By Guy Gugliotta
Washington Post Staff Writer
Tuesday, July 5, 2005
PASADENA, Calif., July 4 -- NASA's Deep Impact spacecraft completed a flawless journey to oblivion early Monday, slamming into an onrushing comet to vaporize itself in an Independence Day blaze of glory.
Scientists and engineers here at NASA's Jet Propulsion Laboratory cheered as spectacular images taken by a second flyby spacecraft positioned nearby confirmed that the "impactor" had scored a perfect bull's-eye, smacking into comet Tempel 1 at its lower edge at 1:52 a.m. Eastern time, spewing a column of debris that lighted up the heavens.
"Oh, my God, look at that!" JPL astronomer Donald Yeomans shouted as the first images were posted. "There's considerably more material [debris] than I thought. It looks enormous."
By assessing the shape and size of the crater and chemically analyzing the debris that belched from it, scientists hope to gain new insights into the composition of the solar system at the time of its formation 4.5 billion years ago.
The flyby spacecraft, stationed 5,350 miles from the comet at impact, used two cameras and an infrared spectrometer to record the event and its aftermath for 13 minutes, then turned away in "shield mode" as the comet passed it only 310 miles away, traveling at a relative speed of 23,000 mph.
In addition, the impactor itself carried a camera that sent back crystal-clear pictures of ridgelike features, apparent craters and sinkholes, and other pockmarks that grew to dominate its field of vision as the spacecraft closed on the comet at 6.4 miles per second. The last image was sent only three seconds before the crash.
"It was just phenomenal. We didn't have to exercise one contingency plan," said the project manager, Rick Grammier of JPL. "We're minus one spacecraft: The impactor has been totally vaporized." But the flyby spacecraft emerged 40 minutes after impact none the worse for its close encounter with the comet.
Twelve hours after the explosion, the comet continued to spew a plume of debris thousands of miles into space. University of Maryland astronomer Michael A'Hearn, the project's lead scientist, said "the outgassing could last for weeks," as carbon dioxide and carbon monoxide ices within the comet vaporize in the sun.
Brown University crater specialist Peter Schultz said early analysis of the collision suggested an effect much like an armor-piercing artillery shell, with the impactor piercing a hard crust or thick sheet of surface dust and then diving into the comet, throwing up an almost vertical plume of debris.
Heat and pressure built as the impactor plunged deeper into the comet, finally causing the 820-pound projectile to explode, tossing an additional, much larger fan of debris into space. Project scientists have speculated about what the crater will look like once photo technicians subtract the debris from the images, but Schultz said final results will not be known for a week.
"I think it's big," Schultz said. "I think it's bigger than house-sized." But he would not guess about the prediction of many scientists: that it could reach the size of the Rose Bowl, the stadium only a few miles up the road from JPL.
Besides the spacecraft images, a network of about 60 Earth- and space-based telescopes and thousands of amateur astronomers were standing by to participate in the first-ever globally coordinated effort to watch an object dig a crater in a comet. Early results showed that the impact had caused the comet to brighten fivefold.
Comets, made mostly of dust and ice, periodically migrate from deep space, their outer layers burning away as they approach the sun. To get to the ancient material within, Deep Impact needed to punch through the boiling crust.
Deep Impact, with the impactor attached to the flyby spacecraft, was launched Jan. 12 for an Independence Day rendezvous with Tempel 1, about 83 million miles away and hurtling through space at 66,000 mph.
At 2:07 a.m. Eastern time Sunday, and with the comet 547,000 miles away, the flyby spacecraft released the impactor and then did a 14-minute "divert burn" both to move itself out of harm's way and get into position below the comet so it could watch its erstwhile companion be obliterated 24 hours later.
The released impactor locked on to Tempel 1. Grammier said both the impactor and the flyby spacecraft were only a little more than half a mile from their preferred tracks, "phenomenal" accuracy after nearly six months in space. The spacecraft were traveling at 43,000 mph, with the comet overtaking them at a relative speed of 23,000 mph.
Throughout Sunday, the flyby spacecraft relayed the impactor's position and its own to JPL via the Deep Space Network. At JPL, two teams of engineers (red shirts for the impactor, blue shirts for the flyby spacecraft) evaluated the information and saw no problems: "It's an understatement to say that the flight team is excited," Grammier said.
Two hours before impact, the spacecraft took control of their own destinies, using "autonavigation" to make the mission's final decisions. Ground-based engineers needed 7 1/2 minutes to send and receive signals from the spacecraft -- too long to wait as the time to collision dwindled.
Ninety minutes away, the impactor made the first of three scheduled course corrections, using its camera to point at the comet's brightest spot. Thirty-five minutes away, the impactor made a second correction.
"The first correction actually pulled it off the comet," Yeomans said. "The second one put it back where it started."
And the third, with only 12 1/2 minutes left, aimed the impactor at the lower right corner of the comet, a bright spot with plenty of sunlight for the flyby spacecraft's imagers. The flyby spacecraft focused on the same spot.
The comet grew in the impactor's camera. It looked like a giant potato, pockmarked and gouged, but also had what appeared to be broad, smooth undulating surfaces. Cross hairs in the camera focused on a smooth spot between two craters with what looked like squiggly ridges above it and to the right.
"The comet is very different in shape from other comets we've seen," A'Hearn said. "This is the first time we've seen things that look like impact craters, and we don't understand what produces the flat surfaces."
Ten minutes out, controllers announced that the last course change had brought the impactor on target with an error of only 0.23 percent.
Twenty seconds to impact, then the time had passed.
The impactor's radio signal was lost, but there would be no confirmation that the spacecraft had fulfilled its mission until the flyby cameras produced a picture of the event.
During the wait, images from the impactor continued to transmit: "Our spacecraft's doing remarkably well for something that's about to be vaporized," said Yeomans, providing commentary as events unfolded. "Our brave little spacecraft is in a very hostile environment."
Five minutes later, the first flyby picture appeared. The comet's lower right quadrant had blossomed in a brilliant, unmistakable explosion of light. Cheers erupted in the JPL control room as picture after picture flashed onto screens lining the walls, each more spectacular than the one before.
"That's plenty of confirmation, no question about that," exulted Yeomans. "I can't believe they pay us to have this much fun."