Deep Impact (spacecraft)

• 1. Introduction

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  At 05:52 UTC on the 4th of July, 2005, Deep Impact drove a copper-tipped probe into the nucleus of comet Tempel 1 at more than ten kilometres per second. The spacecraft had crossed 429 million km in 174 days to reach that single moment. No mission had ever deliberately fired a projectile into a comet's nucleus to expose its interior. Scientists at mission control held widely differing views on what would happen. Some believed the probe would punch straight through and exit the other side. Others predicted a conventional crater. Others thought the nucleus might open up to reveal a hollow interior. What the collision actually found challenged basic assumptions about what comets are made of, where they come from, and how the solar system looked in its earliest years. A spacecraft built for one critical shot would go on to study other comets for nearly a decade before going silent in 2013.

• 2. The Copper Bullet

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  A comet-impact mission was first proposed to NASA in 1996. Engineers at the time doubted whether any probe could be aimed with enough precision to strike a fast-moving nucleus. In 1999, a revised proposal was accepted and funded through NASA's Discovery Program of low-cost spacecraft. Ball Aerospace & Technologies in Boulder, Colorado, built both major sections of the craft. The mission's Principal Investigator was Michael A'Hearn, an astronomer at the University of Maryland. The Flyby spacecraft weighed 601 kg and was designed to observe the collision from a safe distance. The Impactor weighed 372 kg. Writing the flight software alone took 18 months and produced 20,000 lines of code running across 19 separate application threads.

  The Impactor's core payload was a 113 kg block of pure copper, which formed 49% of the total Impactor mass. It was chosen not for explosive force but for scientific neutrality. Copper was not expected to be found on a comet, so scientists could set aside any copper signature appearing in their spectrometer readings. No explosives were needed. At a closing speed of 10.2 km/s, the Impactor's kinetic energy alone was equivalent to 4.8 tonnes of TNT.

  The Impactor also carried a camera called the Impactor Targeting Sensor, which was optically identical to the main navigation camera on the Flyby section. During the final approach, it transmitted images in real-time to the Flyby spacecraft, resolving detail as fine as 0.2 m. The Impactor captured its last photograph 3.7 seconds before striking the nucleus.

  The probe was originally scheduled to launch on the 30th of December 2004. NASA delayed the date to allow more time for software testing. The launch came on the 12th of January 2005, from pad SLC-17B at Cape Canaveral, lifted by a Delta II rocket. Within the first day in orbit, the spacecraft switched itself to safe mode. The cause was a misconfigured temperature limit in the fault protection logic for its thrusters. On the 13th of January 2005, NASA announced the probe had recovered.

  By the end of the commissioning phase in March 2005, engineers had found a further complication. The High Resolution Imager had lost proper focus after undergoing a bake-out procedure. On the 9th of June 2005, less than a month before the planned impact, the team announced a solution. Mathematical deconvolution could process the blurry images and recover much of the expected resolution. On the 23rd of June 2005, flight controllers executed the first targeting maneuver, directing the Impactor toward a window in space just 100 km wide.

• 3. Stranger Inside

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  Don Yeomans, a member of the science team, confirmed the Impactor had struck exactly where intended. JPL Director Charles Elachi said the result had exceeded expectations. Mission control did not know the Impactor had succeeded until 05:57 UTC, five minutes after the actual collision. Lucy McFadden, one of the mission co-investigators, captured the outcome precisely. "We didn't expect the success of one part of the mission to affect a second part," she said. "But that is part of the fun of science, to meet with the unexpected."

  A press briefing at 08:00 UTC on July 4 showed the first processed photographs. They revealed existing craters on the comet's surface but not the newly formed one. That crater was later measured at about 100 m wide and up to 30 m deep. Swift X-ray telescope analysis showed the comet continued releasing gas for 13 days after impact, peaking five days in. The collision expelled approximately 5 million kg of water and between 10 and 25 million kg of dust. Hubble, Chandra, Spitzer, and XMM-Newton all observed the event from orbit. Europe's Rosetta spacecraft was about 80 million km from the comet at the time. It used its own cameras and spectroscopes to analyze the composition of the gas and dust cloud.

  Approximately 4,500 images from the HRI, MRI, and ITS cameras were radioed to Earth over the following days. They ran counter to almost every prediction. The excavated material was far less icy than researchers had expected. Scientists compared its texture to talcum powder rather than sand. Observations showed the comet was about 75% empty space. One astronomer compared its outer layers to a snow bank. Of the structural models for cometary nuclei, scientists could rule out with confidence only those treating comets as very porous, loose aggregates.

  Spectroscopic study of the ejecta revealed clays, carbonates, sodium, and crystalline silicates. Clays and carbonates ordinarily require liquid water to form, making their presence a puzzle. Sodium is rare in space. Based in part on the presence of ethane, astronomers hypothesized that Tempel 1 formed in the Oort cloud region near Uranus and Neptune. Comets born that far from the Sun are expected to carry more ices with low freezing temperatures, and ethane fit that pattern.

