NASA Deep Impact Probe to Comet 9P/Tempel
The Deep Impact probe performed a flyby of comet 9P/Temple and had the extra feature of an impactor to create a fresh crater in order to see a little bit inside the comet. The probe took photos and did spectroscopic analysis.
The Deep Impact probe was part of the NASA Discovery program, approved in 1999 and launched in early 2005. The travel time to the comet was half a year. The impactor separated from the probe a day before and went ahead. The impactor had its own camera so that it could take close-up photos and transmit them back to the probe before impact. The impactor had a mass of 370 kg (820 pounds) and was traveling at 10.3 km/sec (6 miles per second) when it impacted the comet. The probe flew by just a minute later at a safe distance of 500 km. Earth based and Earth orbiting telescopes were also watching, as well as the European comet probe Rosetta which was 80 million miles away but had additional spectroscopic equipment.
The impact caused the comet to brighten to about 6 times its normal brightness. However, the impact kicked up a lot more dust than expected so that the probe could not see the impact site as it flew by. One of the results of the experiment was that the comet was found to be a lot dustier than expected. Included in the material blown off was an estimated 5 million kilograms of water but also around 10 to 25 million kilograms of dust. The dust was also extremely fine and powdery. The crater continued to outgas for 13 days, with a peak in the outgassing after 5 days.
Six years later, in 2011, another NASA probe, the Stardust spacecraft, flew by comet 9P/Tempel and observed that the crater created by the impactor was about 150 meters wide and 30 meters deep.
After Deep Impact completed its impact and flyby in 2005, an extended mission was approved to do a flyby of another comet, whereby the probe would fly past Earth for a gravity assist. (The extended mission was called EPOXI, for Extrasolar Planet Observation and Deep Impact Extended Investigation, as the probes equipment was useful for some other astronomical observations as well.)
The original target for the next rendezvous was Comet Boethin. However, while Deep Impact was still en route to Earth, astronomers could no longer find Comet Boethin, which means it probably broke up and would become a difficult mission with considerable risk of failure. Therefore, a different target was settled upon, whereby Deep Impact would target Comet 103P/Hartley aka Hartley 2, which would take 2 years longer than Comet Boethin would have taken.
Deep Impact did a flyby within 700 km of Comet Boethin in late 2010, taking good photos which showed several bright gas jets, and collecting spectroscopic data.
The Deep Impact mission was extended again to fly by a third target in the year 2020, the asteroid 2002GT.