Near-Earth asteroid 4179 Toutatis will be passing within 7 million kilometers of Earth on December 12. It’s visited us several times before, with a close pass every four years in December. As near-Earth asteroids go, it’s a good-sized one, an elongated and lumpy object about 2 by 2 by 4 kilometers in extent.
As with all encounters since 1992, it will be the target of an imaging campaign from some of the world’s great radio telescopes, including those at Goldstone and Arecibo. In fact, the Goldstone observations have already started. At the Minor Planets Mailing List, Lance Benner posted that they expected to achieve resolutions of 7.5 meters per pixel, and hoped to achieve 3.75 as the asteroid gets very close, with very high signal-to-noise ratio. I’m looking forward to those close-approach images!
This particular close approach by Toutatis is extra-special, because Chang’E 2 (China’s erstwhile lunar orbiter) is on its way to a flyby, with a close approach on December 13. According to radio astronomer Michael Busch, Chang’E 2 will fly within a few hundred kilometers of Toutatis.
Chang’E 2 produced beautiful photos of the Moon, but it will be a major challenge for it to obtain photos of Toutatis. Chang’E 2’s camera, like most mapping cameras on orbiting spacecraft, is a pushbroom-style imager that is designed to take advantage of the spacecraft’s predictable, steady orbital speed to sweep an array of pixels along the ground, with the ground a fixed and predictable distance away from the spacecraft. This asteroid encounter bears no resemblance to an orbital mapping mission. Chang’E 2 will be passing Toutatis at a high relative velocity of 11 kilometers per second, which means that the distance to the target will be changing very rapidly. In order for Chang’E 2 to get a photo, it will have to very carefully aim its narrow-angle imager in the correct direction and slew the spacecraft in order to scan the linear camera detector across the asteroid and therefore acquire an image.
It helps that the orbit of Toutatis is extremely precisely known (thanks to all those previous radar observations), but still, it will not be easy for Chang’E 2 to succeed. If it does succeed, it will obtain at most two images, one on approach and one on departure, with resolutions of a few tens of meters. This isn’t any better than the radar resolution, but images would identify albedo patterns and could help disambiguate radar-derived models of Toutatis’ shape. This is an extremely challenging thing for China to attempt, especially given that this is their first deep-space encounter. They only recently brought online a radio telescope of the kind you need to perform the necessary deep-space communication and spacecraft tracking and navigation.