Deep Impact (spacecraft)

Template:Infobox Spacecraft Deep Impact is a NASA space probe designed to study the composition of the interior of the comet Tempel 1. At 5:52 UTC on July 4 2005, one section of the Deep Impact probe successfully impacted the comet's nucleus, excavating debris from the interior of the nucleus. Photographs of the impact showed the comet to be more dusty and less icy than expected. The impact generated a large, bright dust cloud that obscured the hoped-for view of the impact crater.

Previous space missions to comets, such as Giotto and Stardust, were fly-by missions, only able to photograph and examine the surfaces of cometary nuclei from a distance. The Deep Impact mission was the first to eject material from a comet's surface.

Mission profile

Following its launch on January 12, 2005, the Deep Impact spacecraft traveled 429 million kilometers in 174 days to reach Comet Tempel 1 at a cruising speed of 28.6 km/s (103,000 km/h or 64,000 mph). Once the spacecraft reached the vicinity of the comet on July 3, 2005, it separated into two portions, an impactor and a flyby probe. The impactor used its thrusters to move into the path of the comet, impacting 24 hours later at a relative speed of 10.3 km/s (37,000 km/h or 23,000 mph). The impactor, with its mass of 370 kilograms (814 pounds), delivered 1.96 × 10¹⁰ joules of kinetic energy- the equivalent of 4.7 tons of TNT. Scientists believe that the energy of this high-velocity collision was sufficient to excavate a crater up to 100 m wide (larger than the bowl of the Roman Colosseum), although the crater has not yet been spotted in post-impact images as the cloud of debris resulting from the impact is obscuring the view.

Just minutes after the impact, the flyby probe passed by the nucleus at a close distance of 500 km, taking pictures of the crater position, the ejecta plume, and the entire cometary nucleus. The entire event was photographed by Earth-based telescopes and orbital observatories, including the Hubble, Chandra, Spitzer and XMM-Newton. The impact was also observed by cameras and spectroscopes on board Europe's Rosetta spacecraft, which was about 80 million km from the comet at the time of impact. Rosetta should determine the composition of the gas and dust cloud kicked up by the impact.

After this flyby of Tempel 1, Deep Impact was retargeted to comet Boethin. On July 20, 2005, a trajectory correction maneuver was performed to place the spacecraft on a trajectory to carry it to the Earth and use a gravitational slingshot to target another comet and the follow-on mission was approved October 31, 2006.

Scientific goals

The mission's Principal Investigator was Michael A'Hearn, an astronomer at the University of Maryland. He led the science team, which included members from Cornell University, University of Maryland, University of Arizona, Brown University, Belton Space Exploration Initiatives, JPL, University of Hawaii, SAIC, Ball Aerospace and Max-Planck-Institut für extraterrestrische Physik.

The Deep Impact mission was planned to help answer fundamental questions about comets, including information about the composition of the comet's nucleus, the age of the materials within the nucleus, and if spacecraft can be used to deflect comets off course.

Spacecraft design and instrumentation

Spacecraft overview

Cameras of the flyby spacecraft, HRI at right, MRI at left

The spacecraft consists of two main sections, the 370 kg copper-core "Smart Impactor" which impacted the comet, and the "Flyby" section, which imaged the comet from a safe distance during the encounter with Temple 1.

The Flyby spacecraft is about 3.2 meters long, 1.7 meters wide, and 2.3 meters high. It included a solar panel, a debris shield, and several science instruments for imaging, infrared spectroscopy, and optical navigation to its destinatino near the comet. The spacecraft also carried two cameras, the High Resolution Imager (HRI) and the Medium Resolution Imager (MRI). The HRI is an imaging device that combines a visible-light camera, infrared spectrometer, and an imaging module. It has been optimized for observing the comet's nucleus. The MRI is the backup device, and was primarily used for navigation during the final 10-day approach.

The impactor section of the spacecraft contains an instrument that is optically identical to the MRI, called the Impactor Targeting Sensor (ITS). Its dual purpose was to sense the Impactor's trajectory, which could then be trimmed (adjusted) up to four times, and to image the comet from close range. As the impactor neared the comet's surface, this camera took high-resolution pictures of the nucleus (as good as 0.2 meters per pixel) that were transmitted in real-time to the flyby spacecraft before it and the Impactor were destroyed. The final image taken by the impactor was snapped only 3.7 seconds before impact.

