…(wait for it)…water. We have been working with soil since forever — poking in it for potatoes, heaping it into shelters against the wind, digging wells in it to get water, crossing it on the backs of camels, irrigating it for our crops after we learned farming, and so on. And over the centuries we have developed many equations to help us predict how this ubiquitous granular material will behave. (The very first soil mechanics experiment was by none other than Charles-Augustin de Coulomb in 1776. The first in low gravity was by Buzz Aldrin in the Apollo 11 mission, 193 years later.) Nevertheless, our equations are all semi-empirical, meaning they are based on our experiences and not completely from first principles of physics. That’s a problem, because our experience with soil here on Earth doesn’t help very much when we go to other planets where gravity is less, the atmosphere thinner, the soil less weathered, and so forth. Granular matter behaves differently depending on what environment you put it into. That’s why we need equations from first principles and not just from our experience on Earth.

Performing experiments shooting jets of gas into different granular materials in a variety of gravities.

I have done many experiments with sand and with simulated lunar and Martian soil in the reduced gravity aircraft (what people still call “the Vomit Comet”, even though it’s not the same aircraft). In low gravity, sand starts to act more clingy, like powdered confectioner’s sugar. It’s remarkable the first time you see that happen. For example, we had a container of simulated lunar soil, and in Earth’s gravity you could tilt it and the soil would slide down into a heap just like regular sand. But in lunar gravity in the aircraft, you could turn the container completely upside down and the soil would hang in the top of the container. You had to shake the container to get it to fall down, and when it did it would first form cracks like a fracturing solid, and then it would bust apart, fall, and splash around the bottom of the container like agitated, black milk. ‘Twas amazing to behold.

So here’s the situation: granular matter is one of the two materials we’ve used the most in this world, and yet we can’t predict it’s behavior outside this world, and it is everywhere we want to go. And on top of that, it’s where most of the resources in space are located, so we need to work with granular matter more than any other substance in space.

What We Must Do with Granular Matter

When we go to another world in space, we must:

Small mining robots supply resource needs to a Martian outpost. Concept art by Pat Rawlings, drawn for the KSC Swamp Works.

Land on granular matter with rockets
Drive on it
Dig in it
Process it for resources
Build with it as a construction material, and
Study it for science

When I was at NASA, this is what I spent all my time working on. We were developing technologies to do all of these things with granular matter. (That’s why the lab was called the Granular Mechanics and Regolith Operations Laboratory.) I’m still working on it now at the university, and so are my colleagues in the Swamp Works and elsewhere in NASA, along with many other researchers in academia and business.

Now it’s one thing to develop a technology intended to work with regolith on another world. It’s another thing to prove that it will really work after the technology arrives there. I’ve seen technologies that worked great in Earth’s gravity that didn’t work at all when you put them into the reduced gravity aircraft. And there are some technologies that are simply too big or too slow to test inside an airplane, which provides only 20 seconds of low gravity at a time. And some technologies require hard vacuum as well as low gravity, but sadly we can’t fit better than a tiny vacuum chamber inside the airplane. So how do we test the interaction of technology with granular matter as it will behave in the full glory of the space environment?

I think the best strategy is to develop simulation algorithms that will predict how granular matter behaves on other planets. Then, we can set the environment in our virtual world to anything we like. We can set it to the conditions of the reduced gravity aircraft, or the lab, or the field test on top of a volcano, or to the distant planet itself. Then, we can compare how the technology fared in the reduced gravity aircraft and all the other test locations against how the software predicted it would fare. If the software predicted the behavior correctly in each case, then we can feel more confident that it predicts the behavior on the planet correctly, as well. Thus, the simulations allow us to cobble together a test program using whatever inadequate test conditions we can afford, in airplanes or on volcanoes or in the lab, and despite the inadequacy give us some real confidence before we fly the mission.

Now, this strategy only works if the simulation software is good. But remember, we don’t have any fundamental equations of granular material behavior. We have made lots of progress, but we have lots of work still to do. So this is why we have communities of people trying to solve fundamental granular physics. When we have been forced to build things without adequate software, we sometimes have hardware failures in space. You might remember that Mars rover Spirit became stuck in sand, as did the Chinese lunar rover Yutu. And when Curiosity landed on Mars, the Sky Crane’s thrusters blew gravel onto the rover and that apparently broke one of its two wind sensors. I could go on and on telling of problems with granular materials defeating our space technology. These kinds of problems could prove fatal if we have a mission with humans on a faraway world, and that makes it much more important.

So we had a great conference this past week in Erlangen. We talked about 3D printing entire buildings out of lunar soil, how sand dunes migrate on Saturn’s moon Titan, how planets form by grains colliding in space, and why Brazil nuts rise to the top of a mixed nut container when shaken. (Hey, it’s important to know if you want to predict how granular materials will behave!) It was great seeing some colleagues whom I have not seen in many years. We made some plans for collaboration. I hope to fill you in on some of the progress we are making, soon.

(For now, if you want to see some of my prior work on granular physics theory, check out this paper here, which was published here. Or if you want to see some experimental work, check out this one.)

Author information

Phil Metzger

The post Space: the Ultimate Sandbox appeared first on Philip Metzger.

Moonrakers: Getting the Big Cheese!

Phil Metzger — Wed, 11 Sep 2013 16:36:51 +0000

In near-Earth space there are three places to mine. The first is the Earth itself. Some of the challenges to mining only on the Earth are (1) too much mining on Earth messes up habitats for life, (2) some of the best resources on Earth are already becoming scarce so their costs are rising, and (3) if we want to use Earth’s resources for anything we are doing in space then we have to launch them into space, and that is very expensive!

The second place to mine in near-Earth space is on a Near-Earth Asteroid (NEA) that we will bring back to orbit the Moon. We hope to start doing that very soon! This has been discussed in some of the prior posts.

The third place near the Earth where we can mine is the Moon. Unlike NEAs it’s always nearby, and the space mining enthusiasts (including myself) will tell you it is loaded with resources. Others will scoff at this idea and tell you that the transportation costs are so high that even if the Moon were made of pure gold it would be unprofitable to bring any back to Earth.

The Moon: is mining it an illusion or a lucrative opportunity?

There is an ancient proverb that tells of a simpleton who, seeing the Moon’s reflection in a pond and thinking it was a floating cheese, reached for it and fell into the water. And there is a true tale from the 1700’s telling how the people of Wiltshire, England became known as the Moonrakers after they were caught raking a pond at night to retrieve smugglers’ contraband that had been hidden under water. They did some quick thinking and fooled the law enforcement authorities by acting as the proverbial simpletons, raking at the reflected image of the Moon because (they said) they thought it was a floating cheese.

