Deep Impact started out as a neat, straightforward mission.  The idea was to get close to a comet, hit it with a projectile, and observe the ensuing dust cloud for scientific data.  The spacecraft was launched on January 12, 2005, from Cape Canaveral.  After about seven months in flight, it reached its destination, the comet Tempel 1.  On July 2, Deep Impact released its “impactor,” which had its own power source and was designed to operate autonomously for just one day, long enough to move itself into the path of the comet and hit it.  About 24 hours later, the impactor successfully performed this maneuver, taking some spectacular pictures during the approach.  The actual moment of impact was recorded by the larger Deep Impact probe, which was watching from about 300 miles away.

The impact caused a brilliant flash of light, illuminating the side of the comet facing the probe: a battered little world covered with craters and other scars.  The cloud of debris was bright and larger than anticipated.  While planetary scientists were expecting the collision to throw up some liquid water with chunks of rock and ice, what they actually saw was more fine and powdery.  Because of this, it was not possible to see the resulting crater, and its exact size remained a mystery.

But it certainly made a nice plume, and this was observed by the Deep Impact probe itself and by the Spitzer Space Telescope, which is in an Earth-trailing solar orbit.  Scientists will be mining information out of this data for a long time, but already they have gotten some interesting facts.  Spitzer obtained spectrographic information, and analysis of this has revealed the signatures of a list of chemicals- they’re calling it “comet soup.”

Some of the ingredients are not surprising: silicates (sand) which were already known to be standard comet components.  But here’s a real head-scratcher: the plume from Tempel 1 also contained clay and carbonates.  What’s strange about them is that they are only supposed to form in water.

Commenting on this, Dr. Carey Lisse of Johns Hopkins University’s Applied Physics Laboratory said, “How did clay and carbonates form in frozen comets?  We don’t know, but their presence may imply that the primordial solar system was thoroughly mixed together, allowing material formed near the sun where water is liquid, and frozen material from out by Uranus and Neptune, to be included in the same body.”

This goes along with findings from the Stardust probe (featured in one of our earlier articles) which took samples of dust from comet Wild 2,  in which materials that could only have formed in extreme heat were found.  Since these particles were encased in the ice of the comet, it is obvious that the different components of the comet were formed in different places, and then somehow combined.

Spitzer also spotted some substances that have never been seen in a comet before, such as iron-bearing compounds and aromatic hydrocarbons, which can be found more commonly in car exhaust and barbecue pits.

We will certainly hear more from the ongoing analysis of that data, but meanwhile, Deep Impact is moving on to bigger and better things.

The probe is still operational, and NASA has big plans for it.  It has been renamed EPOXI, and is actually two missions combined: a search for extrasolar planets and another comet investigation.

In July 2007, NASA announced that the Deep Impact mission would be extended to include a second encounter, this time with the comet Boethin.  That would be an impressive accomplishment, but there might be a problem: the orbit of this comet was not known with absolute certainty.  It had only been seen twice ever, and the most recent sighting was more than twenty years ago.  To make the necessary course alteration, the probe was going to make a flyby of Earth and use the planet’s gravity to bend its path.  In order do this accurately, it would have to make its approach to Earth at exactly the right angle, so that it would emerge from the maneuver headed in the right direction.  The probe was in hibernation following its encounter with Tempel 1, and would only wake up when it was close to Earth.  The NASA scientists would have to find the comet quickly and calculate exactly what the angle of the flyby should be.

Unfortunately, it didn’t work.  When the probe came out of hibernation, the ground crew looked for comet Boethin, and it wasn’t there.  The approximate orbit of the comet had been calculated from the two sightings that had been made, but the calculations were obviously off.

Time was wasting; the flyby of Earth was approaching, and NASA couldn’t find its target.  A desperate decision was made: pick another comet, quick!

They picked a comet called Hartley 2.  This was actually a better target because it had been extensively observed, and its orbit was known accurately.  However, the new course would take two years more than the mission to Boethin would have, and the cost would be correspondingly higher.  That cost had not been taken into account when the original budget was drawn up, so the extension would require new funding.

There was no time for a budget meeting.  The mission team made the Earth flyby and sent the probe off toward Hartley 2, thus obligating NASA to pay for two more years of mission time.  Luckily, their bosses were understanding, and NASA increased the mission’s funding to include the extra expense.

That encounter will happen in November of this year.  Until then, EPOXI has its work cut out for it- and here we get into the other part of the extended mission.  In a complete departure from its original purpose, the probe is going to do some searching for planets around other stars.

It all came out of an earlier exercise in the Deep Impact itinerary.  When the probe was near Earth, it performed a series of observations of this planet.  As much as we know about our home world, there’s always room for more knowledge, and it was thought that Deep Impact might be able to yield some interesting science.

It certainly did.  The NASA scientists found that by analyzing the sunlight glinting off the Earth, they could tell what kind of terrain was passing by.  Not surprisingly, the oceans reflected a lot more sunlight than the land did, and water, soil, vegetation etc., all reflect different wavelengths.  By analyzing the light, it was possible to tell whether land or water was passing underneath the probe.  It was also easy to detect the presence of vegetation, as this caused a bright glow in the infrared just above the visible light range.

This was something new.  Reading the accounts at the NASA website, you get the feeling that the space boys were really surprised at the degree of detail and accuracy that they were able to achieve, and it suggested a whole new purpose for this mission.  On the way out to comet Hartley 2, EPOXI can stay busy by scanning stars and looking for glints of sunlight from them.  A brilliant gleam can only mean one thing: water.  In nature, only water, either in its solid or liquid form, is smooth and reflective enough to do that.  Even if we don’t see that, slight changes in the light reflecting from an extrasolar planet may tell us what kind of terrain it has, or maybe even reveal the presence of vegetation.

This research has already yielded some interesting results.  The probe has scanned seven stars, and while it hasn’t found the telltale glint of oceans, it has found a new planet.  It’s a “hot Neptune,” a planet roughly comparable to Neptune in size, but orbiting very close to its parent star.  While extrasolar planets aren’t the big news that they once were, there is always the chance that EPOXI will turn up something really big.  An extrasolar planet with water oceans would be a revolutionary discovery, and this could be the way we find one.

Considering the fact that this angle of research had not even been thought of when the mission started, the NASA folks certainly deserve high marks for resourcefulness and ingenuity.

Check back here for updates.  When EPOXI encounters Hartley 2 in November, we will cover it, of course.  Stick with us, and you won’t miss a thing.

Sources:
Deep Impact: MIssion to a Comet at NASA website:  nasa.gov/mission_pages/deepimpact/media/spitzer-di-090705.html

Mission news: NASA’s Deep Impact Films Earth as an Alien World at NASA website:  nasa.gov/topics/solarsystem/features/epoxi_transit.html

News Releases: NASA Gives Two Successful Spacecraft New Assignments at NASA website:  nasa.gov/home/hqnews/2007/jul/HQ_07147_Discovery_missions.html

Lakdawalla, Emily: Deep Impact Snaches Science Data From Earth-Moon Flyby at Planetary Society website:  planetary.org/blog/article/00001276/

Lakdawalla, Emily: DPS Meeting: Saturday: Studying Extrasolar Planets with Planetary Spacecraft at Planetary Society website:  planetary.org/blog/article/00001691/