Tempel 1: Another look
Published on Oct 19, 2007 at 12:23 am.
3 Comments.
Filed under comets.
On April 3, 1867, astronomer Ernst Wilhelm Liebrecht Tempel discovered a rather dim unremarkable comet. Tempel discovered or co-discovered 21 comets and 5 asteroids. The comet of 1897, Comet Tempel 1 (9P/Tempel) is the first comet credited to Tempel. Comet 9P/Tempel has never been very impressive, reportedly never being brighter than about magnitude 11, making it a telescopic comet.
Over the years, Comet Tempel 1 was lost, found, lost again, and found again, and has been observed each time around the Sun since 1972. It currently has 5.5 year orbital period (that has changed over time due to shifts in its orbit imposed by the gravity of the planet Jupiter). In 1999, NASA gave a green light to a proposed mission to a mission to send a spacecraft to Comet Tempel 1 and blow a hole through its crust by hitting it with a projectile. The mission, named Deep Impact, was a success. Launched January 12, 2005, the spacecraft encountered the comet during the summer of that same year, with the impact occurring July 4, 2005. As expected, the impactor blasted a hole in the comet. The resulting crater is estimated to be about 100 meters across and about 30 meters deep. There were some surprises, though. The impact ejected far more dust and solid debris than had been expected. This dust cloud actually hid the crater from the view of the Deep Impact flyby spacecraft. The primary goal of the mission was to study the debris excavated by the impact, but a secondary goal was to study the crater itself. Though studies of the crater didn’t work out as planned, the ejected material yielded far more data than had been expected. The Deep Space spacecraft was following an orbit that intercepted the orbit of 9P/Tempel, not on an orbit that matched the orbit of the comet. So, the flyby spacecraft could not hang around until the dust cleared, meaning that the man-made impact crater did not get the detailed studies that astronomers had hoped for.
But, while all this was going on, another comet mission was in progress. The Stardust spacecraft was launched February 7, 1999, on a mission to fly past Comet Wild 2 (81P/Wild), a comet discovered by the astronomer Paul Wild in 1978. The goal of the Stardust mission was to fly near enough to the comet to collect some samples of material thrown outward by outgassing of the comet’s nucleus. This was successfully accomplished on January 2, 2004. Stardust then continued on its orbit, which once again brought the spacecraft to the vicinity of Earth. The samples collected were then delivered to Earth by a sample return capsule that landed in Utah on January 15, 2006. The rest of the spacecraft continued on its way with nothing much else to do, again returning to Earth’s vicinity in January of 2009.
However, on this next passage by Earth, the spacecraft’s orbit can be slightly adjusted so that Earth’s gravity hurls the craft off on an orbit that intersects that of Comet Tempel 1. This past summer, NASA gave the go-ahead for this extension of Stardust’s mission, called the New Exploration of Tempel (NExT). The orbital maneuver, though, must be extremely precise, and there is little or no room for error. Stardust does not have enough fuel to make major or multiple orbital changes. They have to get it right the first time, but NASA has a wonderful track record of using gravity assist maneuvers. After the maneuver, Stardust will be on an orbit to fly past 9P/Tempel on February 12, 2011. Assuming that everything goes well for the spacecraft (it wasn’t intended to last for that length of time), then we’ll be able to see the crater made by the impactor. Then, the detailed observations of the crater may yield additional information on the comet.
Studies of the crater may yield some surprises. For example, we don’t really know the interior structure of comets. So, analysis of the crater might yield some clues. Is the material homogeneous to a depth of tens of meters? Or is there a thin surface layer covering unaltered material below? Will the excavated portion of the comet be amorphous, or will it be layered? The answers to these questions can tell us a lot about the nature of comets, or at least about the nature of this particular comet. So far, only a handful of comets have been seen up close, and each one has strikingly different visual characteristics. It is unclear if these bodies are truly similar, but look different, or if comets are a much more diverse class of body than we’ve thought for so many years.
But, NExT, if successful, may yield an extra bonus. As I said, only a handful of comets have been visited by spacecraft (1P/Halley, 19P/Borrelly, 81P/Wild, and 9P/Tempel). None of them have been visited a second time. Only a handful of asteroids have been visited by spacecraft, either. However, asteroids are believed to be relatively slow to change. Comets shed material and undergo changes quite frequently. Solar heating causes the ices of the comet to sublimate and blow outward through vents and jets. This blows cometary material out with the gasses. Comets have been known to have sudden outbursts, and some have even broken apart. Every time around its orbit, a comet is slightly different. But, no one really knows how much change a comet undergoes each orbit, because no comet’s nucleus has been imaged on successive passages around the Sun. The NExT mission will change that. Comet Tempel 1 will be seen again. So, even if somehow Stardust does not see the crater caused by the Deep Impact impactor, there will still be useful science done just by imaging the nucleus of the comet again.
Oh, and Stardust is not the only spacecraft to be recycled and used in a new mission. After the encounter with Comet Tempel 1, the Deep Impact spacecraft was finished with its mission. That was all that it had been designed to do. However, NASA has also decided to OK the extension of that mission, too. Deep Impact is now slated to visit the comet 85P/Boethin in December 2008. So, if all goes well, NASA will have two multi-comet spacecraft missions completed, all for the price of two low budget missions, because both Stardust and Deep Impact were Discovery Program missions, which are very low budget space missions.
-Astroprof
A Ler…-- Rastos de Luz on October 19, 2007 at 10:05 am: 1
[…] Tempel 1: Another look no Astroprof’s Page […]
Scott on October 19, 2007 at 4:16 pm: 2
Your link to comet Beothin mistakenly points to 19P/Borrelly.
There is some concern about sending Deep Impact to Comet Boethin due to the absence of observations during the expected apparition in 1997. That decision may get reversed.
Astroprof on October 19, 2007 at 11:21 pm: 3
Thanks for catching that. I fixed the link.