So what should people think about us space miners who want to rake the actual Moon for its resources, as though it really were a big “cheese” floating in the pond of the sky? Are we the proverbial simpletons embodied, raking at an illusion until eventually we fall in like fools? Or are we like the Moonrakers of Wiltshire, getting ready to pull out the hidden goods and have the last laugh?

One Giant Ore Body

Elemental composition of lunar soil

When I first joined the resource utilization community some years ago I was told there are no veins of gold on the Moon; no concentrated platinum deposits; no special locations of anything worth mining. (How that view has changed! More on that in a later post…) I was also told, however, that there is one very valuable resource that is available literally all over the Moon, everywhere you go. It is oxygen. The minerals of the Moon are more than 40% oxygen by mass. That is similar to the ordinary beach sand here on Cocoa Beach, for example. The grains of sand on the beach are composed of the mineral quartz, a crystalline structure of silicon dioxide — two oxygen atoms for every silicon atom. Oxygen has an atomic weight of 16, and silicon is 28, so oxygen comprises (16+16)/(16+16+28) = 53% of the mass of the sand on Cocoa Beach. The beach is more than half oxygen everywhere under your feet, and the Moon is very much like that. To get oxygen from the Moon you don’t need to seek out special veins or concentrations of it, special “ore bodies” in the lunar mountains or deep down in the crust. The whole Moon is one giant ore body of oxygen, one great big cheese. The cheese of lunar resources is oxygen. All you have to do is rake up the soil and use a simple chemical reaction to get the oxygen atoms out from the minerals. This is worth doing, because oxygen just happens to be the most valuable commodity in space when you want to conduct exploration missions!

The Most Expensive Part of Space Flight

Why is oxygen so valuable for conduction space exploration missions? Well, if you don’t plan to use the resources of space, then you have to launch everything from the Earth, and the most expensive single part of space missions is providing the rockets and the human labor to launch oxygen from the Earth, because oxygen is so heavy and we need so much of it for a mission. For example, consider the weight of propellants in the Space Shuttle’s big orange external tank. It contained hydrogen and oxygen, and they burned together to produce H₂O – water! The Space Shuttle launched into space on a flame of water! So how much of the weight in that gigantic external tank was due to the hydrogen and how much was due to the oxygen? Oxygen has a molecular weight of 16 and hydrogen has a molecular weight of 1. Two hydrogens combine with one oxygen to make water, so that means 16/(16+1+1) = 89% of the mass in the tank was oxygen! And the propellants in the tank were roughly half the mass of the entire Space Shuttle at liftoff, so it turns out the overall mass of oxygen was about 38% of the total mass of the Space Shuttle and its cargo at liftoff. Now that’s the oxygen that it takes to get into orbit. If you have another rocket designed to go from Earth orbit all the way out to Mars, then similarly high amounts of oxygen must be used, and all that oxygen needs to be launched from Earth at high expense. Then, if you want to actually land on Mars, you need even more propellants to run the engines in the Mars lander, and you need even more propellants just to send the lander’s propellants from Earth out to Mars. And if you want to launch back off from Mars at the end of the mission, then you need even more propellants, and you need even more than that just to land the propellants on Mars, and you need even more propellants to send all the above propellants from Earth out to Mars. And if you want to come back from Mars to Earth, then you need even more. It all adds up: oxygen being used to deliver oxygen being used to deliver oxygen, and it all has to be launched from Earth at great expense. But if you play the Moonraker and get some of the oxygen “cheese” for a Mars mission from lunar soil (and get more of it from Martian soil), then the studies show you can cut the cost of a Mars mission by a factor of 3 or maybe even by a factor of 5. Oxygen derived from minerals in space truly is a big cheese.

Dust to Thrust

Technology to convert dust to thrust during the 2010 field test on Mauna Kea.

This is something we really can do! The technology to rake the Moon and get oxygen atoms out of the minerals is fairly mature. The space resources community has developed several different chemical reactions to make it possible and has built several prototype chemical reactors and taken them to a volcano in Hawaii for rigorous field testing. I was there in 2010 on Mauna Kea when one of these systems was tested. The site was chosen because it is very lunar-like with rugged terrain, volcanic minerals somewhat like those in the lunar soil, and lots of dust. We had robotic excavators raking up the dust, a regolith conveyance system moving the dust into a chemical reactor, solar concentrators collecting sunlight to melt the dust, a chemical reactor extracting oxygen from the melted dust, an oxygen liquefaction system to store the oxygen after it was extracted, and a small rocket thruster to burn the oxygen with methane fuel. Therefore, the system took volcanic dust and converted it into rocket thrust: “Dust-to-Thrust”! And it all worked.

So there you have it! The big cheese of lunar resources isn’t just an illusion. The Moon is a giant ore body providing a resource we need to accomplish Mars missions, and we have the knowledge and technologies to go up there get it. We’re not simpletons raking at a reflection, we’re going to be like the real-life Moonrakers of Wiltshire. And like them, we’re going to have the last laugh!

In the next post I will discuss another highly valuable resource on the Moon.

“Moonraker” lunar excavator in dust-to-thrust field test, Manua Kea 2010, delivering tephra to extract oxygen.

Click here to tweet about this blog post!

Author information

Phil Metzger

The post Moonrakers: Getting the Big Cheese! appeared first on Philip Metzger.

13 Things to Do with Space Water

Phil Metzger — Tue, 28 May 2013 04:21:44 +0000

NASA astronaut Kevin Ford watches a water bubble float freely in the International Space Station. Image Credit: NASA

Mining Space Water

There are several companies that plan to mine asteroids and several more that plan to mine the Moon. In addition, Elon Musk wants to start a colony on Mars, and that will necessitate the mining of Mars. Furthermore, government space agencies plan to do missions to Mars and those missions will be far more affordable if they include mining of the resources of space as part of the mission. So get ready for mining in space!

There is broad consensus that the first and most economic resource to be mined in space will be water. On Earth we don’t call it mining when we get some water. We call it digging a well. However, everywhere we want to go in space the water will be frozen, and since it is usually chemically bonded inside minerals, or at least buried under regolith, and sometimes cryogenically cold and thus harder than granite, getting water in space really is mining.

And where do we get space water in space? It can be obtained from the hydrated minerals of certain types of asteroids. It can be obtained from the polar regions of the Moon. It can be obtained just beneath the surface of Mars. Or we can get it by catching a comet or another icy body from the outer solar system. Water is abundant in our solar system.

What to Do with Space Water?

