Planets, planets, everywhere

After 10 days of vacation and a week of conferences, it's time to settle in to work for the first time in what seems like ages. This week, I'll work on catching you up on news from the 211th meeting of the American Astronomical Society. Let's start with our Solar System.

The big news is that Mars will almost certainly not be hit by an asteroid later this month. It took many nights of observing spread over a few months to verify this, but it seems that Mars will be missed by about 26,000 miles.

The Mars story should serve as an illustration for what would likely happen if we were to find an asteroid that was on course to graze by the Earth. Let's briefly review what happened. A small asteroid was discovered heading toward Mars, with pretty low chances of hitting the planet. As time went on, the odds of an impact seemed to go up, and then, suddenly, they dropped to almost zero. And this scenario would likely replay itself in regard to Earth -- in fact, it already has, with regard to the asteroid Apophis's close brush with Earth in 2029, when it will miss Earth by a mere 18,300 miles.

Why do the odds of an impact go up and go down? It all has to do with errors. When an asteroid is first found, its orbit is unknown. After a few days, a preliminary orbit can be estimated. But there are still big uncertainties in the orbit. Where it will be on a given time in the future can only be described as a circle or oval (more accurately, a spheroid), kind of like a target at a shooting range. The less we know about the asteroid, the larger the target area has to be to describe all the possible locations. So, when an asteroid is first discovered, that target area is quite huge. If a planet (say Earth or Mars) falls within that target area, then there is a chance that the planet will be hit. But the chance is pretty small, because the target is very large, and, in terms of outer space, planets are tiny. Since the asteroid can land anywhere within the target, the chance of a hit is tiny.

Now, suppose we get better measurements of the asteroid. The size of the target will shrink by quite a bit. But, if Earth or Mars stays within the target area, the chances of a hit go way up. The size of the planet remains constant, but the area where the asteroid may go is much smaller than it used to be. As time goes on and we get more data, the target area continues to shrink. And, most often, any planet in the target area will soon be outside the target area, and the chances of a hit go to zero.

If you watch the game show "Deal or No Deal," you can see the same phenomenon. In the game show, there are suitcases with prizes hidden inside ranging from a penny to a million dollars. At the start of the game, the contestant chooses a suitcase at random. Maybe the suitcase contains a million dollars; maybe it doesn't. Then, the contestant chooses remaining suitcases to open, showing off what is inside. Now let's forget the actual gameplay, and think about what happens as suitcases are opened.

We know that one suitcase contains a million dollars. As we start opening suitcases, chances are that we won't immediately open a suitcase with a million dollars in it. That means that the million dollars is still around, and may still be inside the contestant suitcase! The odds that the contestant's suitcase contains a million dollars goes up. But, eventually, the million dollars is likely to be found in one of the other suitcases, and the chances that the contestant will win a million dollars goes to zero. If you play enough times, eventually the contestant will win a million dollars. But, most times, the contestant won't win.

Now, back to asteroids and planets. Where the asteroid actually goes is like the dollar amount in the contestant's suitcase. At first, we don't really know where the asteroid is going, but as we open up suitcases by making more observations of the asteroid, we learn where the asteroid will not be. And, if one of those suitcases contains a planet about to be hit by an asteroid, the odds of a hit may seem to go up as more and more suitcases are opened. But, almost always, the suitcase containing the asteroid hit will be opened up by more observations, and the planet is safe. However, where "Deal or No Deal" uses only 26 suitcases, the cosmic version of the game uses millions of suitcases.

As we play enough times, eventually we will "win" by getting an asteroid impact (which some might consider losing the game). In fact, in 1994, the planet Jupiter "won" when it was hit by Comet Shoemaker-Levy 9. But, while "wins" happen, they are few and far between.

So, next time you hear about an asteroid with a chance of hitting the Earth, remember what happens on TV and what happened with Mars. The chances may go up at first, and they can climb pretty high. But, in the end, the chances of a hit almost always go to zero. Still, the wait can be frustrating and nail-biting in the meantime.

Finally, in other planet news, the planet Mercury will be visited in just two hours by the MESSENGER spacecraft, the first space probe to visit Mercury since 1975. MESSENGER will fly past Mercury three times in the next few years, using Mercury's gravity as a brake each time, before the probe will be slow enough to go into orbit around Mercury in 2011. This is a tough mission -- temperatures that close to the sun can melt lead, so a special sunshield had to be designed to protect the instruments. Still, the heat from Mercury's surface is enough to damage the cameras on the craft if too many pictures are taken at once. So, the success of this mission hinges as much on clever technicians and engineers as on the scientists behind the mission. Good luck, MESSENGER team!

Spinning up asteroids

This weekend I read this article about how scientists have measured an asteroid's rate of rotation (spin rate) increasing due to sunlight. This effect is called the YORP effect, after the scientists who first described it: Yarkovsky, O'Keefe, Radzievskii, and Paddack.

The effect is very subtle. (When astronomers say "subtle," we often mean "complicated and hard to wrap your brain around.") When sunlight hits an asteroid (or any planet), much of that sunlight is absorbed and converted to heat. This is why the temperature warms up in the daytime! That heat is then radiated back into space as infrared photons.

Photons carry momentum. They carry very small amounts of momentum, but when you have gazillions of photons, that momentum can start to add up, especially for small things. Particles of dust in space near the earth spiral into the sun in less than 3000 years due to momentum from this infrared radiation.

In order to change the spin of an asteroid, it has to be odd-shaped. Think of this kind of like a windmill. If you put a ball on top of a tower, the wind won't cause it to spin. But it you put up odd-shaped blades, you can make a windmill that spins very rapidly. The effect is similar for asteroids. Round asteroids (and planets) don't get spun up by sunlight, but odd-shaped asteroids can. In fact, many small, funny-shaped asteroids spin very fast; this theory explains why.

So, what is the news here? Until recently, this effect was a hypothesis. We had never measured any asteroid spinning faster and faster. But now Stephen Lowry, an astronomer from Queens University in Belfast, has led a team that measured the spin rates of two asteroids: 2000 PH5 (this asteroid doesn't have a "real" name yet) and Apollo. And the spins of these rocks are increasing at a rate consistent with theory.

What does this mean for you? Another effect similar to the YORP effect, called the Yarkovsky effect, claims that the actual orbits of asteroids could be changed from the momentum of light. If this theory is true, if we find an asteroid that will one day in the future hit the Earth, we could change its orbit by "painting" part of the asteroid with white dust rather than needing to launch a nuclear weapon or a large explosive. However, we need to be sure we understand the effect first, or else such deflection may not work or may even work in reverse, causing a rock to hit us all the sooner.