Thursday, October 30, 2008

Be sure to vote (no, not just for president)

A couple of weeks ago, I discovered a new widget for adding polls to the blog. So, for fun, I made a poll about astronomy & taxes and posted it. Go ahead and try it out! I promise, no pop-up ads or special offers or computer viruses will come from that.

On Monday, I'll discuss the results. And then I'll try to think of something new.

On Tuesday, if you're a US citizen, please go out and vote (as long as you haven't already). And, in addition to the presidential race, pay attention to your state and local races, too. I'm amazed how many smart people in my own department don't care about a lot of those. But, if you don't vote in the local elections and your favorite local park disappears, or your property taxes double, or your kid's school falls apart, or you end up on trial in front of that crooked judge, you'll find yourself wishing that you had paid closer attention. Government begins in your backyard.

Greetings from Ohio State

Today I am visiting the Department of Astronomy at the Ohio State University in Columbus, OH. I'm here to give a talk about my research and to visit with some old friends and acquaintances. When most people think of astronomy, they may think of Caltech, or Harvard, or one of the observatories in the southwest, but not Ohio State. However, the faculty and students here at Ohio State are top notch, doing lots of important research on the lives of stars and galaxies. And Ohio State is a partner in the Large Binocular Telescope, a new, big telescope in Arizona.

So, Ohio State has smart astronomers, and now they have a big telescope. We should rightly expect big things from Ohio State astronomy in the next several years.

Monday, October 27, 2008

It'll be a quiet week. Yeah, too quiet.

This week, I need to work on three or four separate papers and applications, all while taking a 2-day trip to Ohio State and trying to keep up on other, more mundane tasks. So, in short, I probably won't be writing much this week. That won't mean I'm slacking, though.

Friday, October 24, 2008

Comments policy reminder

Yesterday I made some behind-the-scenes changes intended to make it easier for people to comment on the blog without having to sign up for a Blogger account. My efforts were rewarded by being spammed overnight, which I've had to spend quite some time undoing. As they say, no good deed goes unpunished.

Before anyone complains about how I erased their 600 bajillion page theory of everything, here is my comments policy, which has been (and remains) publicly available. 'Tain't my fault if you didn't read it:
Because we value your thoughtful opinions, we encourage you to add a comment to this discussion. However, don't be offended if we edit your comments for clarity or to keep out questionable matters, and we may even delete off-topic comments.

(Thanks to Dave Taylor for crafting this disclaimer and letting us use it!)

Thursday, October 23, 2008

Hubble seems to be a little healthier today

NASA announced today that it thinks it has solved the glitches that caused last week's hardware repair to fail. One instrument failure was related to a bad software patch (kinda like when my Windows machine went down last month), and the problem booting the spare data computer has been called an "electrical event of unknown origin," so they tried again and it seems to be working. Maybe a squirrel got into Hubble's transformer; that seems to be the cause of most electrical events in my neighborhood.

Anyway, in a few more days, we'll know if Hubble's main cameras are back online or not. In the meantime, Hubble is still doing science, measuring the positions of some interesting stars to very high precision. It doesn't make for good pictures, but it's good fundamental astronomy.

One star can just be an oddity, but three makes a class

In May, my collaborators and I announced we had discovered a new kind of star. Our findings were bolstered this week when another group found two more stars of the same kind. This was a breath of fresh air for us, because one object can always be explained as "weird" or "unique." But three objects means that we've truly discovered a new class of star.

As a reminder, our discovery was of variations in light from a relatively newly-discovered type of white dwarf, one with carbon atmospheres. White dwarfs are the leftover ashes from stars that have used all of their nuclear fuel. 99.9% of white dwarfs have a thin atmosphere made of hydrogen or helium, but one in a thousand white dwarfs seem to have pure carbon atmospheres. This is hard to explain, because hydrogen is everywhere, and it is so light, it will float to the top of any white dwarf star. So, the stars are mysterious to begin with.