The impactor's payload, dubbed the "Cratering Mass", was 100% copper (Impactor 49% copper by mass) to reduce debris interfering with scientific measurements of the impact. Since copper was not expected to be found on a comet, scientists can eliminate copper from the spectrometer reading. If the impactor was loaded with other materials such as explosives, it would create a significant amount of organic vapor.

Mission events

Before launch

Simulation: The collision of comet 9P/Tempel 1 and the Deep Impact impactor, simulated by Celestia software using pre-impact information. The sun and the earth are on the right side. Note: The Deep Impact itself faces the wrong direction. The solar array should face the sun and the high-gain antenna should point to the earth.

A comet-impact mission was first proposed to NASA in 1996. However, NASA engineers were skeptical that the target could be hit. In 1999, a revised and technologically-upgraded mission proposal, dubbed Deep Impact, was accepted and funded as part of NASA's Discovery Program of low-cost spacecraft. The two spacecraft (Impactor and Flyby) and the three main instruments were built and integrated by Ball Aerospace & Technologies Corp. in Boulder, Colorado, USA. Developing the software for the spacecraft took eighteen months and the application code consisted of 20,000 lines and 19 different application threads. The total cost of the spacecraft reached $US300 million. The name of the mission is shared with the Deep Impact film, in which a comet strikes the Earth; but this is coincidental, as the scientists behind the mission and the creators of the movie devised the name independently of each other, at around the same time.

Launch and commissioning phase

The probe was originally scheduled for launch on December 30 2004, but NASA officials delayed its launch, in order to allow more time for testing the software. It was successfully launched from Cape Canaveral on January 12 2005 at 1:47 p.m. EST (1847 UTC) by a Delta 2 rocket.

Deep Impact's state of health was uncertain during the first day after launch. Shortly after entering orbit around the Sun and deploying its solar panels, the probe switched itself to safe mode. The cause of the problem was simply an incorrect temperature limit in the fault protection logic for the spacecraft's RCS thrusters. The spacecraft's thrusters were used to detumble the spacecraft following third stage separation. NASA subsequently announced that the probe was out of safe mode and healthy.

On February 11, Deep Impact's rockets were fired as planned to correct the spacecraft's course. This correction was so precise that the next planned maneuver for March 31 was canceled. During the "commissioning phase" all instruments were activated and checked out. During these tests it was found that the HRI images were not in focus after it underwent a bake-out period. After mission members investigated the problem, on June 9, it was announced that by using image processing software and the mathematical technique of deconvolution, the HRI images could be corrected to provide the resolution anticipated.

Cruise phase

Comet Tempel 1 imaged on April 25 by the Deep Impact spacecraft

The "cruise phase" began on March 25, immediately after the commissioning phase was completed. This phase continued until about 60 days before the encounter with comet Tempel 1. On April 25 the probe acquired the first image of its target at a distance of 64 million kilometers (39.7 million miles).

On May 4 it executed its second trajectory correction maneuver. Burning its rocket engine for 95 seconds the spacecraft speed was changed by 18.2 kilometers per hour (11.3 miles per hour). Rick Grammier, the project manager for the mission at NASA's Jet Propulsion Laboratory, reacted to the maneuver stating that "spacecraft performance has been excellent, and this burn was no different...it was a textbook maneuver that placed us right on the money."

Approach phase

The approach phase extends from 60 days before encounter (May 5) until five days before encounter. Sixty days out was about the earliest time that the Deep Impact spacecraft was expected to detect the comet with its MRI camera. In fact, the comet was spotted ahead of schedule, sixty-nine days before impact (see Cruise phase above). This milestone marks the beginning of an intensive period of observations to refine knowledge of the comet's orbit and study the comet's rotation, activity and dust environment.

On June 14 and June 22 Deep Impact observed two outbursts of activity from the comet, the latter being six times larger than the former. The spacecraft studied the images of various distant stars to determine its current trajectory and position. Don Yeomans, a mission co-investigator for JPL pointed out that "it takes 7½ minutes for the signal to get back to Earth, so you can't joystick this thing. You have to rely on the fact that the Impactor is a smart spacecraft as is the Flyby spacecraft. So you have to build in the intelligence ahead of time and let it do its thing." On June 23, the first of the two final trajectory correct maneuvers (targeting maneuver) was successfully executed. A 6 m/s (13.4 mph) velocity change was needed to adjust the flight path towards the comet and target the impactor at a window in space about 100 kilometers wide.

