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A Valentine's Day Rendezvous

Week of February 14, 2000

At first glance, the asteroid 433 Eros might seem just like every other asteroid. It's potato shaped, about 40 x 14 x 14 kilometers in size, and rotates every 5.27 hours. When it was discovered it 1898, though, it became quickly known that this was no ordinary rock: it came very close to the Earth.

Most asteroids orbit the Sun at a safe distance from us, in between the orbits of Mars and Jupiter. But Eros was the first of what later became a sizable class of what we call ``Near Earth Asteroids'', or NEAs for short. Its elliptical orbit carries it nearly as close to the Sun as the Earth gets, and sweeps it out nearly 1.8 times as far at the other end of the orbit. As NEAs go, it doesn't get all that close, but it is one of the largest member of this group, and worth studying closely.

composite of four NEAR images of Eros Very closely. At 10:33 a.m. Eastern US time, the Near Earth Asteroid Rendezvous, or NEAR, went into orbit around Eros. This is amazing for many reasons: for one, it's the smallest body by far in our solar system to have a spacecraft orbiting it. While we have seen asteroids up close before, those have been flybys: the probe has passed close and moved on (for example, see my webpage about when NEAR passed the asteroid Mathilde). NEAR was designed specifically to orbit the tiny lump for a year, taking a tremendous amount of data.

Another reason this is amazing is because NEAR almost didn't make it. A rocket misfire in 1998 put NEAR onto the wrong track, and it missed Eros. Quick thinking on the part of the engineers in charge allowed an interesting solution to the crisis. They let the probe circle the Sun for another year, letting it once again catch up to Eros.

A third reason isn't so much amazing as it is funny. By luck or coincidence (or just possibly by design; the engineers admit they had a little flexibility in the scheduling), the rendezvous with Eros happened on February 14, Valentine's Day. In the US, Valentine's Day is a day set aside to show our loved ones that we do indeed love them. Why is that funny? Because Eros was the ancient Greek god of love! So it's an appropriate date for date, so to speak.

I think the most interesting thing we hope to learn from this mission is just how asteroids like Eros form. The only samples we have of asteroids are meteorites; parts of asteroids whose paths crossed the Earth. We have analyzed these meteorites and found that there are several classes of them. Some, called chondrites, appear to be pretty much unchanged since they formed 4.5 billion years ago. Others look like they underwent a process called differentiation, which is when heavy elements separate out from lighter once due to gravity (or spin). Like in the Earth, the heaviest elements settled to the core, while the lighter ones are up here in the crust. Will Eros show signs of differentiation, or is its chemical mix a pristine example of the young solar system? Maybe we'll find out in the next few months.

FOr more info about Eros and NEAR, including tons of pictures (which is where I got the one above), go the source: the NEAR homepage. You can find out just how far NEAR is from Eros, and get the absolute latest updates on the mission.

©2008 Phil Plait. All Rights Reserved.

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