Then, we found that one of these carbon-atmosphere white dwarfs changes its brightness every 417 seconds. We had predicted this, but were still surprised to actually find it. However, our prediction was for a very specific type of variation caused by the surface of the star sloshing back and forth, and we aren't sure that we are seeing this sloshing. Since our discovery, we've thought of lots of other things that could be causing the changes in the star's brightness, from a companion star blocking some of the light, to starspots, to the star ripping material off of an unseen companion. None of these ideas really fit the data we have, and we need new and different observations to really understand this star. Those observations will have to wait, though, because the star is currently behind the sun, out of view until next spring.

So, we still aren't sure exactly what we discovered, and we thought that we had, perhaps, discovered a one-of-a-kind star. With 10 billion stars in our galaxy, even very rare stars should be around. For example, if a certain type of physics should happen in only one in a million stars, then ten thousand stars in our Milky Way galaxy will show that strange physics. Perhaps our star was some one-in-ten-billion oddity.

Some answers in that regard came out last week, when University of North Carolina graduate students Brad Barlow and Bart Dunlap announced the discovery of two more variable carbon atmosphere white dwarfs. Moreover, these two new stars have some of the same peculiarities as our star. So, while I would argue we still aren't positive exactly what we are looking at, we now know that the star my collaborators and I discovered is not a one-of-a-kind star. These stars are a unique class of object.

So, what are these stars? I think there are three viable hypotheses, which I'm ordering by the likelihood I'd give each option. Other people probably would disagree with me, but if they want to argue, they can get their own blogs.

  1. A "sloshing" white dwarf -- This was our first idea, so I'll stick with it as most likely. The main way to test this idea is to look even harder. Most pulsating ("sloshing") white dwarfs slosh at several different frequencies. If we can detect more than one frequency in these white dwarfs, that will seal the deal. One group is already claiming to have detected such additional frequencies, but their paper is not yet published, and it is important enough that we'd like to independently verify it. I'd give this a 60%-70% chance of being the case.
  2. A rapidly-spinning white dwarf with a spot -- If the white dwarf is spinning every 417 seconds, and if it has a starspot (like a sunspot) on it, then we would expect to see the variations we actually do detect. But, most white dwarfs spin very slowly (over periods of months or years if at all), and no starspots have ever been detected on white dwarfs before. But, there's a first time for everything, and it may be possible to relate the fast spinning with the strange atmosphere. I'd give this scenario a 20%-30% chance of being right.
  3. A white dwarf stealing material from an unseen companion white dwarf -- This was our original alternative explanation, and we can't yet rule it out. The exact details of the mass transfer would contradict most ideas on what would happen if two white dwarfs came very close together. But, maybe hypothesis is wrong, as it's never been tested outside of a computer. For that reason, I think this idea has a less than 10% chance of being right.
  4. Something Else -- Maybe there's another explanation. I think we covered the obvious ones, but maybe I overlooked something?

At any rate, these carbon atmosphere white dwarfs are proving to be very mysterious!

Wednesday, October 22, 2008

A pretty picture

Today's Astronomy Picture of the Day is quite stunning:
The spiral galaxy NGC 7331
Image Credit: Vicent Peris, Gilles Bergond, Calar Alto Observatory, APOD

If you were to put your eye up to a big telescope and look at this spiral galaxy, this is not what you would see. This image has been heavily processed to bring out details and subtle colors; the process is described here.

Most color astronomy pictures you see on the web or in print have been processed quite heavily. Some people don't like that; they want what your eyeball would see at the telescope. In that case, here you go:

Sketch of NGC 7331
Image Credit: Wes Stone

Wes Stone's sketch is very impressive (most people would probably just see an oval blob), but you still miss out on a lot of the details that the processed image can get you. The fact is, the reason we astronomers use cameras and computers to process images are to bring out details that the eye, an imperfect light-collecting device, would normally miss. The colors in the first image are giving us a lot of information-- that the big galaxy is making new stars, that the other galaxies are much further away, and even some details about the stars in our own Milky Way that appear in the same picture. Your eye alone would never see these subtle colors, only a faint, ghostly white, and so you would not be able to ever learn what is going on in each of the "island universes" (galaxies) in the pictures.

