Deep Impact (spacecraft)

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.[18] 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[19] 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.[6] The total cost of developing the spacecraft and completing its mission reached US$330 million.[20]

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.[21] It was successfully launched from Cape Canaveral on January 12, 2005, at 1:47 pm EST (1847 UTC) by a Delta II rocket.[22]

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. On January 13, 2005, NASA announced that the probe was out of safe mode and healthy.[23]

On February 11, 2005, Deep Impact's rockets were fired as planned to correct the spacecraft's course. This correction was so precise that the next planned correction maneuver on March 31, 2005, was unnecessary and 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.[24] After mission members investigated the problem, on June 9, 2005, 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.[25]

Comet Tempel 1 imaged on April 25, 2005, by the Deep Impact spacecraft

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• Deep Impact cruising for comet crash

The "cruise phase" began on March 25, 2005, immediately after the commissioning phase was completed. This phase continued until about 60 days before the encounter with comet Tempel 1. On April 25, 2005, the probe acquired the first image of its target at a distance of 64 million km (40 million mi).[26]

On May 4, 2005, 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).[27] 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."[27]

The approach phase extended from 60 days before encounter (May 5, 2005) until five days before encounter. 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 22, 2005, Deep Impact observed two outbursts of activity from the comet, the latter being six times larger than the former.[28] The spacecraft studied the images of various distant stars to determine its current trajectory and position.[6] 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 cannot 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."[29] On June 23, 2005, the first of the two final trajectory correct maneuvers (targeting maneuver) was successfully executed. A 6 m/s (20 ft/s) 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.

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

Deep Impact comet encounter sequence

Impact phase began nominally on June 29, 2005, five days before impact. The Impactor successfully separated from the Flyby spacecraft on July 3 at 6:00 UTC (6:07 UTC ERT).[30][31] The first images from the instrumented Impactor were seen two hours after separation.[32]

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.[23] The Impactor executed three correction maneuvers in the final two hours before impact.[33]

The Impactor was maneuvered to plant itself in front of the comet, so that Tempel 1 would collide with it.[7] Impact occurred at 05:45 UTC (05:52 UTC ERT, +/- up to three minutes, one-way light time = 7m 26s) on the morning of July 4, 2005, 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.[34][35] The energy from the collision was similar in size to exploding five tons of dynamite and the comet shone six times brighter than normal.[36]

A mission timeline is located at Impact Phase Timeline (NASA).

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  Comet Tempel 1, imaged from 4.2 million km at the start of Impact phase
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  Impactor imaged by Flyby spacecraft shortly after separation
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  Image from Impactor
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  Impactor close-up image, taken shortly before impact
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  The moment of impact, as shown on NASA TV
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  HRI movie of impact
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  High Resolution Instrument, Visual CCD (HRIV) during encounter (video)
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  Impactor Targeting Sensor, Visual CCD (ITS) during encounter (video)

Because the quality of the images of the crater formed during the Deep Impact collision was not satisfactory, on July 3, 2007, NASA approved the New Exploration of Tempel 1 (or NExT) mission. The mission utilized the already existing Stardust spacecraft, which had studied Comet Wild 2 in 2004. Stardust was placed into a new orbit so that it passed by Tempel 1 at a distance of approximately 200 km (120 mi) on February 15, 2011, at 04:42 UTC.[46] This was the first time that a comet was visited by two probes on separate occasions (1P/Halley had been visited by several probes within a few weeks in 1986), and it provided an opportunity to better observe the crater that was created by Deep Impact as well as observing the changes caused by the comet's latest close approach to the Sun.

On February 15, NASA scientists identified the crater formed by Deep Impact in images from Stardust. The crater is estimated to be 150 meters (490 ft) in diameter, and has a bright mound in the center likely created when material from the impact fell back into the crater.[47]

The image of the impact which was widely circulated 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. 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 Waikīkī Beach.[36]

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."[48]

One day after the impact, Marina Bay, a Russian astrologer, sued NASA for US$300 million for the impact which "ruin[ed] the natural balance of forces in the universe."[49] 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."[50] 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)."[51]

The CD containing the 625,000 names is added to the Impactor

Deep Impact participation certificate of Mathias Rex

The mission was notable for one of its promotional campaigns, "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.[52] 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."[53] The idea was credited with driving interest in the mission.[54]

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."[55]

Two days after the US mission succeeded in having a probe collide with a comet, China revealed a plan: landing a probe on a small comet or asteroid to push it off course. China said it would begin the mission after sending a probe to the Moon.[56]

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."[57] By mid-2007, amateur astronomers had submitted over a thousand CCD images of the comet.[58]

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 2 magnitude increase in brightness after the impact.[59] Another amateur published a map of the crash area from NASA images.[60]

The Deep Impact mission coincided with celebrations in the Los Angeles area marking the 50th anniversary of "Rock Around the Clock" by Bill Haley & His Comets becoming the first rock and roll single to reach No. 1 on the recording sales charts. Within 24 hours of the mission's success, a 2-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 & His Comets performing in 1955 and the surviving original members of The Comets performing in March 2005.[61] The video was posted to NASA's website for a couple of weeks afterwards.