So what will we do with all that water once we mine it in space? Here is a list of 13 things:

1. Rocket propellants

Water is H2O, two hydrogen atoms for every oxygen atom. It can be split into separate hydrogen and oxygen using an electrical current in a process known as electrolysis. The power to drive this process can come from solar cells, capturing the sun’s energy to split the water. Then, the hydrogen and oxygen can be stored and eventually burned together in a rocket engine to provide thrust for the mission. That is the kind of fuel used by the Space Shuttle’s main engines. Most people don’t realize that the Shuttle used to fly into space on a flame of water. Used this way, water is simply an energy storage medium, slowly taking in the energy required to split it apart, storing that energy in concentrated form as chemical energy, and giving it back as thermal energy very rapidly when we burn it, so that the rocket nozzle converts it into kinetic energy by pushing the rocket. Water: the energy food for rockets!

2. Electrical power generation

After we have electrolyzed the water into hydrogen and oxygen, there is more that we can do than burn it for rocket propulsion. We can also use it in fuel cells to generate electricity through a slightly more subdued process. Think of it as electrolysis in reverse. We put the hydrogen and oxygen into the fuel cell and out comes an electrical current and water. Fuel cells are great for generating electrical power when we don’t have enough sunlight, such as when we are in the shadow of a planet or in a permanently shadowed crater or deep underground or during the lunar night. Once again, the water is used as an energy storage medium.

3. Radiation shielding

Water can also stop energy — the energy of high velocity cosmic radiation particles. Cosmic radiation is the biggest hindrance to sending humans to Mars. It can cause cancer such as leukemia at a rate that is too high to accept. Cosmic radiation consists of the nuclei of atoms from far, far away in our galaxy. They were accelerated by the shockwaves of exploded stars. It is hard to understand how they got going so fast, so extremely close to the speed of light. Perhaps they have been bouncing around the galaxy for a very long time, passing through the shockwaves of many different exploded stars, gaining energy every time. In any case, they are going so fast that they are very difficult to stop. Here on the Earth we have a a nice strong global magnetic field to deflect much of the cosmic radiation and a nice thick atmosphere to absorb the rest. On a journey in space we can’t afford to take along tens of miles of atmosphere around our spacecraft and it is hard to make a big enough magnetic field when the spacecraft is supposed to be small and lightweight, so what can we use instead? We can put some other kind of absorber around the spacecraft. It turns out the worst absorber for cosmic radiation would be a heavy element like lead. Lead works great for x-rays, but for cosmic radiation the lead nuclei are shattered by the high velocity particles and then the astronauts are bombarded by all the fragments of the lead nuclei, so the radiation dose would be actually increased instead of decreased! The best radiation shield is hydrogen, an element that consists of just one proton so its nucleus cannot shatter. The second best radiation shield is anything that contains a lot of hydrogen. One excellent material is therefore water, good ol’ H2O. If we can mine enough water in space then we can put nice thick quantities of it around our spacecraft to keep the crew healthy and safe. That will make the spacecraft heavier, of course, but then we can use more of the same water as additional rocket propellant to push it all to Mars (or wherever it is that we want to go).

4. Drinking

Technically we don’t need water in space for drinking because we can recycle our pee. If you’re not really fond of that idea, then you must be very fond of space mining, because otherwise recycled pee is on the menu. It’s really not so bad to drink recycled pee. Here on Earth you do it all the time. It’s just not as easy on a tiny spaceship where the proximity of the recycling equipment forces you to remember where your beverage came from. And recycling on such a small scale as a spaceship is expensive and tricky. Giant spaceships like Earth are better at that stuff. Anyhow, without space mining, you’ll be drinking lots of pee. If on the other hand you have lots of water from mining in space, then your pee can be dumped overboard to make miniature yellow comets in orbit around the sun. Consider it a form of art. So it’s your choice: making space art, or drinking pee.

5. Plant growth

Plants need a lot of water. The water we use for our space plants can be recycled (like our pee) because whatever water the plants transpire through their leaves can be recaptured by equipment that dehumidifies the air, and whatever water is still in the plants when we eat them will be…well, you know. Anyhow, no recycling process is 100% efficient and we will constantly lose some water from our spacecraft into space during long journeys or into the Martian atmosphere at a colony on its surface. And where do we get all that water from, in the first place? It is too expensive to launch all the plants’ water from Earth. It’s much better to get our plants’ water in space: on Mars, or the Moon, or wherever we’re going to be growing them.

6. Breathing

The largest cost of spaceflight is launching oxygen from the Earth. The majority of the oxygen we usually launch into space is used for rocket propellants and for fuel cells. However, we also bring along some oxygen to breath. As our bodies burn carbohydrates for energy, some of the carbon from them is oxidized in our cells to make carbon dioxide (CO2) and we breathe it out into the air around us. We have to remove the CO2 from the air because otherwise it would build up too much and suffocate us. Normally we just sequester the CO2 for the duration of the mission and dispose of it after landing. If we want to recycle the oxygen contained in the CO2 during the mission, then we will need to use energy to split it back into carbon and oxygen. Plants do that for us naturally as part of their photosynthesis process, recycling not just the oxygen but the carbon, too, turning it into plant tissues, which also store up chemical energy. Unfortunately, we would have to take huge forests of plants to support the oxygen needs of just a few astronauts, and that would make the spacecraft to heavy and too expensive to fly. To recycle the oxygen artificially we would need chemical processing machinery to replace the plants, plus a big enough energy source to run it. It might be less weight to simply bring the carbon and the oxygen to use up during the trip. That’s how we do it now. The carbon is brought in little baggies in the form of astronaut food. The oxygen is brought in tanks. The carbon dioxide is cleaned out of the air and sequestered until the spacecraft lands – or it could be vented overboard. Thus, we aren’t currently recycling them in space. And currently we launch all the oxygen needed for the entire mission from Earth. For longer journeys where we will need more oxygen we could do this more affordably if we got the oxygen by mining and electrolyzing water in space.

7. Manufacturing processes

Eventually we need to make more and more things in space rather than launching them all from Earth. Many manufacturing processes require water. As time goes on we will be doing more and more of these processes on the Moon or on Mars or elsewhere, and so we will need an expanding supply of water.

