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Astrophysicist Checks In: A Pair of Near Misses

What your world looks like today The Minor Planet Center hide caption

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The Minor Planet Center

Got this post from our own astrophysicist to the (radio) stars, Summer Ash. She blogs regularly at Newtonianism for the Ladies — and we're hoping to have her on the show later this week.

Special to the BPP from Summer Ash:

Wow, that was close! Did you see that?!? We had not one, but two, near misses in the inner solar system last week. Asteroid TU24 buzzed by Earth last Tuesday while Asteroid WD5 just missed Mars on Wednesday. Who knew there was so much traffic among the planets?!

The Minor Planet Center knew. While both Earth and Mars escaped the events of last week unscathed, near-Earth objects are an actual concern that might one day pose a true danger. As of today, according to scientists there, the inner solar system looks like what you see on the left. The center updates this plot daily; it's hard not to look at it and realize that the statistics may not be in our favor in the long run.

The Minor Planet Center operates from the Smithsonian Astrophysical Observatory under the auspices of the International Astronomical Union. Its scientists are responsible for naming, compiling, checking, and making available all data related to minor bodies in the solar system. These include all natural satellites (moons), asteroids, centaurs, Trans-Neptunian/Kuiper Belt objects and comets.

To be visible through a telescope, even a very powerful one, an object must reflect a fairly large amount of sunlight. Asteroids typically have an albedo, or reflectivity, of less than five percent, meaning they bounce back less than five percent of the sunlight that hits them, much like fresh asphalt. This is what makes them so hard to see.

The first asteroid, Ceres, was discovered in 1801. They weren't of much interest to astronomers — some even termed them "vermin of the skies" — until observers discovered asteroids with orbits very near to Earth. The potential threat of an Earth impact caused astronomers to take notice. With the 1994's crash of Comet Shoemaker-Levy into Jupiter and the increasing acceptance of the proposal that an asteroid impact may have caused the Cretaceous-Tertiary extinction, the threat became real. Now everyone is paying attention and each new discovery of a near-Earth object (NEO) brings with it both the hype and the fear that this may be the big one. One day that may be the case (that's a topic for another post), but today NASA's Near-Earth Object Program Office has it all under control.

Asteroids TU24 and WD5 were discovered by the Catalina Sky Survey on Oct. 11 and Nov. 20, 2007, respectively, but merely seeing an asteroid once means nothing. For astronomers to properly calculate its physical properties such as size, mass, composition, rotation, and orbit, an asteroid must be observed multiple times — the more, the better. Of these, size and orbit matter most when a potential collision with Earth is at stake. Multiple observations over consecutive nights enable astronomers to see how the light reflected by the asteroid changes, thereby helping them to estimate its overall shape and size. Tracking its path with respect to background stars gives a measure of its distance from Earth and its orbital path. Taken together, these parameters help astronomers determine whether the asteroid is likely to pose a danger to us. Potentially hazardous asteroids are put on a watch list and continually monitored all over the world. As a result of detailed follow-up observations of Asteroids TU24 and WD5, we now know much more about them.

Snazzy: Asteroid TU24 Arecibo and Greenbank hide caption

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Arecibo and Greenbank

Meet Asteroid TU24:

This sequence of images was taken by the Goldstone Solar System Radar Telescope out in the Mojave Desert, over the course of a few hours. Though the resolution isn't great, about 20 meters per pixel, the shift of the brightest spot in each image clearly shows how the asteroid is rotating. These images also allowed astronomers to refine their estimate of the asteroid's size to approximately 800 feet (250 meters) in diameter, or about the length of one and a half cross-town blocks in Manhattan. In the end, TU24 passed us at the safe distance of 0.003 astronomical units — 1.4 times this distance to the moon — roughly 334,000 miles. Twelve hours before this closest approach, the Arecibo and Green Bank radio telescopes were able to get the snazzy image on the left.

Like an interplanetary race course NASA diagram hide caption

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NASA diagram

And now for Asteroid WD5:

This one is a bit camera shy. With fewer observations to rely on, we know less about it. But the most important detail — that it would not hit Earth — was thankfully confirmed. Mars was a different story. The red planet was given odds of one in 75 odds for impact, an exceptionally high probability when astronomers primarily deal with statistics closer to one in a million or billion. Still, that's almost 99 percent chance of WD5 missing Mars completely. The asteroid's orbit skims close to Earth's orbital path (intersecting long after we had been there) and continues on to intersect Mars', as illustrated in the diagram at left, released by NASA in mid-December. Upper limits estimated WD5 to be traveling 13-and-a-half kilometers per second, which left some uncertainty as to whether a collision with Mars was inevitable.