On July 5, 2005, the surviving original members of The Comets (ranging in age from 71–84) 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.[62] In February 2006, the International Astronomical Union citation that officially named asteroid 79896 Billhaley included a reference to the JPL concert.[63]

Its first extended visit was to do a flyby of Comet Boethin, but with some complications. 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.[65]

The original plan was for a December 5, 2008, flyby of Comet Boethin, coming within 700 kilometers (430 mi) 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."[66] The $40 million mission would provide about half of the information as the collision of Tempel 1 but at a fraction of the cost.[66][67] Deep Impact would use its spectrometer to study the comet's surface composition and its telescope for viewing the surface features.[65]

However, as the December 2007 Earth gravity assist approached, astronomers were unable to locate Comet Boethin, which may have broken up into pieces too faint to be observed.[68] Consequently, its orbit could not be calculated with sufficient precision to permit a flyby.

Comet Hartley 2 on November 4, 2010

In November 2007 the JPL team targeted Deep Impact toward Comet Hartley 2. However, this would require an extra two years of travel for Deep Impact (including earth gravity assists in December 2007 and December 2008).[68] On May 28, 2010, a burn of 11.3 seconds was conducted, to enable the June 27 Earth fly-by to be optimized for the transit to Hartley 2 and fly-by on November 4. The velocity change was 0.1 m/s (0.33 ft/s).[69]

On November 4, 2010, the Deep Impact extended mission (EPOXI) returned images from comet Hartley 2.[64] EPOXI came within 700 kilometers (430 mi) of the comet, returning detailed photographs of the "peanut" shaped cometary nucleus and several bright jets. The probe's medium-resolution instrument captured the photographs.[64]

Deep Impact observed Comet Garradd (C/2009 P1) from February 20 to April 8, 2012, using its Medium Resolution Instrument, through a variety of filters. The comet was 1.75–2.11 AU (262–316 million km) from the Sun and 1.87–1.30 AU (280–194 million km) from the spacecraft. It was found that the outgassing from the comet varies with a period of 10.4 hours, which is presumed to be due to the rotation of its nucleus. The dry ice content of the comet was measured and found to be about ten percent of its water ice content by number of molecules.[70][71]

At the end of 2011, Deep Impact was re-targeted towards asteroid (163249) 2002 GT which it would reach on January 4, 2020. At the time of re-targeting, whether or not a related science mission would be carried out in 2020 was yet to be determined, based on NASA's budget and the health of the probe.[72] A 71-second engine burn on October 4, 2012, changed the probe's velocity by 2 m/s (6.6 ft/s) to keep the mission on track.[73] Also, there was a 140-second burn on November 24, 2011. Distance of a flyby would not be more than 400 kilometers.

In February 2013, Deep Impact observed Comet ISON. The comet remained observable until March 2013.[74][75]

On September 3, 2013, a mission update was posted to the EPOXI mission status website, stating "Communication with the spacecraft was lost some time between August 11 and August 14 ... The last communication was on August 8. ... the team on August 30 determined the cause of the problem. The team is now trying to determine how best to try to recover communication."[71]

On September 10, 2013, a Deep Impact mission status report explained that mission controllers believe the computers on the spacecraft are continuously rebooting themselves and so are unable to issue any commands to the vehicle's thrusters. As a result of this problem, communication with the spacecraft was explained to be more difficult, as the orientation of the vehicle's antennas is unknown. Additionally, the solar panels on the vehicle may no longer be positioned correctly for generating power.[76]

On September 20, 2013, NASA abandoned further attempts to contact the craft.[77] According to chief scientist A'Hearn,[78] the reason for the software malfunction was a Y2K-like problem. August 11, 2013, 00:38:49.6, was 2³² tenth-seconds from January 1, 2000, leading to speculation that a system on the craft tracked time in one-tenth second increments since January 1, 2000, and stored it in an unsigned 32-bit integer, which then overflowed at this time, similar to the Year 2038 problem.[79]