8. Spacecraft cooling

Everything that does useful activity produces waste heat. Even if you are just huddled in a furry jacket in the howling winter winds of Antarctica thinking about your financial situation, your body needs — needs — to get rid of heat. A perfect jacket would kill you. That is one of the laws of thermodynamics. Heat is what carries away increased entropy from our bodies, and unless our bodies are continuously producing and shedding entropy then we aren’t doing anything useful; we would be dead. Well, space is really cold, but the vacuum of space is like an excellent jacket that keeps us from losing heat very easily. And so after a little while our spaceship will get too hot and be dead. We solve this problem by using radiator panels. When a spacecraft has its radiator panels pointed toward the cool darkness of space, it can radiate away its waste heat and entropy in the form of infrared radiation. On the Space Shuttle the radiators were just inside the payload bay doors. So when the doors were closed, how did the Space Shuttle get rid of waste heat? By dumping heated water into space! That can be an important way to shed waste heat when you can’t get a radiator pointed toward cold space or when you have a huge amount of heat that needs to be dumped in an emergency. Water to the rescue! Dumping it for thermal control will probably always be a part of how we explore space. Our bodies also use water this way, too; it’s called sweating.

9. Scientific research

Already we are studying water’s properties in space, as illustrated in the photograph above. We will also want to mine water in space to study what’s in the water and learn more about our solar system and galaxy and universe. Also, some scientific instruments rely on large amounts of water. One proposal, championed by Alex Ignatiev at the University of Houston, is to create a huge bath of water on the Moon to detect neutrinos. Melt the lunar ice; form a giant swimming pool; look for neutrinos passing through the water. Very cool!

10. Bathing

Not only neutrinos, but also humans would like to get wet in space. I wonder what it’s like to take a shower in zero-g? It must be really fun! And it’s necessary. Astronauts get dirty, too.

11. Swimming and recreation

And getting wet isn’t just to get clean. When we have space resorts, we will want them to have swimming pools! In lunar gravity you could swim so easily, floating with only a tiny amount of your body underwater. But on the other hand, the tiniest disturbance would create huge waves so that might make lunar swimming a bit more adventurous. Come to think of it, we could have excellent surfing resorts with artificial wave generators on the Moon! And when we do, remember to thank a space miner.

12. Spacecraft life support for aquatic astronauts

Will humans be the only species to leave planet Earth? I sure hope not! All species bound to the Earth will eventually go extinct unless they find a way into space. That’s because eventually the sun will expand and burn up the Earth, and before then the Earth will get too hot and boil its oceans away, and before then a blackhole might swing by and eat us up, and before then a comet might hit the Earth and wipe out life, and before then the Yellowstone supervolcano might blow, etc., etc. So the question is, do you really want to save the whales? If you do, then start making plans to turn them into whale astronauts. We will need giant spaceships for these whales, with giant amounts of ocean water inside. Or we can just take baby whales…with giant amounts of ocean water inside. When we have ocean spacecraft, we can take whales to other stars where we will terraform new ocean-bearing worlds where they can live. You might think this is a crazy idea, but space mining will take our civilization to the next level where we will have millions or even billions of times the capacity to do amazing things. When that day comes, crazy-sounding ideas like saving all the species of the Earth and spreading them to more worlds will suddenly not be crazy any more. Remember, if you told the ancient hunter-gatherers about an iPhone or an airplane or a gasoline-powered car, they would consider you crazy. And yet here we are.

13. Terraforming planets

And that brings us to the biggest need for water of them all: terraforming. Huge amounts of water will be needed to terraform a dry planet. As Freeman Dyson once pointed out, we can quickly soak a dry world by crashing an icy moon from the outer solar system into it. Maybe we won’t want to do that with our own dear Mars for sentimental and historic reasons — I mean, who wants to destroy one of our own solar system’s moons? But we can certainly do it at another solar system where we aren’t so attached to all its stinking little moons. One way or another, space resources will lead us to the day when we can engineer new worlds complete with oceans and water cycles, where we and the whales and all the other species from Earth can carry on.

Summary

As the miners like to say: if you can’t grow it, you have to mine it. Mining water in space is a first step toward establishing a space economy, taking civilization to the next level, and becoming a multi-world species.

Tweet this

Author information

Phil Metzger

The post 13 Things to Do with Space Water appeared first on Philip Metzger.

The Type of Asteroid to Mine, Part 3

Phil Metzger — Mon, 20 May 2013 13:50:24 +0000

Astronauts visiting a Near Earth Asteroid. Image credit: NASA

For near-term mining in space, we can mine the Moon or we can mine Near Earth Asteroids (NEAs). This series of posts discusses asteroids. What type of asteroid is best for space mining? As we saw previously, asteroids can be completely different from one another in their composition because of the different ways they formed. Some came from undifferentiated (“primitive”) bodies while others came from protoplanets big enough to differentiate. That is, they were big enough to melt and separate into metal, heavier minerals, and lighter minerals forming a core, mantle, and crust. Among the basic types of asteroids there are many, many subcategories. Much of what we know about asteroids comes from studying meteorites that have fallen onto the Earth. These do not give us a complete sampling of all the asteroids, so for those we cannot physically reach yet we must learn what we can with telescopes. Therefore, they are classified according to the pattern of how they absorb and reflect light (what we can see in the telescope), their spectrum. That tells how much sunlight is reflected by an asteroid at each wavelength of the light.

Asteroid Spectral Types

Just like the classification of meteorites found on the Earth, the classification of asteroids has several major categories and many subcategories. Research is still ongoing to understand asteroids and so there are competing classification systems.

Understanding the asteroid spectra is complicated by something that occurs in space known as space weathering. That is, solar wind wears material off the asteroid, and foreign material accumulates by gravity onto the asteroid, changing the appearance of its outer surface. Fortunately we have been making progress in understanding how space weathering changes the spectra, so we are getting better at subtracting out these effects.

There is no need to go into too much detail. We need to know only a few main points for mining the asteroids and they are pretty straight-forward. So here they are.

Asteroid Types for Space Mining
Spectral Type	Similar to What Meteorite Type	Resources	Purpose
Type C	carbonaceous chondrite	water + metal + organic compounds	rocket propellants and other consumables for space missions + metal for 3D printing of hardware to use in space + making rubber or plastic or methane for rocket fuel or CO2 for plants
Type S	LL chondrite	platinum group metals	sell on Earth for use on Earth
Type M	iron meteorite	metals including platinum group metals	manufacturing large hardware items in space for colonization or larger missions and/or for sale on Earth

C Type

This class include asteroids that look dark through a telescope and have spectra indicating they are composed of carbon compounds. They are therefore called carbonaceous asteroids and are believed to be made of the same sort of material as the carbonaceous chondrite meteorites. The dwarf planet Ceres has a spectrum like a carbonaceous asteroid (at least on its surface). C Type accounts for about 75% of all asteroids. They formed in the outer portions of the asteroid belt closer to the frost line where it was cooler so carbon compounds could condense. They also contain hydrated minerals so we can get lots of water out of them. Some estimates are that we can get over 20% of the mass of the asteroid out in the form of water. Since chondrites are undifferentiated they also contain primitive metals, sometimes as much as 40% by mass. These would be extremely good asteroids to mine, having water, metals, and carbon compounds, all three in one. Because they are dark, they are rather hard to find.

