I blame NASA for a lousy night's sleep.Last night the agency's Stardust spacecraft made a close flyby of the periodic comet 9P/Tempel 1. The two were closest, about 110 miles (178 km) apart, for a few moments just before 11:40 p.m. EST (3:40 UT on February 15th). From engineering telemetry, it was clear that the spacecraft had performed exactly as expected. It had turned on cue during the flyby to keep the comet's nucleus centered and snapped frames every 6 seconds as it zipped by at 6.8 miles (10.9 km) per second.
But then I learned that the spacecraft wasn't going to relay its best images to Earth for another a few hours. So I set the alarm for 3 a.m. — only to find out that Stardust had disobeyed orders and started sending the most distant of its 72 close-approach images first.Only this late morning did the most detailed pictures reach Earth, and I quickly forgot about the loss of sleep. The craft's sole camera, pieced together from a spare lens from the Voyager mission and a detector from Galileo, recorded a diverse and structured icescape, including features that have puzzled planetary scientists ever since Tempel 1 got a visit (and took a celebrated wallop) from the Deep Impact mission 5½ years ago. The flyby also recorded a large swath of the nucleus that hadn't been seen during the 2005 visit.
Scientists are most keen to see ground zero for the strike made by Deep Impact's 815-pound (370-kg) copper-core cannonball. The impact released an unexpectedly huge cloud of gas and dust.
It's not obvious to me that Stardust's images show an obvious crater or splash. But to the trained eyes of planetary geologists, telltale hints are evident.
"We did get it, there's no doubt," says Peter Schultz (Brown University), who describes the crater as subdued and about 150m across with a small mound in the center. "It looks as if stuff from the impact went up and came back down," Schultz explains, burying most of the just-formed depression. "It's telling us that this part of the surface is weak and fragile."That the impact zone was seen at all, with excellent lighting just as Stardust came closest, is a testament to a years-long effort by ground-based observatories to nail down the comet's rotational state and spin rate. This allowed Jet Propulsion Laboratory dynamicists to time the encounter for optimum viewing. (Stardust has almost exhausted the fuel for its thrusters, so there was little margin for error.) By all account the nucleus had rotated to within just a few degrees of where the team had hoped it would be. More broadly, as lead scientist Joseph Veverka (Cornell) points out, this is the first time a comet has been seen up close before and after its perihelion with the Sun. Orbiting the Sun every 5½ years, Tempel 1 reached perihelion just a month ago. The science team will closely compare images from Deep Impact and Stardust for changes in the 5-by-3-mile (8-by-5-km) nucleus, particularly a flat tongue of debris, about 2 miles long, that appears to have shrunk since Deep Impact's visit.
Don Brownlee (University of Washington), who guided Stardust's initial science team, comments that the spacecraft's dust detector recorded several thousand hits as it cruised through the comet's tenuous coma of gas and dust. But rather than slowly rising to a crescendo and then falling smoothly, the recorded hits reveal that dust peppered the craft in rapid-fire bursts, a consequence of small "clods" disintegrating into countless smaller flecks after leaving the nucleus.
NASA now can claim two spacecraft that have each visited a pair of comets. Just two months ago, on November 4th, Deep Impact got its second rendezvous with a successful flyby of Comet 103P/Hartley 2. And Stardust's claim to fame was its fly-through of Comet 81P/Wild 2 in 2004, where it snagged dust samples from the coma in a "catcher's mitt" of aerogel that it later dropped off at Earth.
After the wild success with Wild 2, someone at NASA decided it would be clever to rename the spacecraft Stardust-NeXT, adding an appendage that stands for "New Exploration of Tempel" in recognition of its second cometary flyby. (Am I the only one who finds the space agency's name-game annoying? It's gotten really bad the past few years.)
In any case, sending Stardust to a second comet was a bargain, adding only $29 million to the mission's $300 million total cost. The craft will keep Tempel 1 in its sights as the two distance themselves over the next several weeks. But with only a few pounds of propellant remaining, the two-timing comet hunter has little more to give. NASA and Lockheed Martin, which built the spacecraft, plan to end the mission on April 28th.