Deep Impact is a NASA space probe which was launched on January 12, 2005. It was designed to study the composition of the comet interior of 9P/Tempel, by releasing an impactor into the comet. At 5:52 UTC on July 4, 2005, the impactor successfully collided with the comet's nucleus. The impact excavated debris from the interior of the nucleus, allowing photographs of the impact crater. The photographs showed the comet to be more dusty and less icy than had been expected. The impact generated a large and bright dust cloud, which unexpectedly obscured the view of the impact crater.

Previous space missions to comets, such as Giotto and Stardust, were fly-by missions. These missions were 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, and the mission garnered large publicity from the media, international scientists, and amateur astronomers.

Upon the completion of its primary mission proposals were made to further utilize the spacecraft. Consequently, Deep Impact flew by Earth on December 31, 2007 on its way to an extended mission, designated EPOXI, with a dual purpose to study extrasolar planets and comet Hartley 2.

Scientific goals

The Deep Impact mission was planned to help answer fundamental questions about comets, which included what makes up the composition of the comet's nucleus, what depth the crater would reach from the impact, and where the comet originated in its formation. By observing the composition of the comet, astronomers hoped to determine how comets form based on the differences between the interior and exterior makeup of the comet. Observations of the impact and its aftermath would allow astronomers to attempt to determine the answers to these questions.

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.

Spacecraft design and instrumentation

The spacecraft consists of two main sections, the 370-kg (815-lb) copper-core "Smart Impactor" that impacted the comet, and the "Flyby" section, which imaged the comet from a safe distance during the encounter with Tempel 1.

The Flyby spacecraft is about 3.2 meters (10.5 ft) long, 1.7 meters (5.6 ft) wide and 2.3 meters (7.5 ft) high. It includes a solar panel, a debris shield, and several science instruments for imaging, infrared spectroscopy, and optical navigation to its destination 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 used primarily 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 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 (0.7 ft) 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 could eliminate copper from the spectrometer reading. Instead of using explosives, it was also cheaper to use copper as the payload.

The name of the mission is shared with the 1998 Deep Impact film, in which a comet strikes the Earth. This is coincidental, however, as the scientists behind the mission and the creators of the film devised the name independently of each other at around the same time.

Mission profile

Following its launch on January 12, 2005, the Deep Impact spacecraft traveled 429 million kilometers (267 million mi) in 174 days to reach comet 9P/Tempel 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, a 370-kilogram (820-pound) copper projectile, delivered 1.96 × 10¹⁰ joules of kinetic energy—the equivalent of 4.7 tons of TNT. Scientists believed that the energy of the high-velocity collision would be sufficient to excavate a crater up to 100 m (328 ft) wide (larger than the bowl of the Roman Colosseum). The size of the crater was still not known one year after the impact.

Just minutes after the impact, the flyby probe passed by the nucleus at a close distance of 500 km (310 mi), 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 (50 million mi) from the comet at the time of impact. Rosetta determined the composition of the gas and dust cloud that was kicked up by the impact.

Mission events

Before launch

A comet-impact mission was first proposed to NASA in 1996, but at the time, 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. Developing the software for the spacecraft took 18 months and the application code consisted of 20,000 lines and 19 different application threads. The total cost of developing the spacecraft and completing its mission reached $330 million.

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 thruster catalyst beds. 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. The "commissioning phase" verified that 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 restore much of the resolution anticipated.

Cruise phase

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 9P/Tempel. On April 25 the probe acquired the first image of its target at a distance of 64 million km (40 million miles).

On May 4 the spacecraft executed its second trajectory correction maneuver. Burning its rocket engine for 95 seconds, the spacecraft speed was changed by 18.2 km/h (11.3 mph). 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 extended from 60 days before encounter (May 5) until five days beforeencounter. Sixty days out was 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, 69 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 (62 mi) wide.

File:DI_MRI_T1_doy150.jpg|May 30, 2005, 35 days from impactFile:DI_MRI_T1_doy166_log.jpg|June 15, 19 days from impactFile:DI_MRI_log_T1_doy172.jpg|June 21, 13 days from impactFile:DI_MRI_log_T1_doy178.jpg|June 27, 7 days from impact, near end of approach phase

Impact phase

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.

The impactor was maneuvered to plant itself in front of the comet, so that 9P/Tempel would collide with it. 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 4,500 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 shone six times brighter than normal.

File:DI_MRI_log_T1_doy180.jpg|Comet 9P/Tempel, imaged from 4.2 million km at the start of Impact phaseFile:DI_MRI_Impactor_-24hr.jpg|Impactor imaged by Flyby spacecraft shortly after separationFile:Deep_Impact_Impactor_1.jpg|Nucleus imaged by the ImpactorFile:Tempel_Impactor_150Km.jpg|Image from ImpactorFile:Deep_Impact_Impactor_3.jpg|Impactor close-up image, taken shortly before impactFile:Deep Impact approach 2.jpg|The moment of impact, as shown on NASA TVFile:Tempel1-impact.jpg|Full scope of the impact plumeFile:121520main_HRI-Movie.gif|HRI movie of impact

Impact Phase Timeline (NASA)

Results

Mission control did not become aware of the impactor's success until five minutes later at 0157 ET. Once news of a successful impact had taken place, the mission control team members applauded and hugged each other. Don Yeomans confirmed the results for the press, "We hit it just exactly where we wanted to" and JPL Director Charles Elachi stated "The success exceeded our expectations."