S Type

These asteroids are rather brighter than the C Types and appear to be “stony” in composition. They can be either primitive — having chondrites (usually) — or from differentiated bodies — so not having chondrites. The LL chondrite meteorites are especially interesting for platinum mining because, although the LL means “Low iron content and Low total metal” they nevertheless are wonderfully high in platinum content. That could be important if we plan to bring the mined metals back to Earth to sell on the terrestrial metals market. If we can find a way to reduce the launch and landing costs, then this could be a quick revenue stream to support the space mining company. However, if we plan to use the metal in space then there is no need to go after just platinum. It would make better sense to go after a different class of S Type asteroid, those in the H chondrite family since H means “high” metal content. Or better yet we would go after an M Type asteroid.

M Type

M Type asteroids are moderately bright and are usually metal but sometimes metal-stone mixtures. Some of them are surely the same composition as the iron meteorites that have fallen to Earth. They are believed to have come from the cores of differentiated planetoids that were later broken apart. These asteroids are interesting for mining because a pure metal asteroid is a lot of metal and can be made into a lot of spacecraft. A quick estimate shows that the asteroid belt has a billion times more metal than all the high grade metal ore in the crust of the Earth.

Summary

Three types of asteroids are most often discussed for space mining. One is Type C, another is a certain subcategory of Type S, and another is Type M. Type C is probably the best all-around asteroid for mining in the early stages of space industry because it provides lots of water for rocket fuel, which can be sold to NASA or others to do various space missions. That might be a good strategy for initially starting up space industry. Type C also provides metal and organic materials that can be used for making things in space. Type S asteroids of the LL chondrite class may be good for platinum mining for sale on Earth to provide a revenue stream for the mining company. That will be a good strategy if we can lower the operational costs so that it becomes economic. Type M may provide platinum, too, as well as vast amounts of nickel and iron that, in the long run, will be needed for solar system civilization.

Author information

Phil Metzger

The post The Type of Asteroid to Mine, Part 3 appeared first on Philip Metzger.

The Type of Asteroid to Mine, Part 2

Phil Metzger — Thu, 16 May 2013 18:05:46 +0000

Space mining is critically important to the future of human civilization. This series of posts discusses the best type of asteroid to bring back to Earth to mine. The previous post (Part 1) discussed why the asteroids formed near Jupiter and why their chemistry is different than the chemistry of Earth. This post (part 2) will now explain why there are such different types of asteroids.

Differentiation

As the protoplanets were forming in the asteroid main belt, some grew bigger than others. When they got bigger, they got hotter. Just as two cold people in a winter forest are warmer if they huddle together, and a hundred huddling people are warmer still — at least those in the middle are — so it is with rocks in space. They generate some heat because the radioactive materials decaying within, and the more they clump together the hotter their middle becomes. When the protoplanets are bigger than some critical size, their insides melt, and if they are big enough then the protoplanets melt entirely, including their outer layers. When that happens, a protoplanet becomes a giant liquid ball floating in space, and so fluid mechanics come into effect. Heavier liquids — the molten metals — sink to the inside and become the protoplanet’s metallic core. Lighter liquids — the silicate minerals — float to the outside and become its rocky crust. Intermediately heavy liquids — typically heavier minerals with lots of iron in them, like olivine (the volcanic green glass in Hawaii) — float to the middle and become the protoplanet’s mantle. This process of separating into a core, mantle, and crust is called differentiation. Some of the asteroids like Ceres and Vesta are differentiated bodies. Others are not; they are too small so they never melted. How do we know this? They have a special tell-tale sign that gives them away.

Chondrules and Chondrites

The largest of these chondrules is less than a centimeter across. Source: NASA

Very early in the solar system’s history, tiny solid spheres formed in the gas cloud that surrounded the sun. These are called chondrules. The chondrules were eventually caught up in the material that formed rocks in protoplanets. Whenever a protoplanet got big enough to melt and differentiate, the chondrules inside it were melted, too, so they are no longer visible in the rocks of those protoplanets. However, the protoplanets that never got big enough to melt still have the chondrules in them. The presence of chondrules is a give-away sign that the rock came from a protoplanet that is undifferentiated. It is composed of the primordial material of our solar system.

A chondritic meteorite. The spheres in it are chondrules. Source: Wikipedia

Some chunks of these undifferentiated meteoroids and asteroids have fallen to Earth. We call meteorites that have chondrules still in them chondritic meteorites. The ordinary chondrites are the ones that are formed of stony material (with bits of metal mixed in). Some have high iron content and so are called H-chondrites (“H” for “High iron”). Others have low iron and are called L-chondrites. Still others have low iron and low total metal content and are called LL-chondrites.

The Murchison meteorite is an example of a carbonaceous chondrite. Source: NASA

In addition to these ordinary chondrites there are carbonaceous chondrites. They are a darker family of meteorites that contain organic material (complex carbon compounds, like PAHs and amino acides), perhaps up to a few percent by mass. The organic material is like the kerogen found in Earth-rocks, but on Earth it comes from decomposed life that collected over millions of years. Kerogen-bearing rocks are mined on Earth for the extraction of natural gas. The organic material in carbonaceous chondrites appears to be non-biological in origin, having formed in the molecular cloud that surrounded our sun and in reactions within water inside the protoplanets. Apparently these carbonacous asteroids formed in the farther reaches of the asteroid main belt, closer to the frost line, because the carbon compounds needed cooler conditions to condense than did the stony material of the ordinary chondrites. Carbonaceous chondrites are further separated into many different sub-categories depending on the details of their composition. In addition to the ordinary and carbonaceous varieties there are other types of chondrites as well.

Without Chondrules

Other meteorites are achondritic because they came from bodies that had differentiated (melted) and thus lack chondrules. The question is, which part of the differentiated body did they come from? This is what gives the asteroids so much variety! Some of the achondritic meteorites come from a differentiated protoplanet’s crust, so they are silicate in composition and are called stony achondrites. Others come from the core, so they are metal in composition and are called iron meteorites. Others come from the protoplanet’s mantle near the core, so they are stony-irons, having a matrix of iron-nickle with crystals of the iron-rich mineral olivine embedded inside. The fact that we get meteorites from a protoplanet’s core, mantle, and crust tells us something important and sad about that protoplanet: somewhere along the way toward planet-hood it got smashed. But this is good news for space mining. Here on Earth most of the metal is inaccessible in the planet’s core. In the movies, you can tunnel directly into the core of the Earth. In real life, not really. Almost all the Earth’s metal is completely inaccessible. In the asteroid belt, however, the core of one or more entire protoplanets has cooled, hardened, and busted apart into bite-sized chunks waiting to be converted into the means of an amazing civilization. Some of them have been flung into the inner solar system as NEAs where we can mine them.