The image processing that we do to make pretty pictures is not done to fool anyone, or to take away the "naturalness" of an image (if any picture made by sticking a digital camera on the back of a heavy piece of machinery is "natural"). Rather, it is to bring out the true beauty of stars and planets and galaxies, to enable each of us to look on worlds that our eyes alone would never be able to reveal.

Monday, October 20, 2008

Gyrochronology, the dating of spinning stars

the gyro
Image source: Delia's Restaurants

I like the English language. And it's not just because I'm a native speaker, but also because you can invent words to describe new inventions or ideas, and if it is a good word, people will use it.

A newer word I heard for the first time last week was "gyrochronology." You might guess from the word that it has to do with figuring out how long the food on display in a Greek restaurant has been sitting there -- from the Greek words chronos, meaning "time", logos, meaning "study", and gyro, meaning a tasty sandwich (pictured above). And you'd be close, but not quite right, because gyro is actually a word meaning "to spin," like the rotating spits of meat from which a gyro's filling are cut. So, gyrochronology is actually a word meaning, roughly, the study of ages determined from spinning. Huh?

At the meeting on the ages of stars I attended last week, I learned quite a bit about stars and how we get ages for them. One technique that was talked about was, in fact, gyrochronology.

Most things in the Universe spin. The Earth spins every 24 hours (actually every 23 hours and 56 minutes, but who's counting?). The sun spins about every 25-36 days, depending on whether you are looking at the equator or the poles. The Milky Way spins every 250 million years. And neutron stars can spin hundreds of times a second.

Due to a law of physics called the "conservation of angular momentum," once an object starts spinning, it will continue to spin unless some other force works to slow it down. Stars don't have a lot of forces acting on them, so they continue to spin for billions of years.

Many decades ago, astronomers noted that very young stars tended to spin very fast, as fast as once a day or more. But the sun takes almost a month to spin. This seemed odd, as there was no obvious reason why the sun should slow down. After a lot of head-scratching and wacky ideas, astronomers finally came to think that it may be the sun's magnetic field that acts as a break. The sun's magnetic field extends well beyond the orbits of the planets of our Solar System, and so may be able to interact with other magnetic fields, which should slow the sun down.

In recent years, a handful of astronomers have studied a lot of clusters of stars (groups of stars born at the same time, and so we can use many methods to get their ages). These astronomers, such as astronomer Sydney Barnes of Lowell Observatory, have determined that stars seem to slow their spinning at a fairly well-defined rate. Not only that, but only stars with magnetic fields slow their spinning in time. This is a nice confirmation of the solar spin-down idea, and it gives us a tool to get the ages of stars, if we can measure how fast they are spinning. Thus, we have a new field of astronomy -- gyrochronology.

There is still a lot of work to be done. Although the hypothesis that magnetic fields seem to slow the spinning seems to work, we don't quite understand how it works. And since we don't yet understand the details of how it works, we can't yet make a predictions using the hypothesis. For the time being, we have to calibrate the spin rates of stars using stars with known ages (like stars in star clusters, or the sun). And we have more calibrations that need to be done; except for the sun, we have few limits on the ages of old stars.

So, astronomy gets a nifty, if as yet somewhat mysterious, new tool to use for calculating how old a star is (as long as the star has a magnetic field). And the English language gets another new word, which you can now use to impress your friends and neighbors.

Friday, October 17, 2008

Hubble limping along

One of the advantages of being at a conference in Baltimore is that we are just a few blocks from the Hubble Space Telescope's home, the Space Telescope Science Institute (STScI).

This week, STScI is trying to recover Hubble from its recent troubles. As a reminder, the computer on Hubble that prepares and sends the pictures back to Earth failed about 3 weeks ago. Hubble has a spare on board, but it took time to develop plans to bring that online. Those plans were approved this week, and the changeover began. We've been getting updates on this changeover from Institute scientists throughout the week,

The changeover is not going as smoothly as would like. The spare computer (which has been turned off for 19 years!) booted up, but has since shut down due to a problem. We don't know why yet, but scientists are working on diagnosing the problem.

