Heads up: the Deep Impact encounter is coming up!

Less than a week remains before the Deep Impact mission is set to meet its fate at Tempel 1. A mission like this has been a dream for planetary scientists for a long time. There are dozens of impact experiment researchers who've spent their entire careers attempting to simulate impacts, usually by using a gun like the NASA Ames Vertical Gun Range to shoot a tiny pellet at a tiny target at speeds of 5 kilometers per second (10,000 miles per hour). Those experiments can produce structures that superficially look similar to impact craters visible on the Moon and other planets, but still it seems like quite a conceptual stretch to perform such tiny experiments and then scale them up to the size, energy, and speed of real planetary-crater-producing impacts.

Consequently there are huge battles between the experimenters and the modelers, people who use computer code to calculate what's going on physically in large impacts. Some people do both kinds of work. But because nobody has ever actually witnessed a reasonably-sized impact with scientific instruments deployed on the target, there's only so much data available to use to test the theories. Deep Impact is going to change that, at least a little bit. It's actually not the greatest test case for people interested in cratering on the Moon, Earth, and Mars, because Tempel 1 is a small enough object to make its gravity almost negligible. On top of that, we don't actually understand how comets are put together -- whether they are strong, coherent bodies like planets or just porous balls of fluffy, dirty ice, so it is very hard to use physics to model the impact and predict what's going to happen. Scientists are just going to have to watch what happens and work backward from that to figure out what the comet is made of and how it's put together. But Deep Impact will still be the first time we've had tons of scientific assets pointed at the spot where a respectably-sized impact is going to take place, and the data from this encounter will likely be mined for many years to produce a better understanding of what goes on during an impact.

Because so little is firmly known about what's going to happen at 05:52 on July 4, we've decided to have a little fun with the uncertainty. The Planetary Society has a contest going to invite people to guess the size of the crater that the impactor will make on Tempel 1. This kind of game is very popular among the people who work on missions. On the Mars Exploration Rover mission, the game was to guess where in the landing ellipse each rover was going to fall -- there were giant posters in the hallways outside the missions operations areas at JPL, and everybody who walked by could put their mark on the poster where they thought the rover would land. On Huygens, the game was to guess at exactly what time the probe would hit the surface of Titan. These games aren't just random pools, like raffles or lotteries; the outcomes depend upon precise but unknown physical properties of the new worlds that the spacecraft visit. For the rovers and Huygens, the big unknowns were how the atmospheres of Mars and Titan were going to slow the spacecraft's decent. For Deep Impact, the big unknowns are the physical properties of the comet. Whoever wins the guessing game is lucky, but in their lucky choices they have zeroed in on a previously unknown, or at least poorly understood, piece of data.

There will be a lot of action surrounding the Deep Impact encounter; The Planetary Society is hosting an event here in southern California on July 3 for people to wait and watch and see what happens. I will be at the Jet Propulsion Laboratory with the news media, waiting and watching and reporting everything I see through this Weblog. It's a packed timeline, with most of the action happening in the few hours before and after the impact.

But I'll also be looking forward to the aftermath, months to years from now, as the results come out in scientific papers. The group of scientists who are investigators on Deep Impact are an all-star team among cometary researchers and small body geologists, and they'll have a lot to say about what the comet is made of. But the presence of two researchers in particular caught my eye: Peter Schultz, of Brown University (my alma mater), and Jay Melosh, of the University of Arizona. These are two of those distinguished career impact experimenters I mentioned earlier, and they are -- ahem -- not always in agreement about how impact processes work. I think it's great that they are both on the same science team, because you can be sure that between the two of them all of the details of what happened during the impact are going to be hashed out. Every argument that either one of them makes is going to be thoroughly worked over by the other. And both of them really care about reaching out to the public, so we are going to get to watch these arguments play out. It's going to be fun.