• 
  May 30, 2005, 35 days from impact
• June 15, 19 days from impact
  June 15, 19 days from impact
• June 21, 13 days from impact
  June 21, 13 days from impact
• 
  June 27, 7 days from impact, near end of approach phase

Impact phase

Deep Impact comet encounter sequence

Impact phase began nominally on June 29, five days before impact. The impactor successfully separated from the Flyby spacecraft at 6:00 (6:07 Ground UTC) July 3 UTC. The first images from the instrumented Impactor were seen two hours after separation.

The Flyby spacecraft performed one of two divert maneuvers to avoid damage. A 14-minute burn was executed which slowed down the spacecraft. It was also reported that the communication link between the flyby and the impactor was functioning as expected. The Impactor spacecraft executed three correction maneuvers in the final two hours before impact.

Impact occurred at 05:45 UTC (05:52 Ground UTC, +/- up to three minutes, One-Way Light Time = 7m 26s) on the morning of July 4, within one second of the expected time for impact.

The Impactor returned images as late as three seconds before impact. Most of the data captured was stored on board the Flyby spacecraft, which radioed approximately 4500 images from the HRI, MRI, and ITS cameras to earth over the next few days. The energy from the collision was similar in size to exploding five tons of dynamite and the comet shined six times brighter than normal.

• Comet Tempel 1, imaged from 4.2 million km at the start of Impact phase.
  Comet Tempel 1, imaged from 4.2 million km at the start of Impact phase.
• 
  Impactor imaged by Flyby spacecraft shortly after separation.
• Nucleus imaged by the Impactor.
  Nucleus imaged by the Impactor.
• 
  Image from Impactor.
• 
  Impactor close-up image, taken shortly before impact.
• 
  The moment of impact, as shown on NASA TV.
• Full scope of the impact plume.
  Full scope of the impact plume.
• 
  HRI movie of impact.

Impact Phase Timeline (NASA)

Results

Astronomers did not become aware of the impactor's success until five minutes later at 0157 ET. In the post-impact briefing at 0100 Pacific Daylight Time (08:00 UTC) on July 4, 2005, the first processed images revealed existing craters on the comet. NASA scientists stated they could not see the new crater that had formed from the impactor. The only models of cometary structure they could positively rule out were the very porous models which had comets as loose aggregates of material.

Data from the mission are still being analyzed, but initial results were surprising. The material excavated by the impact contained more dust and less ice than had been expected. In addition, the material was finer than expected; scientists likened it to talcum powder rather than sand.

Analysis of data from the Swift X-ray telescope showed that the comet continued outgassing from the impact for 13 days, with a peak five days after impact. A total of 250,000 tonnes of water was lost.

Public interest

Media coverage

This image was circulated widely in the media.

The impact was a substantial news event reported and discussed online, in print, and on television. There was a genuine suspense because experts held widely differing opinions over the result of the impact. Various experts debated whether the impactor would go straight through the comet and out the other side, would create an impact crater, would open up a hole in the interior of the comet, and other theories. However, twenty-four hours before impact, the flight team at JPL began privately expressing a high level of confidence that, barring any unforeseen technical glitches, the spacecraft would intercept Tempel-1. "All we can do now is sit back and wait," said one senior personnel. "Everything we can technically do to ensure impact has been done." In the final minutes as the impactor hit the comet, more than 10,000 people watched the collision on a giant movie screen at Hawaii's Waikiki Beach.

Experts came up with a range of soundbites to summarize the mission to the public. Iwan Williams of Queen Mary, University of London, said "It was like a mosquito hitting a 747. What we've found is that the mosquito didn't splat on the surface; it's actually gone through the windscreen." One of the NASA investigators, Dr. Jessica Sunshine, explained the mission by analogy with how a geologist examines a rock: "He doesn't just look at it, he gets his hammer out and hits it, to find out about what it's like inside and how it's put together: is it a loose association of particles or is it solid?"

Send Your Name To A Comet!

The mission was notable for one of its promotional activities, "Send Your Name To A Comet!". Visitors to the Jet Propulsion Laboratory's website were invited to submit their name between May 2003 and January 2004, and the names gathered - some 625,000 in all - were then burnt onto a mini-CD, which was attached to the impacter. Dr. Don Yeomans, a member of the spacecraft's scientific team, stated "this is an opportunity to become part of an extraordinary space mission...when the craft is launched in December 2004, yours and the names of your loved-ones can hitch along for the ride and be part of what may be the best space fireworks show in history." The idea was credited with driving interest in the mission.