Two of Hubble's three main cameras are also having some problems. The power supply to an ultraviolet camera is acting up, and the cooling system to the infrared camera is also acting up. The engineers don't think these are related, but diagnosis is ongoing.

In the meantime, Hubble is actually working. The Fine Guidance Sensors, which tell Hubble where it is pointing, have also proven useful for science in the past several years. These sensors can actually detect the wobble of stars in the sky due to Earth's motion around the sun. These sensors don't use the broken data computer to talk to Earth, so all of the approved science using those sensors is being done now, while the other problems are being troubleshooted.

As I said before, Hubble is not completely broken. But it is limping along, and it is unclear how well we can fix it before the upcoming Hubble servicing mission next spring. It is quite possible that, with a few more weeks of troubleshooting, Hubble will be back to work with its full complement of cameras. In a worst case, it will have to just use its Fine Guidance Sensors for science until the repair mission.

Best of luck to the Hubble scientists and engineers as they continue to struggle with Hubble!

Wednesday, October 15, 2008

Bubble, bubble, toil and trouble: How convection affects stars

Image Credit: http://www.germes-online.com/

As I mentioned yesterday, this week I'm at a conference in Baltimore on the ages of stars. Many of yesterday's talks centered around calculating the ages of stars from theory. We do this by "building" a star in a computer, letting the computer age the star according to physics, and seeing what happens.

As a general idea, this is a fairly simple concept. You need to tell the computer how massive the star is, what material the star is made out of, and then just give it a list of physics: nuclear reactions, gravity, temperature and pressure laws, and other such stuff. The computer then calculates the life cycle of the star. Essentially, at the core of the star, nuclear fusion converts hydrogen to helium. When the hydrogen runs out, the star begins to die.

But the devil is in the details. One of the biggest issues that we've struggled with for decades is called mixing. Parts of stars undergo a slow roiling, like the material in a lava lamp. Blobs of hot material rise up, cool off, and sink back down. The motion of these blobs stirs up the material in a star, and can bring some fresh fuel down to the nuclear reacting core of a star. The problem is, this motion is very hard for computers to model. We can put simplified versions of mixing into computers, but it's a bit of a guess as to how much.

Physics doesn't tell us how much things will get mixed up by this roiling, because we don't have enough information about the exact conditions in a star. So, we just put a knob on the computer models, and slowly turn it from no mixing to a lot of mixing. When astronomers do this, the age of a given star can change by almost a factor of two. And that's not so great; it's like you asking me how old John Q Public is, and I respond that he's between 40 and 80 years old.

But all is not lost. The mixing does more than just bring some fresh fuel to a star's nuclear furnace. It also subtly changes what the star looks like from the outside. So, the job then falls to people like me, an observer, to measure what large numbers of stars look like and to compare that with the various computer models. And, when we do this with stars, we find that there needs to be some moderate mixing, which makes stars a little older than we thought. So, I could use that to tell you that John Q Public is in his mid to late 60s. That may not be as accurate as you'd like, but it's better than the earlier answer.

This was part of the point of this conference, to bring both theorists and observers together to compare our data and computer models, and to let each other know our newest and best results.

Tomorrow, we talk about my main topic of interest -- white dwarfs.

Tuesday, October 14, 2008

The Ages of Stars, or My Week in Baltimore

his week I am in Baltimore for a conference entitled "The Ages of Stars." So, as time (and internet) permits, I'll post some of the things I learn during the conference.

Most of the work we do as astronomers involves needing to know how old things are. The ages of things give us tight constraints on the type of physics needed in a certain situation. The classic example of this is our sun.

Throughout history, there have been various guesses as to what powered the sun. Ancient mythology often portrayed the sun as a flaming chariot crossing the sky, while ancient Greek scientists imagined the sun as.

In the late 1800s, we were starting to learn that the Earth was old. At least millions of years old. Physicists by the name of Lord Kelvin and Hermann von Helmholtz came up with the idea that the sun was powered by gravity. If the sun starts as a large cloud of gas, gravity will try and make the gas cloud contract. When a gas contracts, it heats up, causing its pressure to increase. So, the sun would contract under gravity until its internal pressure balances the force of gravity. If you do some basic calculations, you find that the temperature of this gravity-powered sun would be roughly the same as the sun's actual temperature!

