Derby and District Astronomical Society

The Journal of the Derby and District Astronomical Society
September - December 2005

Bullseye! Deep Impact at Comet Tempel 1
By Anthony R. Southwell

On July 4th 2005 a quite unprecedented event occurred some 268 million miles from Earth, for the first time, humankind was to get a glimpse inside a cometary nucleus by smashing a portion of a spacecraft into it. The comet in question was Comet Tempel 1 and the spacecraft was the Deep Impact probe.

Deep Impact is part of NASA’s Discovery Program of space science missions, which are low-cost highly focused science missions; a proposed mission must stay within the $299 million price cap set by the Discovery Program. Other missions in this program have included:

The Deep Impact Launch. So, as you can see, Deep Impact comes from a very distinguished stable. Planning and design for the Deep Impact mission took place from November 1999 through to May 2001. As pictured at left, the spacecraft was launched atop a Delta II booster from Cape Canaveral on January 12th 2005. Encounter with its target comet was scheduled to take place on July 4th 2005.

Deep Impact was going to shoot an 820-pound instrumented projectile into the nucleus of Comet Tempel 1. It was hoped that the 23,000 mph collision would form quite a sizable crater, and that the flyby spacecraft will be able to observe the stages of the crater’s development, how deep it gets and how wide it becomes. It was hoped that Deep Impact would be able to look into the newly formed crater for almost 15 minutes before the spacecraft rapidly moves away from Tempel 1 on to an orbit around the Sun. It was expected that the collision of the impactor with Tempel 1 would cause a plume of gas and dust to spray out of the newborn crater, and the spacecraft was expected to measure and record what the plume does to the comet’s atmosphere.

The Deep Impact Spacecraft. The mission’s main scientific objectives were:

So why pick Comet Tempel 1 as the target for Deep Impact? Choosing Tempel 1 as the target had a lot to do with timing. The mission planners needed a comet that would be relatively easy to reach at about the time the spacecraft was ready for launch. The comet has an orbital period of 5.5 years and it was 268 million miles away from Earth at the time of encounter. Also Tempel 1 has a big nucleus (3.1 miles wide by 6.8 miles long). The impact should form a crater, and not obliterate the comet. Tempel 1 also has an orbit that would allow the spacecraft to reach the comet wit a high velocity and on the sunward side, so that the impact would be sunlit and visible from Earth. So, as it turns out, Tempel 1 was the prime candidate for Deep Impact because it was in the right place at the right time.

What did Deep Impact find on July 4th 2005? Twelve hours earlier Deep Impact released the Impactor probe on its one-way journey of discovery. The Impactor was completely on its own from that point on, all the navigational and course correction manoeuvres needed to keep the Impactor on target for Tempel 1 were conducted autonomously. And the Impactor did a sterling job. The images sent back by the Impactor as it approached Tempel 1 were absolutely astonishing. Both the Impactor and the flyby spacecraft took around 4,500 images during the encounter. As we got closer to the nucleus we began to see surface features down to about 12 feet in diameter. There were depressions, and what the author interpreted as a scarp feature, and what for all the world looked like craters! Now we must be very careful here, there were indeed craters imaged on the surface of Tempel 1’s nucleus, but whether these craters were formed by impact events or by sublimation effects each time that Tempel 1 comes close to the Sun, is still a matter for some debate. The author favours the latter hypothesis for the formation of the craters (I favour the former, as some have definite rims - webmaster ML)!

View of Tempel 1 nucleus from Deep Impact.
View of Tempel 1 nucleus from the impactor probe.  Image Credit: NASA/JPL.

Close Up View of Tempel 1 nucleus from Deep Impact.
A closer view of the Tempel 1 nucleus. Note the appearance of what look like craters.  Image Credit: NASA/JPL.

As for the preliminary results of the collision of the Impactor, a plume was generated from the impact site (see below), but the major surprise to researchers on the ground was the opacity of the plume and the light that was given off by it. That finding suggested that dust excavated from the comet’s surface was extremely fine, more like talcum power than beach sand, this means that the surface of Tempel 1 is weak, weaker than the finest powder snow.

