NASA Succeeds In Crashing Craft Into Comet

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."