As the gravity-powered sun would be as hot as the real sun, it would release heat into space. This heat loss would drop the sun's pressure a bit, and gravity would then cause the sun to contract a little more, causing it to heat back up again, and again stopping its shrinking. This process would continue until the sun finally shrank so small that it would disappear.

If you work out the time it would take the sun to go from a big cloud of gas to an infinitely small ball due to this contraction, it works out to be about 30 million years. For Lord Kelvin & Herrman von Helmholtz, that was great, because the Earth was old, but still younger than the total lifetime of the sun.

Then, early in the 20th century, we learned about radioactive decay. This allowed us to get the true ages of rocks on Earth. And geologists found that the Earth was not millions, but billions of years old. About 4.5 billion years old, to be more exact. Suddenly, Lord Kelvin's contracting sun wouldn't work. The sun needed another energy source, one that could last billions of years.

So, in stepped Sir Arthur Eddington, a British astronomer. Eddington proposed that, since the interior of the sun is so hot, there must be nuclear reactions occurring in the sun. And those nuclear reactions (specifically, the fusion of hydrogen atoms into helium atoms) would release a ton of energy. The sun would have enough energy to last for up to 100 billion years under fusion. It took decades to prove Eddington right (that's a story for another day), but he was right. And his research was spawned by knowing how old the Earth (and therefore the sun) had to be.

In one of those strange twists of nature, it turns out that Kelvin and Helmholtz weren't completely wrong. When stars are formed, they contract from a big cloud of gas to a star, and this contraction follows the basic laws and time scales of Kelvin and Helmholtz.

There are many other examples of the ages of stars and galaxies forcing us to reconsider certain physics. For that reason, we continue to try and determine more accurate ages of stars. And this conference is an attempt to bring everyone who works in this field up to date on the latest research.

Thursday, October 09, 2008

Planetariums and presidential politics

In Tuesday's presidential debate, John McCain accused Barack Obama of asking for $3 million for the Adler Planetarium to get a new "overhead projector." Now, for me, an overhead projector looks like this:
A typical overhead projector
If three million dollars were being spent on that, I'd agree it is a horrid waste of money, since I can buy one of those machines for $160 at OfficeMax.

BUT, that is not what the Adler Planetarium wanted. They needed a new Zeiss star projector, which looks something like this:

Zeiss projector
complete with computer controls and precision optics. Combined with other multimedia equipment, these machines are the centerpiece of any planetarium. And such technology ain't cheap; it takes more than a 60 watt light bulb and a screen to get this puppy to work.

There are many other bloggers (three separate links there) who discuss this issue in more detail; I won't repeat their arguments here.

Basically, the money was a request for a science education project in Chicago. If people want to debate if that money is worthwhile, fine. If people want to debate if and how projects should be specified in the federal budget, fine. But to state that this request is for an "overhead projector" is an intentional effort to mislead the public, the sort of mis-statement that McCain has claimed to oppose throughout his career (a stance I admired him for). And this seemingly throw-away statement does far more than just try and make Obama look bad -- intentionally or not, it belittles the fine folks at Adler Planetarium who have dedicated their lives to science education, at the cost of being grossly underpaid and underappreciated for their significant talents in both science and education.

Finally, let's look at this year's federal budget. The budget deficit for fiscal year 2008 was about $438 billion. So, let's take this three million off of there. Now the federal budget deficit would have been $437.997 billion. In fact, let's take off every earmark "pork" project in the 2008 budget, about $10 billion. Now the budget would be down to $428 billion. Hmm, not much better. In fact, we would have to cut 90% of the non-military, non-mandatory federal budget ($487 billion in fiscal year 2008) to solve the budget deficit. 90% cuts in homeland security, education, energy, transportation, health, science -- everything, except the military, social security, medicare, and interest payments on the debt. Face it, folks, pork barrel spending is barely a drop in the bucket of our federal budget deficit.

