Two asteroids caught in the act of colliding?

An asteroid collision would be an amazing event to watch. Astronomers say the average impact speed of the collision would be more than 11,000 miles per hour, or five times faster than a rifle bullet!

Astronomy is a science for people with good imaginations. Let’s face it. No one has ever yet witnessed an actual collision between asteroids. But – on January 25 and 29 – the Hubble Space Telescope photographed a comet-like object that some astronomers are saying may be fresh from an asteroid collision.

See the two images above? When you look at the larger image, be aware that asteroids are generally thought to be rocky bodies. Most don’t leave trails of debris (although some asteroids may, for example, 3200 Phaethon, the mysterious hybrid asteroid-comet that spawns December’s annual Geminid meteor shower).

In the smaller image above (the same object on a different day, btw), notice the complex X-pattern of filamentary structures near the “nucleus,” or center of the object. Astronomers say this mysterious X-shaped debris pattern and the streamers of dust trailing from the asteroid that suggest a head-on collision between two asteroids.

At the time of the Hubble observations, the object was approximately 90 million miles from Earth. By contrast the moon is only about a quarter million miles away.

The nucleus or central area of this object is estimated to be 460 feet in diameter.

This object is called P/2010 A2 by astronomers. It was first discovered by the Lincoln Near-Earth Asteroid Research, or LINEAR, program sky survey on January 6.

“This is quite different from the smooth dust envelopes of normal comets,” said principal investigator David Jewitt of the University of California at Los Angeles. “The filaments are made of dust and gravel, presumably recently thrown out of the nucleus. Some are swept back by radiation pressure from sunlight to create straight dust streaks. Embedded in the filaments are co-moving blobs of dust that likely originated from tiny unseen parent bodies.”

Asteroid collisions – indeed, collisions of all kinds – were believed to have been common in the early years of our solar system, billions of years ago. And astronomers have long thought the asteroid belt is being ground down through collisions. But they’ve never seen an actual collision.

Why is P/2010 A2 thought to be an asteroid? Doesn’t it look more like a comet with that flowing debris trail? Yes, perhaps … but normal comets come to our part of the solar system from distant icy reservoirs far from our sun, in the Kuiper Belt and Oort Cloud.

P/2010 A2 orbits much closer to the sun, in the inner regions of the asteroid belt where its nearest neighbors are dry rocky bodies lacking volatile materials. In other words, much like Phaethon 3200, this object looks like an asteroid because of its orbital path, but it behaves like a comet in leaving behind a debris trail.

According to these astronomers, this leaves open the possibility that the complex debris tail is the result of an impact between two bodies, rather than ice simply melting from a parent body.

“If this interpretation is correct, two small and previously unknown asteroids recently collided, creating a shower of debris that is being swept back into a tail from the collision site by the pressure of sunlight,” Jewitt said.

The main nucleus of P/2010 A2 would be the surviving remnant of this so-called hypervelocity collision.

The astronomers also point out that the orbit of P/2010 A2 is consistent with membership in the Flora asteroid family, whose characteristics are not well understood at present, but which is thought by some astronomers to have been produced by collisional shattering more than 100 million years ago. In other words, where we now see this family of asteroids, there was once a larger parent body that was ground down, as astronomers like to say, by collisions.

One fragment of that ancient smashup might have struck Earth 65 million years ago, triggering a mass extinction that wiped out the dinosaurs, according to astronomers.

Remember that timescales in the solar system are vast in contrast to our puny human timescales, and that the solar system continues to evolve to this day. I believe these astronomers are trying to say that P/2010 A2 looks as if it has undergone one of the collisions associated with the Flora family of asteroids – a delayed reaction from the group of collisions that created the Flora family millions of years ago – in a solar system that remains dynamic in ways we humans cannot truly comprehend.

Go here for Hubble images and more information about the possible asteroid collision.

February 2, 2010

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