The dust plume generated by the Impactor had an unexpected effect - it obscured the crater that was produced at the impact site. The plume remained hanging above the surface of Tempel 1 due to the extremely low gravitational field that Tempel 1 creates, which is 1/10,000 of the Earth’s. Scientists have estimated that the crater produced was probably at the high end of pre-mission expectations (150 to 750 feet in diameter). The impact did not affect the orbital path of Tempel 1 - the Impactor did not have enough mass to be able to do that.

Although the dust plume was blocking the view of the crater produced by the impactor, the nature of the dust is providing valuable information on the composition of Tempel 1. The Deep Impact flyby spacecraft carried spectrometers that looked at the chemical composition of the dust plume and detected vaporised water, carbon dioxide and hydrocarbon molecules. Liquid water cannot exist on the surface Tempel 1 due to the lack of an appreciable atmosphere, but the gathered data may indicate the presence of ice or dry ice. Researchers will be analysing the Deep Impact data for many years to come.

The impact on Comet Tempel 1 as imaged by the Deep Impact flyby spacecraft.
The impact on Comet Tempel 1 as imaged by the Deep Impact flyby spacecraft.  Image Credit: NASA/JPL.

The ejecta plume generated by the impact as imaged by the Deep Impact flyby spacecraft.
The ejecta plume generated by the impact as imaged by the Deep Impact flyby spacecraft.  Image Credit: NASA/JPL.

In addition to the Deep Impact spacecraft observing the impact on Comet Tempel 1, other instruments in space and on the ground observed the fireworks. They included:

The Hubble Space Telescope conducted spectroscopic observations of Tempel 1 during impact.

The Chandra X-Ray Observatory was to look for X-ray emissions.

The Spitzer Space Telescope observed Tempel 1 before, during and after impact in the infrared.

The Galaxy Evolution Explorer used ultraviolet wavelengths to watch for changes in carbon monoxide and carbon dioxide before, during and after encounter.

The European Space Agency’s Rosetta spacecraft observed Tempel 1 and utilised a NASA microwave instrument that measured the comet’s water signature before and after impact. In addition a NASA ultraviolet imaging spectrometer analysed gases in the coma and tail and measured the comet’s production rates of water, carbon monoxide and carbon dioxide.

Finally, the Submillimeter Wave Astronomy Satellite observed the comet during the month’s of June and July 2005 to monitor any change in water production before, during and after the impact.

Ground-based observatories also observed Tempel 1 during the Deep Impact encounter. The comet was best placed for observation during the encounter for observers in the Pacific, most specifically for the Hawaiian Islands, although mainland observatories were also used. These included Palomar Observatory in California, Kitt Peak National Observatory in Arizona, and the European Southern Observatory (ESO) at La Silla in Chile to name but a few. ESO monitored the comet for a week following the impact. At the time of impact Tempel 1 was low in the twilight sky in the constellation of Virgo, and was a magnitude 9.5 object. It was hoped that it would brighten following the Impactor collision.

The ESO reported that the impact did not produce a new active region on the comet, and furthermore, once everything had calmed down, the comet simply carried on as before. The impact had not made the slightest change in how Tempel 1 behaves. The author has yet to find any data on what Tempel 1’s visual magnitude eventually became at the time of impact and period immediately following it, and would be very interested to hear from the readership if they have any data on this.

Following the impact phase of the mission, NASA gave permission on July 20th 2005 for the Deep Impact team to perform a course correction burn that would put the spacecraft on a trajectory to fly past Earth in late December 2007. The manoeuvre allows NASA to preserve the spacecraft for a possible future cometary encounter mission.

So the Deep Impact mission has been a fantastic success. We have inspected the inside of a cometary nucleus and the data provided by the spacecraft will open up a new understanding of the most primitive bodies in the Solar System. By studying comets we can travel back in time to see how our Solar System was formed, and quite possibly, if complex organic molecules are found in abundance within comets, how the building blocks of life were transported to the early Earth.

There is another spin-off from the Deep Impact mission. On July 4th 2005 mankind took out an insurance policy. Deep Impact showed that we can successfully rendezvous with a comet (that has been done before, granted), but not only can we rendezvous, we can actually ‘touch’ these bodies. The data that the Deep Impact spacecraft has sent back from its encounter with Comet Tempel 1 could tell us how we could, one day, deal with the threat of a comet on a collision course with Earth.

To see the Society's 'Send Your Name to a Comet' certificate from NASA click here.