We've got big budget problems. While $3 million may sound like a lot, it's peanuts compared to the choices that will have to be made if we want to cut the deficit significantly. This country has big problems with finances, infrastructure, external security threats. Whether Adler Planetarium gets its $3 million or not will not affect any of those problems in the slightest. Instead of railing against science education, let's have our presidential candidates talk about things that are important, like the trillions of dollars that have been lost in the stock market. (Let's see, for $3 trillion, I could buy a million Zeiss overhead projectors...) Or the 10 million people who are unemployed (if we cancel the Adler planetarium outlay, we can give each of them 33 cents). Or the 30,000 American soldiers that have been wounded in Iraq. Or the fact that Osama Bin Laden is still a free man, despite having killed over 2700 Americans. Compared to these issues, the Adler Planetarium doesn't seem like an issue that should decide who our next president should be.

It's bad when even astronomers are talking finances

Don't Panic
Image © 2004, Buena Vista Home Entertainment

We astronomers can be a fairly unflappable group, perhaps because our science gives us a sense of proportion, sometimes a bit too much so. On September 11, 2001, a handful of my colleagues were hard at work, figuring "things would sort themselves out." When Hurricane Ike was approaching the Texas Coast and had a chance of striking Austin, our lunchtime and break time discussions were centered around that week's astronomy colloquia.

So, it was a little astounding at lunch yesterday, when the lunchtime discussion was mostly related to the U.S. financial crisis. A couple of times, a couple of people tried to steer the talk into a discussion of magnetic hoop stresses and the magneto-rotational instability (topics I don't understand very well), but even the die-hard theorists' eyes kept flitting back to the restaurant's TV screen, as the Dow went from -60 points to +60 points to -50 points to positive territory again, all in about 45 minutes.

Bad economic times do not bode well for science. State budget problems mean fewer positions at state universities. The 30% drop in the markets takes a significant chunk of money out of university endowments, donated monies that are invested, with the profits being used to fund research, scholarships, and jobs. The federal science budgets will almost certainly not rise greatly, if at all, except in a few specific fields (like energy technology). The extent our future money problems is extremely uncertain. So, we're nervous, too.

Tuesday, October 07, 2008

Fireball from Earth-targetting asteroid likely spotted

As I mentioned last night, a small asteroid, named 2008 TC3, was discovered on a collision course with Earth. Because the rock was small (less than 5 meters, or about 15 feet, smaller even then the numbers in my original post), all that was expected was a streaking meteor and a likely explosion high in Earth's atmosphere.

Well, at least two reports have come in, one from an Air-France KLM jetliner somewhere over Africa, and another from a spotter in central Europe. Both described the event as lasting just about 1 second, terminating in a bright flash. This likely means the rock didn't survive very long, exploding very high in Earth's atmosphere.

More than anything, this event proves that our various near-Earth asteroid surveys are capable of detecting even very small asteroids, and that our computer simulations that determine the trajectories work. We thought that they would, but they had yet to be tested under extreme conditions, such as very close passage to a major planet, when the pull of gravity gets very strong very quickly.

Congratulations to the discoverers of 2088 TC3, to those who successfully predicted its path despite just a single day's worth of data, and to all who witnessed both the incoming asteroid and the resulting flash. Newton's theory of gravity wins again! (I always worry about that, since gravity is only a theory, complete with strengths and weaknesses. :-P )

Monday, October 06, 2008

No need to worry, but a small asteroid will hit the Earth tonight

As I mentioned in my last post, asteroids come in a range of sizes. A few are big, maybe a thousand miles across. Most are small. The smaller you go, the more there are. And there's no formal lower limit on the size of an asteroid, so rocks just a few feet across could be called asteroids.

I mention this so that when I say the next sentence, you don't worry (because there is no need to worry). But, in just a few hours an asteroid is going to hit the Earth.

Don't worry, this is not one of those dinosaur-murdering, ten-mile-wide monsters. No, this rock is probably about 30 feet across, which is so small that it is likely to explode and burn up in Earth's atmosphere.

