It was a mid-autumn morning in the U.S. shortly after a major election. While the news was still brimming with the big Democrat upset, a small spacecraft found itself in the realm of icy vapors far from all of the political commotion.
The Deep Impact spacecraft flew within 435 miles of Comet Hartley 2 this past November 4th to reveal a tiny, but energetic comet full of jets, poisonous gas, and highly varied terrain. Even though the comet is only a little more than a mile across, the flyby proved to be very exciting and rewarding. Comet Hartley 2 was found to be shaped like a chicken drumstick with rough terrain and active jets at both lobes bound together by a smooth bridge of finer rock or dust. The comet may be a contact binary, or two comets, or nuclei, just close enough for the material that spews from the jets to fall back to the surface and collect between the lobes and create a smooth-looking connecting bridge. Jets were seen everywhere, even on the night side. This was unexpected as it has been theorized that the heat from direct sunlight triggers the jets. These jets were found to be powered by carbon dioxide instead of water meaning this comet is different than most. The theory has been that water ice powers the jets of comets, so the origin of Comet Hartley 2 must be different. Comet Hartley 2 also spewed a huge volume of cyanide gas briefly in September before Deep Impact arrived.
The Comet Hartley 2 flyby was not planned in the original itinerary for Deep Impact and is a bonus encounter. Deep Impact was launch on January 12, 2005 with the goal of deploying an 820-pound copper probe that would smash into Comet Temple 1 on July 4, 2005 with the hopes of raising a huge cloud of dust and ice to study. It was a huge success and Deep Impact was reprogrammed to fly by Comet Boethin in December 2008, but the comet was nowhere to be seen and probably broke up and disappeared. Fortunately Comet Hartley 2 was in the right place at the right time for a close encounter with Deep Impact. Although Deep Impact had no more probes to deploy and impact the comet, it was too good of an opportunity to pass up. The extended mission after the Comet Temple 1 encounter is called EPOXI, derived by the two acronyms of its two science objectives: EPOCh (Extrasolar Planet Observation and Characterization) and DIXI (Deep Impact Extended Observations). The original Deep Impact mission to impact Comet Temple 1 cost $252 million and the extended mission to Comet Hartley 2 cost only $45 million. This is a bargain as a separate mission to Comet Hartley 2 alone would cost around $450 million. Deep Impact is almost out of fuel, so its life is probably over, but will go down in history as a productive mission.
Another mission that has been extended is Stardust. This spacecraft collected dust particles from Comet Wild on January 2, 2004 when it flew through the coma. It ejected the collection capsule when it flew by Earth on January 15, 2006 and it is now safely in a lab. Stardust is presently on its way to fly past Comet Temple 1 on February 14, 2011 where it will photograph and study the impact crater that the Deep Impact probe created in 2005. The impact threw up so much dust and ice that Deep Impact could not photograph the resulting crater in time before it flew by. This should be another rewarding encounter on a budget by using a spacecraft beyond its primary goal.
In a tough economy it is always wise to get the most for the money. Using spacecraft to fly past comets and asteroids before or after their primary missions is the new way of being economical, with the promise of missions being more rewarding than they would otherwise be.