Reaction from China

Chinese researchers used Deep Impact mission as an opportunity to highlight the efficiency of American science because public support ensured the possibility of funding long-term research. By contrast, "in China, the public usually has no idea what our scientists are doing, and limited funding for the promotion of science weakens people’s enthusiasm for research."

Two days after the U.S. mission succeeded in having a probe collide with a comet, China revealed a plan for what it called a "more clever" version of the mission: landing a probe on a small comet or asteroid to push it off course. China will begin the mission after sending a probe to the Moon.

Contributions from amateur astronomers

Since observing time on large, professional telescopes such as Keck or Hubble is always scarce, the Deep Impact scientists have called on "advanced amateur, student, and professional astronomers" to use small telescopes to make long-term observations of the target comet before and after impact. The purpose of these observations is to look for "volatile outgassing, dust coma development and dust production rates, dust tail development, and jet activity and outbursts." Since 2000, amateur astronomers have submitted over a thousand CCD images of the comet.

The comet is currently too dim to be seen with anything smaller than a large backyard telescope, but it was thought possible that the impact on July 4 could brighten the comet substantially, making it visible through binoculars toward the star Spica (visible even to the naked eye in areas with low light pollution).

One notable amateur observation by the students of King's School Canterbury, Kent, UK who during a press conference took live images using the Faulkes Automatic Telescope in Hawaii (the students operate the telescope over the internet and were in the UK) they were one of the first groups to get images of the impact.

One amateur astronomer reported seeing a structureless bright cloud around the comet, and an estimated 2 magnitude increase in brightness after the impact. Another amateur published a map of the crash area from NASA images.

Musical tribute

The Deep Impact mission coincided with celebrations in the Los Angeles area marking the 50th anniversary of "Rock Around the Clock" by Bill Haley and His Comets becoming the first rock and roll single to reach No. 1 on the recording sales charts. Within twenty-four hours of the mission's success, a two-minute music video produced by Martin Lewis had been created using images of the impact itself combined with computer animation of the Deep Impact probe in flight, interspersed with footage of Bill Haley and His Comets performing in 1955 and the surviving original members of The Comets performing in March 2005. The video was posted to NASA's website for a couple of weeks afterwards.

On July 5, the surviving original members of The Comets (ranging in age from 71 to 84) performed a free concert for hundreds of employees of the Jet Propulsion Laboratory to help them celebrate the mission's success. This event received worldwide press attention. Later, in February 2006, the International Astronomical Union citation that officially named asteroid 79896 Billhaley included a reference to the JPL concert.

Future activities

On July 21, 2005 Deep Impact executed a trajectory correction maneuver that placed the spacecraft on course to fly past Earth on December 31, 2007. The maneuver allows the spacecraft to use Earth's gravity to begin a new mission in a path towards another comet. The proposed $500,000 extended mission is called EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) and in January 2008 will have Deep Impact begin studying the stars around several known extrasolar planets to attempt to find other nearby extrasolar planets. The telescope on the spacecraft would attempt to find the planets using the astrometry and transit method. In December of the same year, the spacecraft would come within 435 miles (700 kilometers) of Comet Boethin. Instead of using another impactor to collide with the comet (which the spacecraft does not have), the spacecraft will observe the comet to compare it to various characteristics found on Temple 1. Deep Impact will use its spectrometer to study the comet's surface composition and its telescope for viewing the surface features.

See also

• Unmanned space mission

References

• "Deep Impact: Our First Look Inside a Comet." June 2005 issue of Sky and Telescope magazine, pp. 40-44. PDF file.
• "Deep Impact encounter press kit." PDF file.
• "Deep Impact: Mission Science Q&A." NASA. [1]

External links

Official websites

• Deep Impact at JPL
• Deep Impact at NASA
• Deep Impact Mission Profile by NASA's Solar System Exploration
• Deep Impact at the University of Maryland, College Park
• Deep Impact at Ball Aerospace & Technologies Corp.

Additional information

• Space.com Technology page on Deep Impact
• Watch a mini-doco about Deep Impact from Australian TV, 4 August 2005
• Deep Impact Reveals Comet's Components - Scientific American

Maps, photos, and other images

• Real-time image viewer
• Real-time Java viewer of Deep Impact's position

Other languages

• Croatian: Deep Impact images and information
• French: Photos and videos of the impact