The asteroid, with the exciting name of 2008 TC3 was discovered last night by astronomers at the Mount Lemmon Observatory outside of Tucson, Arizona. It was quickly confirmed by other observers. The size of the rock is estimated from how bright the object appears and how far away it is. For being on a track to hit the Earth, it is still very faint, over 10,000 times fainter than what your unaided eye can see.

That will change tonight, though, when the rock enters Earth atmosphere over Sudan. Friction from the atmosphere will cause the rock to heat up and start to burn away, producing a brilliant fireball. The fireball could explode, which could create a loud sonic boom. And all of this while the fireball passes over a war zone...

We have no clue whether this asteroid will survive its trip, reaching the Earth's surface as a shower of meteorites, or whether it will completely disintegrate in the atmosphere. At any rate, people in northern Africa and the Arabian Peninsula all have a good chance of seeing a fireball tonight (at 02:46 UT, or about 10:46pm Eastern Daylight time).

Asteroids this size probably hit somewhere over the Earth every few months. The difference with 2008 TC3 is that we know it is coming. And, for the first time, we might be able to train scientific instruments on the fireball and learn something about it, like its composition, whether it is a solid rock or a conglomeration of even smaller rocks, and perhaps even where in the Solar System it came from (though this article from New Scientist says there probably won't be any time for an official observing expedition to be put together).

So, again, no need to panic. I'm not fleeing for the hills; in fact, I'm a bit disappointed that I will miss out on the show. I look forward to seeing pictures and video of this event!

Thursday, October 02, 2008

Nuking asteroids

Nuclear weapons test Image Credit: U.S. Government, found on the Nuclear Weapon Archive website

This week, we had one of the more interesting colloquia in a while. A former University of Texas student and current employee of Lawrence Livermore Laboratory, David Dearborn, gave a talk on how to use nuclear weapons to divert asteroids that are on a collision course with Earth.

This idea has been popularized in movies like Deep Impact and Armageddon. But most of us have had serious doubts that using nukes on asteroids would be a good idea. Apollo astronaut Rusty Schweickart is strongly against this idea. NASA has been considering the possibility of using nukes on asteroids, and thinks the idea may have merit. I have been (and remain) very skeptical. But David's talk did give us a lot of food for thought.

First, let me be clear. There are no known asteroids that will hit the Earth in the foreseeable future. Not one. So we have time to discuss this problem, and we don't need an answer tomorrow. Though we know we can call on Bruce Willis if we need to.

Asteroids are bits of debris left over from the formation of the Solar System, ranging from small rocks up to a thousand kilometers across. Most asteroids are between the orbits of Mars and Jupiter, but a few (maybe a few thousand or so that are bigger than a few hundred feet across) get close enough to the Earth that they may, one day, hit the Earth.

Asteroids orbit the sun due to gravity, just like planets. So, you would think that we could figure out where the asteroids will be hundreds of years from now, just like we can with planets. But it ain't that easy.

Because asteroids are small, it is fairly easy for them to get tossed about. An asteroid going past the Earth won't do much to the Earth, but the Earth's gravity can fling the asteroid about like it is a toy. The gravity of undiscovered asteroids is enough to change their orbits in ways we can't predict. The strength of the Milky Way Galaxy's gravity varies across the Solar System in ways that affect asteroid positions in the far distant future. But the biggest unknown comes from the Yarkovsky effect.

The Yarkovsky effect is not hard to understand. Think of a typical day on Earth. When the sun rises, the temperature starts to go up. At night, the temperature drops as the Earth re-radiates that heat. The same thing happens on asteroids.

Particles of light, called photons, have momentum, just like any other moving thing. So, when light hits the Earth or an asteroid, the rock gains the momentum of the photon. It's not a lot, but the sun is always putting out photons. When the Earth or the asteroid re-radiates that light (as infrared, heat photons), it has to give the photon a little momentum. In a nice Universe, everything would balance out. But it doesn't, and the Earth and asteroids tend to send out those photons in some directions more than others. And that tends to act like a little tiny rocket engine, slowly pushing the Earth or the asteroid around.