In the post-impact briefing at 0100 Pacific Daylight Time (08:00 UTC) on July 4, 2005, the first processed images revealed existing crater on the comet. NASA scientists stated they could not see the new crater that had formed from the impactor, but it was later discovered to be about 100 meters (328 ft) wide and up to 30 meters (98 ft) deep. Lucy McFadden, one of the co-investigators of the impact, stated "We didn't expect the success of one part of the mission [bright dust cloud] to affect a second part [seeing the resultant crater]. But that is part of the fun of science, to meet with the unexpected." 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 5 million kilograms (11 million pounds) of water and between 10 and 25 million kilograms (22 and 55 million pounds) of dust were lost from the impact.

Initial results were surprising as the material excavated by the impact contained more dust and less ice than had been expected. The only models of cometary structure astronomers could positively rule out were the very porous models which had comets as loose aggregates of material. In addition, the material was finer than expected; scientists compared it to talcum powder rather than sand. Other materials found while studying the impact included clays, carbonates, sodium, and crystalline silicates which were found by studying the spectroscopy of the impact. Clays and carbonates usually require liquid water to form and sodium is rare in space. Observations also revealed that the comet was about 75% empty space, and one astronomer compared the outer layers of the comet to the same makeup of a snow bank. Astronomers have expressed interest in more missions to different comets to determine if they share similar compositions or if there are different materials found deeper within comets that were produced at the time of the solar system's formation.

Astronomers determined that the comet had possibly formed in the Uranus and Neptune Oort cloud region of the solar system. Based on its interior chemistry, astronomers were able to determine that a comet which forms farther from the Sun will have greater amounts of ices with low freezing temperatures, such as ethane, which was present in 9P/Tempel. If comets have similar compositions as Tempel, astronomers believe they could have formed in the same region.

Public interest

Media coverage

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 9P/Tempel. One senior personnel member stated "All we can do now is sit back and wait. 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 day after the impact Marina Bay, a Russian astrologer, sued NASA for $300 million for the impact which "ruin[ed] the natural balance of forces in the universe." Her lawyer asked the public to volunteer to help in the claim by declaring "The impact changed the magnetic properties of the comet, and this could have affected mobile telephony here on Earth. If your phone went down this morning, ask yourself Why? and then get in touch with us." On August 9, 2005 the Presnensky Court of Moscow ruled against Bay, although she did attempt to appeal the result. One Russian physicist said that the impact had no effect on Earth and "the change to the orbit of the comet after the collision was only about 10 cm (3.9 in)."

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 impactor. 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 the 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 called upon "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 was to look for "volatile outgassing, dust coma development and dust production rates, dust tail development, and jet activity and outbursts." By mid-2007, amateur astronomers had submitted over a thousand CCD images of the comet.

One notable amateur observation was by students from schools in Hawaii, working with US and UK scientists, who during the press conference took live images using the Faulkes Automatic Telescope in Hawaii (the students operated the telescope over the Internet) and 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 magnitude 2 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.

Extended mission

Deep Impact is now on an extended mission designated EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), originally planned as a flyby of Comet Boethin, but which has now been retargeted to Comet Hartley 2. On July 21, 2005 Deep Impact executed a trajectory correction maneuver that allows the spacecraft to use Earth's gravity to begin a new mission in a path towards another comet.

The original plan was for a December 5, 2008 flyby of Comet Boethin, coming within 700 kilometers (435 miles) of the comet. Michael A'Hearn, the Deep Impact team leader, explained "We propose to direct the spacecraft for a flyby of Comet Boethin to investigate whether the results found at Comet Tempel 1 are unique or are also found on other comets." The $40 million mission would provide about half of the information as the collision of Tempel 1 but at a fraction of the cost. Deep Impact would use its spectrometer to study the comet's surface composition and its telescope for viewing the surface features.

However, as the Earth gravity assist approached, astronomers were unable to locate Comet Boethin, which may have broken up into pieces too faint to be observed. Consequently, its orbit could not be calculated with sufficient precision to permit a flyby. Instead, the team targeted Deep Impact toward Comet Hartley 2. However, this will require an extra two years of travel for Deep Impact.

See also

• Lunar Crater Observation and Sensing Satellite - Satellite that Impacted Earth's moon in 2009 in an attempt to eject water

Cited references

External links

Official websites

* Deep Impact at JPL

* Deep Impact at NASA

* EXPOXI (Extended Mission 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.

* Deep Impact encounter press kit

* Deep Impact: Mission Science Q&A", NASA

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

* Deep Impact: Our First Look Inside a Comet. June 2005 issue of Sky and Telescope magazine, pp. 40–44

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