Radar Guns Primed for Asteroid Spin Trap
Maybe you’ve heard that an asteroid will make a close call with the Earth this week? Well fear not, doom and gloom it will not bring, but a wealth of new scientific information. A few premier radio telescopes are getting ready to catch a unique “spin” on this particular asteroid.
This particular space rock has the glamorous name of 2012 DA14. The 150-foot (45 meter) wide asteroid will come closer to the Earth than some of our highest orbiting geosynchronous and weather satellites on Feb. 15th. This is still a clear and definite miss for those of us that call Earth home, but it is closer than an asteroid of such a size has come in recent history, and this gives astronomers a unique observing opportunity.
A little while after closest approach, NASA’s 230-foot Goldstone radar dish will transmit a radio signal at the asteroid that will be reflected off of its surface and detected by several telescopes in New Mexico. The recently beefed-up Karl G. Jansky Very Large Array (VLA) will work in tandem with several dishes that make up the Very Long Baseline Array (VLBA) to work as an interferometer and pick up the reflected radio waves from the asteroid in an attempt to measure the asteroid’s spin.
Since an asteroid is not a perfect reflector, the radio waves reflecting off the surface can interfere with each other in a way that produces a “speckle” pattern. This is a well-known optical effect that can be captured with very high time resolution, pushing the capabilities of the VLA’s new correlator. If you’re fast enough, you can catch the speckle pattern rotating in the direction that the asteroid rotates by seeing which pair of antennas in the interferometer catches the speckles first.
The antennas need to be separated by some distance, which is why the VLBA antennas scattered across New Mexico are in use along with the VLA. This gives the precision needed to make an accurate measurement of the asteroid’s spin as it tumbles past.
Map of New Mexico showing approximate locations of the VLA, the Pie Town VLBA antenna, and the Los Alamos VLBA antenna. The Los Alamos dish looks just like the Pie Town one. And yes, Pie Town is known for its pies.
The asteroid spin is necessary to understanding the future orbit of 2012 DA14 and more about the process that can get an asteroid spinning in the first place. Known as the Yarkovsky effect, the sun heats up the side of the asteroid facing it, as you would expect. That side then radiates away heat as infrared radiation, again, as you would expect.
If the asteroid is spinning in a direction opposite to its orbit, the warm side is facing in the direction of motion and can thus slow down the asteroid as it gives off infrared radiation. (See illustration at top.) If the asteroid is spinning in the same direction as the orbit, this effect will cause it to speed up, changing the orbit ever so slightly over time. As principal investigator Michael Busch points out, this effect is always changing the orbit ever so slightly, even when the asteroid is not near Earth.
Gloom and doom? No. Important science? Yes. A chance to use a complicated, interesting, and novel technique to make a difficult astronomical measurement? Definitely yes! This is yet again one of those cases where the real science is far more interesting than the fearful, doomsday “fluff” that can surround the story.
Image Credits: Top – Alexandra Bolling, NRAO/AUI/NSF; Bottom – Google Maps and me
Other sources: VLA proposal to observe 4179 Toutatis using the speckle method. The description here finally got me to wrap my head around the subject!