The Earth is so big that this virtual photon rocket engine doesn't affect the Earth's orbit, not even over billions of years. But asteroids are tiny compared to the Earth, and over 100 years, this rocketing affect can cause an asteroid to drift away from its predicted position by a million miles or more. Worse yet, we can't predict how far or in what direction this drift will happen, because it all depends on how fast the asteroid is rotating, what shape it is, what it is made out of, whether it is a solid rock or a flying pile of gravel, and many other effects.

Now the Earth is only 8000 miles across, so if the Yarkovsky effect can move an asteroid by up to a million miles in a century, it seems unlikely that we can really know if a given asteroid is going to hit us or not more than a hundred years into the future. We can tell which asteroids definitely will not hit us (if they're always more than a million miles away). However, less than a century away, we can rapidly become certain how dangerous any given asteroid is.

One of Dearborn's points on Tuesday was that, should we detect an asteroid that looks like it might hit the Earth over 100 years into the future, we probably shouldn't try and deflect it right away. Since we can't be sure it will hit us, there's always the chance that, by moving the thing, we actually put the rock on a collision course. Instead, we should spend some time getting to know our enemy, sending robots, radio transponders, anything that will tell us: What is the asteroid made out of (stone, iron, granite)? What is the asteroid's internal structure? Is it a pile of gravel, a solid rock, a big rock with a lot of gravel around it, a giant chunk of iron, or something else? What is the asteroid's exact orbit, including effects due to rotation and gravity and other planets?

His second point is, if we find an asteroid that is going to hit Earth in the near future, nuclear weapons may be our only option. A ten or twenty year lead time may not be enough for any other option. And, since there isn't time to learn a lot about the asteroid, we should hit it and hit it hard, launching several bombs and blowing the thing apart. There may still be some dangerous-sized pieces left, but if we can spread them out over an area tens or hundreds of times the size of the Earth, the danger goes down some. And, as much as we hate to admit it, given the choice between a chunk of rock that will cause human extinction and a few chunks of rock that will destroy city-sized areas, most of us will choose the latter.

David's other point that hit home involved the idea of using a nuclear weapon to try and "nudge" an asteroid out of the way. Suppose we have a 100 year lead time, and we need to give an asteroid a nudge. A gravity tractor would work, but requires launching several hundred tons of material on a course to rendezvous with the asteroid. We've never launched so much material. We could also put an efficient rocket engine on an asteroid, and slowly fire it to move the asteroid. Again, this requires putting 10 or 20 tons of material on an asteroid, which isn't impossible, but is hard to do. And how do you keep that rocket engine always pointing in the right direction?

But nuclear weapons are light (about a ton), and we can (and, indeed have) put robots weighing that much anywhere we want in the solar system. And, we know what happens when we blow one up, so we know how much energy we'll be giving the asteroid -- according to Dearborn, a single nuke should be able to push a small asteroid enough to move it out of Earth's way 40 years down the road. And nukes are cheap. We have a lot sitting around, and we have rockets to put them on. So, in Dearborn's opinion, nukes are a cheap, easy, and fully-understood means of trying to push an asteroid out of the way.

I'm still skeptical on this last point. I think all of us are a little leery of using a nuclear device for any reason, and there are a lot of unknowns about asteroids -- I don't think we can be certain that a single explosion would do the trick. And, if it split the asteroid into two or three chunks, now we have twice or three times the trouble to deal with. So, I'm not convinced that nukes are the answer.

But Dearborn did convince me that we shouldn't just throw away the idea of using nuclear weapons to deflect asteroids. It, along with gravity tractors and ion propulsion and laser pulsing and other, even more exotic ideas, are all worth more and careful thought and, perhaps at some point even worth testing. And Dearborn convinced many of us that, should we find a new asteroid that will hit us in a few years' time, we may be able to save ourselves with the very weapons that constantly threaten our existence. Thankfully, with no asteroids on the way that we know about, we don't have to make such a fateful decision now.

After Dearborn's talk, I made a joking comment to a colleague that we should keep the nuclear option under wraps, so that a nation like North Korea or Iran doesn't try and use asteroids as a reason why they need nuclear weapons. I was later surprise to find out that such a claim has already been made.