Next time: part 3, which of these types do we want to mine?

Author information

Phil Metzger

The post The Type of Asteroid to Mine, Part 2 appeared first on Philip Metzger.

The Type of Asteroid to Mine, Part 1

Phil Metzger — Wed, 15 May 2013 20:11:08 +0000

Several space mining companies plan to mine asteroids, and they want to bring back to Earth either the entire asteroid or just the part that they mined from it. Either way, obtaining resources in space will be revolutionary for exploration and for the advancement of human civilization into space, so bringing back an asteroid for the first time may well be one of those special moments in human history that we never forget.

The two previous blog posts discussed what size asteroid we may want to bring back to the Earth and what kind of location and trajectory it needs to be in for us to affordably bring it back. The next few posts will discuss what class of asteroid we want to bring back, i.e., its composition. Here, we discuss the composition of asteroids in relation to where they formed near the solar system frost line.

The frost line lies between the asteroid main belt and the orbit of Jupiter

The Solar System’s Frost Line

Near Earth Asteroids (NEAs) are mostly from the main asteroid belt, kicked into the inner solar system by gravitational disturbances from Jupiter. As such they have a geochemistry that represents the main asteroid belt,somewhat different than the geochemistry of bodies that formed closer to the sun such as our own dear planet Earth. Closer to the sun it was hotter and so fewer volatiles were there while the planets were forming. A volatile is a substance that evaporates in the hot sunlight and is driven outward by solar pressure to the outer, cooler parts of the solar system. It’s like heating a pot of water on the stove during a cold winter’s night: the water is driven from the pot and condenses onto the cold glass of the windows of your home. The “frost line” of the solar system is a hypothetical circle drawn at a radius from the sun just beyond the asteroid belt, just inside of Jupiter. Beyond that circle it was cold enough for volatiles to condense and to form icy and gaseous bodies, like the water that condenses on your cold windows. That is why Jupiter formed where it did; it is composed of the volatiles driven beyond the frost line. And just inside the frost line is the asteroid belt. It is composed of rocky material (mostly non-volatile) that never successfully merged into a planet, and again the frost line explains why the asteroid belt is where it is. Because the frost line caused Jupiter to form so closely nearby, the huge gravity of Jupiter kept stirring up all the other material near the frost line. That caused the material to slam around too quickly so it couldn’t merge into a new planet. Some pieces did grow big, big enough to be called protoplanets, and one of them grew big enough to be called a dwarf planet (that is Ceres), but no object in that region grew big enough to dominate the orbit. Today, the asteroid belt is a collection of protoplanets and smaller rocks including some busted-up protoplanets, still stirring around. So the frost line is straddled by the biggest planet on one side and a failed planet on the other, and the physics of the frost line explains both.

Volatiles in the Asteroid Belt

Now it’s true that most volatiles condensed beyond the frost line, but it’s too simplistic a story to leave it at that. Nature is too wonderfully messy for simplistic explanations. Because lots of protoplanets were forming just inside the frost line where it was almost cool enough for volatiles, they provided some shade and some protection beneath their surfaces from the sun so that plenty of volatiles actually did collect in the asteroid belt, too. And that’s why the chemistry of the asteroids is so different than the chemistry of Earth. The Earth is just silicate materials and metals, plus relatively tiny amounts of water and other volatiles that were brought back into the inner solar system at a much later date by the comets and asteroids. The asteroids contain silicates and metals, like the Earth, but they also contain huge stores of water-bearing minerals and carbon compounds and other volatiles. And that’s why we want to mine them! They’ve got lots of the stuff we need.

Now again, just saying the asteroids have all those kinds of materials is too simplistic a story for this wonderfully messy universe. Not all the asteroids have all these different materials in them. The next post will explain why and will discuss the different types of asteroids that formed.

Author information

Phil Metzger

The post The Type of Asteroid to Mine, Part 1 appeared first on Philip Metzger.

Retrieving Asteroids Near and Far

Phil Metzger — Tue, 14 May 2013 04:25:47 +0000

The previous post discussed retrieving asteroids and what size we want to bring back to Earth. This post discusses the location and trajectory of an asteroid we could retrieve.

Families of Asteroids

Location of the main asteroid belt, Trojan asteroids, and planets.

The vast majority of the asteroids are in the Main Asteroid Belt between Mars and Jupiter. Why? Because (we surmise) the gravitational “stirring” caused by Jupiter, the largest planet, has kept the material in that orbital band from condensing into a planet. The largest asteroid in the main belt is the protoplanet Ceres, classified as a dwarf planet. The Main Belt also includes protoplanets Pallas and Vesta, not quite large enough to be dwarf planets, plus many smaller rocky bodies including fragments of larger bodies that had collided and broken apart.

A little further out in the solar system, at the orbital distance of Jupiter, are the Trojan asteroids that reside in 4th and 5th Lagrange points of the Sun-Jupiter gravity field. To state it simply, they are at the same distance from the sun as Jupiter, but they are clustered in two groups in front of and behind Jupiter, respectively.

Between the main asteroid belt and Jupiter are the Hilda family of asteroids, which are in orbital resonance with Jupiter. Here is a cool video showing the Hildas:

Moving inbounds toward the sun we will notice the Mars-crosser asteroids, which cross (or at least graze) the orbit of Mars. Some of these have very eccentric orbits and so they also cross the orbit of the Earth. All the asteroids that cross the Earth or at least come close to it, whether they also cross Mars or not, are classified as Near Earth Objects (NEOs) or more particularly Near Earth Asteroids (NEAs). Looking even closer to the sun we notice the Venus-crosser and the Mercury-crosser asteroids. Asteroids like all these in the inner solar system aren’t very numerous because they can’t survive very long before crashing into a planet (or the Moon) or before being gravitationally whipped out of the solar system entirely, so it is believed they are continually replenished from the asteroids of the main belt, Jupiter flinging a small fraction of them into the inner solar system. NEAs are therefore a sampling of the main belt and by studying them we can learn about the history of protoplanets in the main belt and therefore about planetary formation processes in general.

The Cost of Bringing One to Earth

As you can imagine, retrieving asteroids from the main belt is probably a bit more expensive than retrieving ones much closer to the Earth. It takes more propellant (i.e., the fuel and oxidizer that are burned in the rocket engine) to move an asteroid from farther away than it does if the asteroid is nearby. And it costs more money to launch all that extra propellant up from the Earth to even begin the mission. The kind of asteroid that can we afford to bring back to Earth — how big and from how far away — can therefore be calculated in terms of how much propellant it takes to go out and bring it back.

