Derby and District Astronomical Society

Aries
The Journal of the Derby and District Astronomical Society
September - December 2005
[Contents]

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:

• Near Earth Asteroid Rendezvous (NEAR) Shoemaker – Launched on February 17th 1996, NEAR Shoemaker was sent out to orbit and investigate the asteroid 433 Eros. NEAR Shoemaker performed its mission flawlessly and at the end of the operational phase of the mission, managers at the Jet Propulsion Laboratory (JPL) took the bold decision to attempt to land NEAR Shoemaker on the surface of Eros. This was successfully accomplished on February 12th 2001. The spacecraft continued sending images back to Earth right up to the moment of landing. Even after the landing JPL still received signals from the spacecraft. The spacecraft was named in honour of Dr Eugene M. Shoemaker, Planetary Geologist and co-discover of Comet Shoemaker-Levy 9. Dr Shoemaker tragically died in a car accident in Australia on July 18th 1997.
  
  
• Mars Pathfinder – Launched on December 4th 1996, the highly successful Mars Lander (Pathfinder) and the tiny Mars rover (Sojourner). Pathfinder landed on Mars in the Ares Vallis region July 4th 1997 and operated on Mars up to September of that year. Sojourner investigated a great number of rocks in the immediate vicinity of the lander, identifying them all as igneous in origin. The Pathfinder Lander was re-named the Sagan Memorial Station once on Mars in honour of scientist, educator, and writer/presenter of the groundbreaking 1980 TV series Cosmos, Dr Carl Sagan, who passed away in December 1996.
  
  
• Lunar Prospector – Launched on January 6th 1998, and entered orbit around the Moon on January 11th 1998. Lunar Prospector’s primary mission was for a one-year, polar orbiting mission dedicated to globally mapping lunar resources, gravity, and magnetic fields, and even out gassing events from the lunar surface. Lunar Prospector found three new gravitational anomalies known as mascons or mass concentrations. These mascons can influence the spacecraft whilst in orbit around the Moon, so the Moon has a rather ‘bumpy’ gravitational field. The most exciting result sent back by Lunar Prospector was the detection of a possible deposit of water ice at the Moon’s North and South Poles, estimates put the amount of water ice at the Poles to be on the order of 10 to 300 million tonnes, but the question remains, how did that water ice get there?
  
  
• Stardust – Launched on February 7th 1999, the mission for this spacecraft was to collect samples of interstellar dust whilst on the way to its primary target, Comet Wild-2 (pronounced Vilt-2), and return the dust sample to Earth. During February to May of 2000 Stardust collected its first set of interstellar dust grains via its collection device, a tennis racket-like affair that had a substance in it called Aerogel. It was within this aerogel that the dust particles would be collected and stored for the trip home. On January 15 2001, the spacecraft flew by the Earth to get enough speed to send it on to Wild-2. During August to December 2002 the spacecraft entered it's second interstellar dust gathering period. On November 2nd, 2002, Stardust flew by the asteroid Annefrank, and on January 2nd 2004 Stardust encountered Wild-2 and collected its cometary particle samples. The spacecraft is due to return to Earth on January 15th 2006.
  
  
• Genesis – Launched on August 8th 2001 and was sent to a point between the Earth and Sun where the gravity of both bodies is balanced. Once there it began it primary mission, which was to collect particles from the solar wind, the stream of charged particles that are constantly being emitted from the Sun. Genesis began its particle collection phase on 3rd December 2001 and completed sampling on 2nd April 2004. The spacecraft released the sample return capsule on 9th August 2004. All did not go to plan, the original landing plan called for the capsule to be caught in mid-air by a helicopter, which did not happen. The capsule’s parachutes failed to deploy and it slammed into the Utah salt flats. Fortunately, the sample container itself was not breached and the researchers say that about 70% of the original science can be recovered. It should take a year or more to analyse the returned particles.
  
  
• Messenger – The MErcury Surface, Space ENviroment, GEochemistry, and Ranging (or Messenger for short) was launched on August 3rd 2004 and after a series of gravity assists, one flyby of Earth (which it completed on August 2nd 2005), one of Venus and a final manoeuvre at Mercury, the spacecraft will be able to begin its investigation of Mercury in March 2011. Three Mercury flybys, each followed about two months later by a course correction manoeuvre, put Messenger in position to enter Mercury orbit in March 2011. During the flybys – set for January 2008, October 2008 and September 2009 – Messenger will map nearly the entire planet in colour, image most of the areas unseen by Mariner 10, and measure the composition of the surface, atmosphere and magnetosphere. It will be the first new data from Mercury in more than 30 years.
  
  
• Dawn – Scheduled for launch on June 17th 2006, the first spacecraft ever planned to orbit two different bodies after leaving Earth during a 9½ - year mission, will orbit Vesta and Ceres, two of the largest asteroids in the solar system.
  
  
• Kepler – Scheduled for a launch sometime in 2007 will search for Earth-like planets via the "transit" method. Kepler will be equipped with a one-meter diameter (39-inch) telescope equipped with the equivalent of 42 high quality digital cameras that will continuously monitor the brightness of 100,000 stars, looking for planets that cross the lines-of-sight between Kepler and their parent stars.

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 mission’s main scientific objectives were:

• Dramatically improve the knowledge of key properties of a cometary nucleus and, for the first time, directly assess the interior of a cometary nucleus by means of a massive impactor hitting the surface of the nucleus at high velocity.
  
  
• Determine properties of the comet’s surface layers such as density, strength, and composition and how porous it is.
  
  
• Study the relationship between the surface layers of the comet’s nucleus and the possibly pristine materials of the interior by comparing the interior of the crater with the pre-impact surface.

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 the impactor probe.  Image Credit: NASA/JPL.

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.  Image Credit: NASA/JPL.

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.

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