"Our cratering experiment went very very well," reported impact scientist Peter Schultz in what may have been the understatement of the weekend. A first look at early science results from the mission suggest that while some events unfolded according to scientists' predictions, Tempel 1 provided many enticing surprises as well.
The mission is not yet over. The spacecraft is still capturing so-called "lookback" images as it recedes from Tempel 1. The spacecraft's point of view now captures the shadowed side of the comet's nucleus. Behind the nucleus, a broad fan of impact ejecta, backlit by the Sun, spreads out into space. This lookback imaging will continue until July 7 at 17:52 UTC (10:52 PDT), 60 hours after the impact.
"The team is very tired," said Rick Grammier, Project Manager for Deep Impact at JPL. "However, they are very excited and feeling very proud at this moment. The flyby spacecraft is in good shape; all subsystems are green. Every iota of memory storage is totally full. It's still performing like a champ. We are working as fast as we can to get all the data downlinked and sent to the science team."
Two exhausted members of the science team, Mike A'Hearn of the University of Maryland and Pete Schultz of Brown University, attempted to explain some of their preliminary impressions of the images and data. The most obvious conclusion that could be drawn from a first look at the data is that the nucleus of the comet did not look like they expected it to, an elongated body similar to Comet Borelly. "You look at that nucleus, and obviously it does not look like a pickle or a cucumber. It looks closer to a muffin," said A'Hearn. "There is a lot of topographic relief. There are things on this comet that look a lot like impact craters to many of us. It looks very different from Wild-2 or Borelly. We don't understand what this means. This comet has had an orbital history that looks pretty much similar to Borelly and yet it looks totally different."
Schultz was clearly thrilled to have caught, in the moment after the impact, an incandescent flare. "At the moment of impact, you heat materials to extremely high temperatures. Some of that is heated vapor. Some of that is melt droplets from within the crater itself. It is like a flashbulb -- material that is glowing so brightly that it illuminates its own picture." Such an incandescent flare had been visible in impact experiments he performed at the NASA Ames Vertical Gun Range facility in order to prepare for the Deep Impact encounter. In fact, Schultz explained, those experiments had contributed to the design of the image sequence for the impact; because of the possibility of an impact flare, the flyby spacecraft was instructed to take images at a high frame rate, once every 50 milliseconds (or a rate of 20 frames per second). The flare is visible in only one of those frames, meaning that the flash lasted for less than 50 milliseconds after the impact.
Following the flare, Schultz said, there was a delay of a few frames before a plume of material can be seen to exit the impact point. The plume actually casts a shadow across the surface of the comet, a shadow that will eventually help the science team pin down the precise location of the impact crater. That few-frame delay indicates a layered structure for the comet, Schultz explained. "My guess is there was soft layering on top, [the impactor] went down, and finally got in contact with ices." The vaporization of ices likely produced that narrow plume. Following the plume, a broad curtain of slower-moving material can be seen to spread out from the comet. "The initial flash is an umbrella-like vapor plume. And then we see the large column. And then the curtain. The curtain later on tells us that the crater is expanding slowly, like you would expect from loose material."
Still, A'Hearn and Schultz tried to emphasize that all of this interpretation is preliminary, based upon the first impressions of sleep-deprived scientists "living on adrenaline." In fact, it may be days before the team will be able to determine whether they managed to image the crater, and how big the crater is. A'Hearn said, "It's going to be at least a week of careful image processing before we understand if the feature we have observed really is the crater itself. The problem is that there is this bright dust in front of it and we are looking through that, and you need to subtract that out of the picture. We may find it more easily by using the infrared spectrometer and looking for temperature differences. That's a very complicated procedure."
Knowing the crater's size is important, of course, because both the science team and the public had a game going to guess how large the crater actually was. "Everybody's pool can't be paid off for another week," A'Hearn said. But Schultz piped up to remind him that they knew one thing about the crater for sure. "I think it's big. I don't think it's house sized, I think it's bigger than that. I think we know whose money we're taking." A'Hearn responded, "Yes, we know some losers for sure."
But the losers of the game have clearly won with the rich trove of science data that Deep Impact produced, and continues to produce. Interpretation of that data will develop slowly over the next few days, as the team recovers from last night's dramatic events. "We have our data," commented spectroscopist Jessica Sunshine after the press conference. "Now we need some sleep."