  Because the impact photographs left the crater's size unresolved, NASA approved a follow-up mission on the 3rd of July, 2007. The New Exploration of Tempel 1, known as NExT, used the existing Stardust spacecraft, which had previously studied Comet Wild 2 in 2004. Stardust passed within approximately 200 km of Tempel 1 on the 15th of February, 2011. Its images measured the crater at roughly 150 m in diameter, with a bright central mound where material had fallen back after the impact. That visit marked the first time any comet had been examined by two separate probes on distinct occasions. Halley's Comet had received several visitors in 1986, but they arrived within weeks of each other.

• 4. The World Watched

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  More than 10,000 people gathered at Hawaii's Waikiki Beach to watch the collision on a giant movie screen. Iwan Williams of Queen Mary University of London described the impact as a mosquito hitting a 747. The mosquito, he said, had not splattered on the surface but gone clean through the windscreen. The event drew genuine suspense. Twenty-four hours before impact, one senior mission official summed up the team's position: "All we can do now is sit back and wait. Everything we can technically do to ensure impact has been done."

  One day after the impact, Marina Bay, a Russian astrologer, filed a lawsuit against NASA. She claimed the collision had disturbed the natural balance of forces in the universe. Her lawyer suggested the impact had altered the comet's magnetic properties and invited anyone whose mobile phone had failed that morning to contact them. On the 9th of August, 2005, the Presnensky Court of Moscow ruled against her. One Russian physicist noted that the collision had changed the comet's orbit by about 10 cm.

  Some 625,000 people had already traveled to comet Tempel 1 before they knew it, their names burned onto a mini-CD and attached to the Impactor. Visitors to the Jet Propulsion Laboratory's website had submitted their names between May 2003 and January 2004. Don Yeomans, a member of the scientific team, described the campaign as an opportunity for the public to become part of an extraordinary space mission.

  The Los Angeles area was celebrating the 50th anniversary of "Rock Around the Clock" by Bill Haley & His Comets when the impact occurred. That song had been the first rock and roll single to reach number one on the recording sales charts. Within 24 hours of the mission's success, producer Martin Lewis had assembled a two-minute music video. It combined impact photographs and animation with archival footage of the original Comets from 1955, alongside footage of surviving members filmed in March 2005. On the 5th of July 2005, the surviving original Comets, aged 71 to 84, performed a free concert for hundreds of JPL employees. In February 2006, the International Astronomical Union officially named asteroid 79896 Billhaley, citing that JPL concert in the formal citation.

• 5. EPOXI and Silence

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  On the 21st of July 2005, Deep Impact fired its thrusters to begin a trajectory using Earth's gravity for a new mission. The craft was then placed into a dormant state. It flew by Earth on the 31st of December, 2007. Its extended mission was designated EPOXI, short for Extrasolar Planet Observation and Deep Impact Extended Investigation. The mission had a dual aim: to observe planets beyond our solar system and to study additional comets.

  The original plan was to fly by Comet Boethin on the 5th of December, 2008, passing within 700 km. Mission leader Michael A'Hearn explained the goal: to find out whether the results at Tempel 1 were unique or shared by other comets. The extended mission was budgeted at $40 million. As the December 2007 Earth gravity assist approached, astronomers found they could not locate Comet Boethin at all. The comet may have broken into fragments too faint to detect, making its orbit impossible to calculate precisely.

  In November 2007, the team retargeted Deep Impact toward Comet Hartley 2. After Earth gravity assists in December 2007 and December 2008, the spacecraft reached Hartley 2 on the 4th of November, 2010. It passed within 700 km. The photographs it returned showed a nucleus shaped like a peanut, with several bright jets.

  From the 20th of February to the 8th of April, 2012, Deep Impact trained its Medium Resolution Instrument on Comet Garradd. Measurements showed the comet's outgassing varied with a period of 10.4 hours, likely matching the rotation of its nucleus. Its dry ice content was about 10% of its water ice content by number of molecules. In February 2013, Deep Impact observed Comet ISON, which remained in view until March.

  Contact with the spacecraft was lost sometime between the 11th and the 14th of August, 2013. The last confirmed communication had been on the 8th of August. By September 10, mission controllers concluded that the spacecraft's computers were continuously rebooting. This left the vehicle unable to issue thruster commands, with its antenna orientation unknown. Chief scientist A'Hearn traced the fault to a Y2K-like software problem. The craft had stored its internal clock as a running count of tenth-second intervals from the 1st of January 2000, recorded in an unsigned 32-bit integer. On the morning of the 11th of August, 2013, that count overflowed. NASA abandoned further contact attempts on the 20th of September, 2013. At the end of 2011, mission controllers had retargeted Deep Impact toward asteroid (163249) 2002 GT. A flyby was projected for the 4th of January, 2020. The spacecraft went silent before it could arrive.