Chang'e 2, a Chinese spacecraft that was orbiting the Moon 18 months ago, has wowed space-watchers around the world by returning detailed images of asteroid 4179 Toutatis taken during a close-in, high-speed flyby.
Confucius actually never said "A picture is worth a thousand words," but that well-known proverb's Chinese attribution (even if unfounded) is ringing true today. That's because we're getting our first looks at asteroid 4179 Toutatis as seen at close range by the Chang'e 2 spacecraft, which flew past the little asteroid two days ago. And those few images say a lot about what this potentially hazardous near-Earth object is all about.
According to Chinese news sources, the spacecraft sped past Toutatis at 6.7 miles (10.7 km) per second and came nearest — a mere 2 miles away! — at 4:30:09 p.m. (8:30:09 Universal Time) on December 13th. The daringly close flyby yielded images that show surface details less than 15 feet (5 m) across.
Launched on October 1, 2010, Chang'e 2 orbited the Moon for 8 months before being redirected last year to the L2 Lagrange point, roughly a million miles on the side of Earth opposite the Sun. But when it left L2 last April, Western observers suspected the spacecraft was heading deeper into interplanetary space. It didn't take long to realize that Chang'e 2 was bound for Toutatis.
This asteroid has been tumbling past Earth the past few days, and astronomers have used their big radar guns in California and Puerto Rico to create detailed radar maps of its lumpy, highly elongated surface. (Check out this short video of radar results, which are described more fully here.)
Measuring about 3 miles (4½ km) long, Toutatis is pretty clearly a loosely bound mash-up of two or more smaller bodies, a notion that's been amply confirmed by Chang'e 2. The spacecraft views, revealed in this report on Chinese television, show that the double-lobed asteroid has a relatively smooth surface with muted craters and scattered clusters of boulders. These views suggest that the surface is blanketed by a fair amount of loose dust. Presumably later versions will be more detailed and color-corrected.
The Chinese Lunar Exploration Program has never been big on giving out detailed information, even after the fact, so it's great to get these early looks at the Toutatis results. Chang'e 2 has a science payload virtually identical to that of its predecessor, Chang'e 1. On board are an imaging spectrometer, gamma- and X-ray spectrometers, laser altimeter, and other instruments. Whether any of those were turned on during the flyby has not been released to the news media.
But we do know that the images sequence seen here was from a small secondary imager mounted on the boxy craft's side, essentially a 1-megapixel webcam with a CMOS detector, that was included to make sure that the spacecraft's solar-cell arrays deployed properly. An interview with Zhou Jianliang, one of the mission's architects, suggests that the camera has a field of view of 7.2° and that it recorded five frames per second for the 100 seconds near closest approach. (I don't speak Chinese, but I found this hour-long special report full of interesting graphics and other insights.)
Even if no other instruments were used, Chang'e 2 might be able to provide a crucial chapter to the Toutatis story. We don't know if flight controllers intended to have such a close flyby, but such a close brush raises the possibility that tracking data collected before and afterward can be used to determine how much Chang'e 2's trajectory was altered by Toutatis and, from that, derive some limits on the asteroid's mass. Knowing the mass and the overall volume (from radar sounding and the flyby images) would yield Toutatis's overall density — key to understanding its bulk composition and internal makeup.
Jon Giorgini, a deep-space dynamicist at the Jet Propulsion Laboratory, thinks pre- and post-flyby tracking could indeed yield a mass estimate. "However, getting a mass would depend on the nature and precision of the Chinese tracking data," he cautions. "Those might not be usable here even if made available."
NASA's Deep Space Network of tracking dishes was not assisting the Chinese, but the Goldstone station's radar system was illuminating the asteroid — and the spacecraft — during part of the flyby. "Our radar signal was not optimized for detecting an object so small," Giorgini explains, but he adds that the Goldstone data are being checked for an echo from Chang'e 2.
In any case, hearty congratulations are in order. China now joins the U.S., U.S.S.R., Europe, and Japan as nations that have successfully conducted an encounter in interplanetary space.