If we know how much the asteroid weighs, and how much velocity change we need to impart to the asteroid to properly change its orbit, and what kind of propellants we want to use to do that, then the rocket equation tells us the mass of propellant we will need to get the job done. It turns out that for a main belt asteroid — let’s say one near Ceres — to fall down toward the sun into an orbit near the Earth, we need to slow it down by about 4.9 km/s. As it falls toward the Earth it will be speeding up again, so to get it to match the Earth’s orbit and rendezvous with the Earth we need to slow it down by another 4.9 km/s after it arrives, so all total it needs its velocity changed by 9.8 km/s. If the asteroid has a mass of 500 tons (and let’s assume the spacecraft mass is negligible by comparison), and if we use liquid hydrogen and liquid oxygen as the propellants, then the total mass of propellant we will need to bring it from Ceres down to Earth is almost 4,000 tons! That’s a lot, and unfortunately it’s not the end of the story. To send that much propellant from low Earth orbit all the way out to Ceres so that we can then bring the asteroid back would require even more propellant. All total, we would need to launch 35,900 tons of propellant from Earth, and assuming a launch cost of $4,000/kg the mission will cost almost 144 billion dollars, not including spacecraft development and mission operations costs! That’s 144 billion dollars in propellant launch costs alone!

To bring back an asteroid from near Mars orbit would take a velocity change of only about 5.3 km/s, but the cost of launching the greater than 3700 tons of propellants would still be about $15B. That’s much less than bringing back an asteroid from the main belt, but still too expensive. And what about bringing back a Trojan asteroid all the way from the distance of Jupiter? Well, since you asked, that would require over 23 million tons of propellant costing over 93 trillion dollars to launch from Earth. It seems we can’t afford those kinds of asteroids.

So, what about the NEAs? Many of the NEAs require a velocity change of 1 or 2 km/s to bring back to Earth. Using our hydrogen/oxygen propulsion system, bringing back a 1.5 km/s NEA would still require 80 tons of propellant at a launch cost of one billion dollars. Even that is rather on the high end. So far, things aren’t looking so good.

Cost of Retrieving a 500-Ton Asteroid with LH2/LOX
Location	Velocity Change	Fuel	Cost
Near Jupiter (Trojan)	24 km/s	23M ton	$93T
Near Ceres (Main Belt)	9.8 km/s	32k ton	$144B
Near Mars	5.3 km/s	3.7k ton	$15B
Typical Near Earth	1.5 km/s	80 ton	$1.1B

Retrieving Asteroids the Inexpensive Way

Asteroid retrieval mission trajectory. Blue: Earth’s orbit. Inner black: asteroid trajectory. Outer black: spacecraft trajectory to quickly rendezvous with the asteroid. Arrows: direction of spacecraft thrust on outbound and return paths. TOF=Time of Flight. C3= specific energy of the spacecraft, i.e., velocity of spacecraft squared when it is well away from planetary influence.

Now for the good news. The Keck Institute of Space Studies did an analysis of an asteroid return mission, and it calculated the fuel cost of bringing an asteroid back to Earth. They found that we can affordably do it if we make three assumptions. One, we have to find an asteroid requiring only about 0.15 to 0.17 km/s velocity change to bring it back, meaning that it is not just a NEA but it is a very special kind of NEA because it is already extremely close to Earth’s orbit. This kind of asteroid needs just a little nudging to steer it gently back to the Earth. Second, even with this ultra-low change in velocity we would need a much better propulsion system than one using liquid hydrogen and liquid oxygen. The study recommends we use a solar electric propulsion system using xenon as the propellant. That kind of engine has almost ten times better performance, although it is slow. Fortunately, for asteroids that are very close to the Earth, the extra mission time is not a problem. Third, the study assumes we use lunar gravity assist to slow down the asteroid when it gets to the Earth so it gets caught in high lunar orbit. With these assumptions, the study showed we could perform the mission with only about 13 tons of propellant! In particular, they looked at the cost of retrieving asteroid 2008 HU4. It is about 7 meter in diameter and has about 1300 tons of mass and requires a velocity change of 170 m/s. At a launch cost of $4000/kg, it would cost only $52M to launch the necessary 13 tons of propellant. That might still sound like a lot, but a typical “Flagship” planetary mission costs between $2B and $3B, so by comparison the $0.052B for the propellant launch is actually quite affordable.

Finally we might ask, if solar electric propulsion is so good, then can we use it to bring back a Trojan after all? Sorry — even with the solar electric engine, retrieving a Trojan would take almost 800 tons of propellant (the xenon) and launch cost would be nearly six billion dollars. Until we can mine our propellants in space rather than launching them, such audacious things aren’t economically viable. But hey, that’s the whole purpose of space mining: to get the mass of propellants and all other such things in space so we don’t need to launch them, so then we really can afford to do all the audacious things! But for now, let’s start simple.

Finding a Good One

So we can afford to bring back an asteroid if it requires only an ultra-low velocity change. A natural follow-on question is, can we find such a special asteroid whenever we want one? We already know about 2008 HU4, but it might not be near the Earth when we are ready to do the mission, so can we find some others? Before we can answer that, we need to know the type of asteroid we will bring back in terms of its composition. Do we want a metal asteroid or a rocky one? Or maybe we want a chondritic asteroid, and if so, what type? When we know what the desirable asteroid is made out of then we can assess its optical properties and then estimate the odds of seeing one as it whizzes past the Earth. That is the subject of the next post. (Hint: it turns out the odds are really, really good!)

Author information

Phil Metzger

The post Retrieving Asteroids Near and Far appeared first on Philip Metzger.

Moon Landings and Breaking into Cars: Solving Unsolvable Problems

Phil Metzger — Sun, 28 Apr 2013 19:28:39 +0000

I don’t believe there are any unsolvable problems. In my job I’m often asked to solve things that seem impossible, but every time it takes just a little creative thinking to overturn the paradigm and solve it. I want to tell you a funny story that really happened to me a few years ago, one that illustrates how we can solve the impossible problems of life.

