Scientists Prove Sunlight Alters Asteroid Orbits

About eight years ago, scientists postulated that something called the "Yarkovsky effect" (or YORP, for short) might be subtly effecting asteroids' orbits, forcing them into the inner solar system. The effect itself was logical enough--a subtle force changes the asteroidal spin rate, thereby changing its path--but the cause was unusual: sunlight. These types of seemingly fantastical theories abound in the sciences; it's hard to tell--with all the strings, the wormholes, dark matter, and new dimensions--which is just flight of fancy and which is reality. Last week, English and Finnish scientists showed that their feet are firmly on solid ground when they proved that sunlight does, in fact, alter asteroids' spins.

Asteroids are usually lumped together with comets and meteors, since they are all basically composed of the same goods, though the method of their formation is very different for each. Comets are composed of a hard ice/rock nucleus. As they near the Sun, the nucleus gets warm, causing the ice to vaporize and some of the nucleus to break off. Some of this dust is blown back by the solar radiation, causing the appearance of a tail. The term "asteroid" is mostly a catch-all: Asteroids are just rocky objects, plain and simple. They can vary in type from being merely a loose collection of rubble, to being a solid chunk of rock or metal, and their diametrical sizes can vary from 1 to 700 km. across. Meteors are pieces of comets or asteroids that have broken off (due to collisions) or been left behind. Once they enter the atmosphere, they become meteorites, or, as we tend to know them, "shooting stars."

Traditionally, asteroids are gravitationally trapped by the combined pulls of Mars and Jupiter, in what's known as the "asteroid belt." For years, astronomers have been trying to explain why it is that some of these objects are pulled into near-Earth orbits while the vast majority stay in such a well-defined region. The difference seems to be in the sizes of the asteroids; the large ones tend to remain in the belt, while the small ones (and, by "small," I mean less than 10 km. in diameter) are drawn in closer to us. In March, 1999, astronomers proposed a theory that sunlight was warming parts of these small asteroids, causing them to re-emit this radiation. Everyone in freshman physics learns that every action has an equal and opposite reaction. In this case, you can think of the pushing out of the radiation from the surface of the asteroid as being the action; the reaction, then, is a tiny recoil. If asteroids were spherical, this reaction would be equal on all sides as it spins on its axis. However, these objects are not always symmetric, and are therefore experiencing this tiny recoil more on some sides than others. The net effect is similar to that of a pinwheel: If you blow straight at it, it stays still; if you blow only at the top, it begins to spin wildly. Why do we care, then, how fast asteroids spin? Everything about an object in orbit determines how and where it stays in orbit; any change in rotation rate can lead to changes in orbital radius, and this change in radius can account for an asteroid's strange presence outside the asteroid belt.

Two separate teams of scientists have now proven this YORP effect to be true. Stephen Lowry, from Queen's University in the UK, and Mikko Kaasalainen, from the University of Helsinki in Finland, have each headed groups charting the spin rate of asteroids. Lowry has discovered that the asteroid "2000 PH5" is speeding up so much in its rotation that it will halve its rotational period in 500,000 years (a mere jiffy in astronomical lingo). Kaasalainen's team has shown that the rotation of the larger "1862 Apollo" is also speeding up, though 100 times more slowly, which is in keeping with the theorized effect.

Asteroids are a pet project of LSU's own Highland Park Observatory, as charting them is the first serious astronomical project on which the team there has embarked. Since May, 1998 the astronomers working at the observatory have been imaging asteroids. This study has led to the discovery of 55 new asteroids already, and there is always more work being done. If you'd like to check out the scientists in action, and be part of the newest and most exciting astronomy story this year, check them out at:

http://www.bro.lsu.edu/index.html

They have open evening sky viewings every weekend, and are only too happy (and capable, I might add) to explain more about this amazing story.