[meteorite-list] Asteroid 4179 Toutatis' Upcoming Encouters With Earth and Chang'E 2

[Ron Baalke]

http://www.planetary.org/blogs/emily-lakdawalla/2012/12061004-toutatis-preview.html

Asteroid 4179 Toutatis' upcoming encounters with Earth and Chang'E 2
Emily Lakdawalla
The Planetary Society
December 6, 2012

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's a process that I described when I posted some Mars Express 
images of Phobos last year.

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. I hope that they 
do succeed, as it'd let me add one more object to my asteroids and comets 
poster!