First, the Deep Space 1 mission.
This was the first of the New Millenium program which is aimed at testing technologies for space science missions. The main technology tested with the Deep Space 1 mission is the ion drive propulsion system (how it works: solar cells produce a voltage which is used to ionize and then accelerate Xenon atoms out the back of the spacecraft to send it forward - as per Newton's 3rd law). There were also several other new technologies tested - plus 2 scientific instruments - a camera (MICAS) and a plasma detector (PEPE) which were pushing the envelope of low mass, low power scientific instruments.
Here's the spacecraft being built 
but here's a "map" showing what's what and here's
a sketch of the flight configuration.
The ion drive allowed the DS1 spacecraft to slowly climb out of the gravitational well of the Sun. The trajectory was adjusted to bring it close to an asteroid - Braille - which was a little disappointing. After coping with a major disaster (loss of a star tracker - the main instrument for pointing the spacecaft in the right direction to allow it to communicate with Earth - and point the cameras at a target - they used the camera as a star tracker) - the spacecraft was targeted at Comet Borrelly.
So, what's the story with Comet Borrelly? Strictly speaking, it is P19Comet Borrelly signifying that it is a PERIODIC comet (i.e. we've seen it more than once and have some idea of its orbit) - discovered by a frenchman in 1905 - vraiment! And the 19 means that it was the 19th periodic comet to be discovered (guess which comet gets P1 - hint: a very famous comet discovered very early). Here's the International Astronomical Union reference.
You can find Borrelly in the sky right here right now (but now a year out of date!). But it is kinda faint (about 10th magnitude). Lots of people have been looking at it with telescopes. The problem is that the telescopic images only see the expanding gas cloud and dust around the comet - the nucleus is hidden inside the bright emission when the comet is in the inner solar system - when it is in the outer solar system it is too small and too dark to be seen (we just have to send a spacecraft to look at it).
Now, if you think these are cool - just wait!
In the meantime, we need to think about Deep Space 1's trajectory and its rendezvous with Borrelly. The first thing is to look in the ecliptic plane - first for the whole mission and then for the last year or so. And, yes, DS1 stays pretty much in the ecliptic. Comet Borrelly, on the other hand, has an eccentric and inclined orbit.
The flyby geometry is a little tricky to work out - the science team waved their arms, used paper cups as models and drew diagrams on napkins over lunch! The comet was moving much faster than the spacecraft so that the primary relative motion is the comet moving from below to above the spacecraft. Or - if you prefer to think of the comet as stationary - the spacecraft moved from above to below the comet - looking down on it at first - and on the Sunward side.
So, what were we expecting to find? Not much, to be honest. The camera had had difficulties, the dust hazard is severe ( (the comet is putting out 2 tons per second) and we were not optimistic that we would see anything as it whizzed by. The telescopic pictures are mostly seeing the coma - the extensive cloud of dust and gas that extends for thousands of times the size of the solid nucleus. Giotto, a European mission, is the only spacecraft to image a nucleus of a comet (Halley in Giotto's case - in 1986 - and Giotto was hit by a small junk of rock which sent it reeling before closest approach).
How did the nucleus get to be the way we think it is? It starts off as a mixture of ice, rock and carbon-based molecules. Icy material that was not accumulated into the cores of the giant planets was kicked outwards (see page 165). Some objects were later perturbed and sent back inwards, perturbed by the giant planets' gravities. These periodic comets have all been traced back to perturbations by Jupiter and are often called "Jupiter Family Comets".
Each time the comet passes close to the Sun it heats up and the volatile components vaporize (carrying off streams of dust) - often in columated jets. This is an artist's picture of the comet nucleus venting. What gets left behind is a crusty surface which is baked and irradiated - mostly rocky-carbonaceous "gunk" - hence the comet nucleus looks very dark. The reflectivity of comets is about 4% (the Albedo = 0.04) - dark, dark black. This means that the equilibrium temperature (Teq=270K [(1-A)/r2]1/4 where r is the distance from the sun in AU - about 1.4 in Borrelly's case at perihelion) is quite high - about 230K. The comet also suffers considerable tidal stresses as it approaches the Sun - comet have been known to break up.
What happens when the gases escape away from the comet's weak gravity? They become ionized by solar photons. Once ionized they become entrained in the solar wind and carried along in a tail of cometary ions - as shown in this sketch:
as well a less
colorful version. The NASA ISEE spacecraft was sent past
Comet Giacobini-Zinner in 1985 and measured these LARGE structures associated
with the interaction of the comet's huge gas cloud and the solar wind - passing
just behind the comet (but carried no camera's to image the nucleus).
OK - after all this build up, time for the results. This is the DS1 picture of Borrelly the week before encounter.
First - there was the relief that the Deep Space 1 spacecraft survived the flyby - here's the flight team, beaming. Then, we start to see the pictures - EVERYONE (including the director of JPL) is hanging around - there's actually about 40 people in the room. The pictures come in a few minutes apart. (The guy with the ponytail driving the computer later told me he took Intro Astro from me at CU years ago!). AND THEN - we get the closest approach picture - and everyone applauds and woops - and grins - the Project Manager (Marc Rayman, in red, ex-CU phD), the camera PI (Larry Soderblom - USGS Flagstaff) and a comet expert (Mike A'Hearn, U of Maryland and PI of Deep Impact mission).
The pictures of the comet can be found at the JPL DS1 website. In particular, shown on this website are the pictures from the Tuesday morning press conference. As you can see, nucleus has all sorts of detailed structure - much more than expected - with the jets coming out of the bright region (which is facing the Sun). In the closest picture pixels are 50 meters across. The comet nucleus is about 9 km long and 4 km wide (at widest).
The plasma data from PEPE are a little harder to explain (but just as cool) - same website - just below the pictures - I recommend reading the captions. Starting about 10 hours before closest approach to the comet, the solar wind began to slow down - indicating that the solar wind momentum was being transferred to the ionized cometary gases. The detector also picked up heavier ions (the top 'chevron' features) - probably ionized products of dissociated water vapor as well as CO. But unlike at Giacobbini-Zinner, the cometary interaction is asymmetric - it is not centered on the comet's nucleus - but offset by 7000 km - that's about 1000 times the size of the nucleus! The only explanation we have been able to come up with - yet - is that the jet is so strong (supersonic) that material travels quite a distance from the comet before being ionized.
After all the excitement, the science team retreat to their tiny room and huddle over their laptops, arguing about the data.
Finally, here's an irreverent version...
