Asteroid flyby reveals a body left over from birth of the solar system

[n2doc]

By John Timmer | Published about 23 hours ago

Many of the asteroids in our solar system are largely piles of rubble, torn apart by collisions and only weakly pulled back together by gravity. But the larger asteroids, the ones 100km and up, will generally survive most collisions with smaller objects intact. As a result, it has been suggested that these larger asteroids will generally have been unchanged since the formation of the solar system. As such, they can provide us with a picture of the early solar system's conditions at the time the first planetesimals condensed, and before they merged to form the rocky inner planets.

In July of 2010, the ESA's Rosetta mission performed a close flyby of the large asteroid 21 Lutetia. For a year afterwards, it was the largest asteroid we've visited (NASA's Dawn mission has since gone into orbit around 4 Vesta. Rosetta sent home fantastic pictures at the time, and now, over a year later, the scientific analysis has been completed. Three papers on Lutetia will be published in today's issue of Science.

One of the papers is devoted to estimating the asteroid's mass. As the probe approached, the gravitational pull of Lutetia accelerated it, creating an additional Doppler shift of Rosetta's signals; in the same way, Rosetta was slowed down once it shot past. All this was picked up by NASA's Deep Space Network, allowing the authors to plug in the values to a model of Rosetta's flyby, and calculate the strength of the gravitational pull, and thus Lutetia's mass. It's 1.7 x 1018 kg, give or take one percent. (Compare that with a bit under 1023 kg for the Moon.)

With that in hand, figuring out the volume occupied by Lutetia would allow the density of its material to be determined. Lutetia is pretty oblong, having major axes of 121, 101, and 75km. But a program called KOALA (Knitted Occultation, Adaptive-optics and Light-curves Approach) could use these dimensions and the images of the asteroid to estimate its volume. That produced a density of 3,400 kilograms per cubic meter, "one of the highest bulk densities known for asteroids."

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http://arstechnica.com/science/news/2011/10/asteroid-flyby-reveals-a-body-left-over-from-birth-of-the-solar-system.ars