One evening I was working late at the Kennedy Space Center. I had to leave by 6:00 p.m. because my wife was planning to go to a meeting and I had to get home to watch the kids. It takes me an hour to drive to Orlando and she had to leave the house by 7, so leaving my workplace by 6 gave me no time to spare. Cutting it close, I walked out to my car a few minutes before 6 and realized in horror I couldn’t find my car keys. Looking through the car’s window in the evening gloom I could make them out lying on the driver’s seat. My co-workers had already left so there was nobody to give me a ride, and if I called a locksmith it would take an hour for them to get badged into the space center, ensuring my wife would miss her meeting. I thought briefly about getting a big rock and smashing the window out. That would be costly to fix. Then the thought occurred, “I’m a physicist, just like Iron Man. He can use physics to create an iron suit. Surely I can break into a car!” Yeah, I know Iron Man is not real. But that thought turned this into a physics problem, just one more unsolvable problem that I could now solve.

I quickly scanned the car for vulnerabilities, and seeing none I realized it was McGyver time: I needed to gather some random, helpful items that would magically empower me to open the car. Fortunately, I had my lab nearby, so I ran back inside to see what I could get. Unfortunately, it’s a lab equipped for studying extraterrestrial regolith, the stuff lying on planets that we Earth-people call “dirt”. While extraterrestrial dirt is very cool, it’s not great for breaking into cars, not even for McGyver. But I snatched the handle from a 5 gallon dirt bucket, got some pliers, a super magnet, a flashlight so I could see what I was doing in the deepening darkness outside, several random items, and ran back out.

All the while, I was fighting the feeling I couldn’t do this, I had too little time, and my wife was going to be very unhappy that I didn’t opt for the big rock. My Iron Man delusion was making things worse, not helping! So to fight these doubts I kept telling myself, “I have a Ph.D. in physics! I work for NASA. If we can land a man on the Moon, surely I can break into a car.”

If we can land a man on the Moon, surely we can break into a car!

That last line is one we Space People hear all the time. Not the part about breaking into a car, but this: “If we can land a man on the Moon, surely we can…” (fill in the blank). Another one we hear all the time is, “Come on, people, this isn’t rocket science!” And of course there is the all-time favorite, “Houston, we have a problem.” If the projector isn’t working in a conference room at NASA, someone will invariably say, “Houston, we have a problem.” Then somebody else will say, “Come on people, this isn’t rocket science!” And finally, someone will helpfully chime in, “If we can land a man on the Moon, surely we can get this projector to work!”

But I was saying those things without any irony, trying to keep my spirits up. I had only 3 more minutes to spare, and that left no time for doubting.

I discovered that new cars are engineered a lot better than the ones I used to break into when I was in high school. Yes, I broke into a lot of cars in high school. No, I wasn’t a car thief. I was a bag boy at a grocery store. Breaking into the cars of customers who had locked their keys inside while their ice cream was melting was an essential skill for bag boys back in the day. But now, I discovered, my bag boy skills had expired. Automotive engineers tightened up the cracks around windows and door jambs. And remember those mushroom-capped pull-posts that used to stick up next to the windows for unlocking doors? Those have been replaced with mechanically-stiff, smooth-surfaced rockers down the sides of the doors where a thin wire can’t effectively push them. So Iron Man needed to invent something new. After feeling all the cracks on the car I discovered that the rear door has softer foam in the crack along one section of its leading edge. Unfortunately, you can’t open the rear door by putting a wire through that crack to pull the rear door handle. Only the front door’s handles will open the door while the car is locked.

So with one minute to spare, I got the dirt bucket’s handle straightened with the pliers and pushed it into the rear door’s slightly larger crack, re-bent it with pliers, pushed some more, rotated it, and bent it again. I snaked it through the door jamb of the rear door, then leftward past the side of the driver’s seat, left again to the front door handle, hooked around the handle, and with a pull from the back door of the car the front door popped open. There was no time to spare. I threw the tools into my car and zoomed off to Orlando. I arrived at exactly 7:00 and my wife went happily to her meeting unaware of the drama in the Kennedy Space Center’s parking lot an hour earlier.

The funny thing is what happened the next morning. I arrived at work early and went straight to the lab to put the tools from my car back into the toolbox. When I walked inside, my colleague (and co-founder of the lab) Rob Mueller was giving a tour to a lady, a man, and two young girls. Being overly proud of myself for the prior night’s heroics, I loudly announced what I had done and made sure to use all the phrases we Space People like to say. “I just kept telling myself, ‘if we can land a man on the Moon, then surely I can break into a car.’ Surely a NASA physicist can break into a car! I wasn’t going to let a simple car stop me. It’s not like it was rocket science, after all!” What a dweeb! The lady and her family stared at me silently with the kind of look that says something is weird here, something is wrong. I noticed the looks, but I was too proud of myself to care.

Later that day I ran into Rob a second time. “Do you know who that lady was, in the lab this morning?” I answered that I did not. “She’s the Chief Engineer at General Motors.”

Oops.

I sure hope she didn’t think that I knew she was from General Motors, that I was oafishly saying those things to try to impress her, to make her think that she needs to collaborate with my lab because we are so much better than her own people when it comes to problems with simple cars, that she needs us because — let’s face it — we know how to do rocket science. But what can I do about that now? It’s just another embarrassing moment in my life, one among the many.

And so, what is the moral of this story? It might be, “don’t brag like a doofus.” Or it could be, “always know your audience before you speak.” But today the moral of the story is this one: the way to solve an unsolvable problem is to honestly believe you can solve it.

Every time I have to solve such a problem, I give myself the internal pep talk. I don’t usually go for the clichés about rocket science and landing a man on the Moon. (Those are reserved for emergencies.) But I do pump myself up, reminding myself that every problem has a solution and that I am going to find it. Honestly, I know I don’t have any special ability to solve problems. It doesn’t require a Ph.D. in physics or experience with NASA or bagging groceries, and it definitely doesn’t require Iron Man. I just believe I can solve hard problems because I believe we all can. All we need is to think outside the box and to keep thinking outside the box until the problem is solved. So learn to turn the problem inside out. Look at it upside down. Step back. Broaden the question. Answer the deeper thing that isn’t being asked. Eventually, you find the solution. Sadly, most people won’t do this. They won’t do it because they don’t immediately see the answer and they give up, believing what the disbelievers have told them.

So try it sometime. Don’t give up on unsolvable problems. Tell yourself that you can think differently and find the solution. Tell yourself that it’s not rocket science. I’ve been doing this for many years, and so far it always works. I don’t believe there are any unsolvable problems.

(By the way, I told this story to my friends on Facebook, and one of them said, “It would be an even better story if your car was made by General Motors.” Well…it was.)

Want some help in solving unsolvable problems? Here’s a great article with practical steps to see problems differently: Unsolvable Problems and How to Solve Them (Einstein’s Secret)

Author information

Phil Metzger

The post Moon Landings and Breaking into Cars: Solving Unsolvable Problems appeared first on Philip Metzger.