Deep Impact was a NASA space probe launched from Cape Canaveral Air Force Station on January 12, 2005. It was designed to study the interior composition of the comet Tempel 1 (9P/Tempel), by releasing an impactor into the comet. At 05:52 UTC on July 4, 2005, the Impactor successfully collided with the comet's nucleus. The impact excavated debris from the interior of the nucleus, forming an impact crater. Photographs taken by the spacecraft showed the comet to be more dusty and less icy than had been expected. The impact generated an unexpectedly large and bright dust cloud, obscuring the view of the impact crater.
Previous space missions to comets, such as Giotto, Deep Space 1, and Stardust, were fly-by missions. These missions were able to photograph and examine only the surfaces of cometary nuclei, and even then from considerable distances. The Deep Impact mission was the first to eject material from a comet's surface, and the mission garnered considerable publicity from the media, international scientists, and amateur astronomers alike.
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 (103P/Hartley). Communication was unexpectedly lost in August 2013 while the craft was heading for another asteroid flyby.
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.
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 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 long, wide and high. It includes a solar panel, a Whipple 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 with a filter wheel, and an imaging infrared spectrometer called the "Spectral Imaging Module" or SIM that operates on a spectral band from 1.05 to 4.8 micrometres. 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. It also has a filter wheel, with a slightly different set of filters.
The Impactor section of the spacecraft contains an instrument that is optically identical to the MRI, called the Impactor Targeting Sensor (ITS), but without the filter wheel. Its dual purpose was to sense the Impactor's trajectory, which could then be adjusted up to four times between release and impact, 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 ) that were transmitted in real-time to the Flyby spacecraft before the camera and 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, with a weight of 113 kg. Including this cratering mass, copper formed 49% of total mass of the Impactor (with aluminium at 24% of the total mass); this was to minimize interference with scientific measurements. Since copper was not expected to be found on a comet, scientists could ignore copper's signature in any spectrometer readings. Instead of using explosives, it was also cheaper to use copper as the payload.
Explosives would also have been superfluous. At its closing velocity of 10.2 km/s, the Impactor's kinetic energy was equivalent to 4.8 tonnes of TNT, considerably more than its actual mass of only 372 kg.
Mission profile
Following its launch from Cape Canaveral Air Force Station pad SLC-17B at 18:47 UTC on January 12, 2005, the Deep Impact spacecraft traveled in 174 days to reach comet Tempel 1 at a cruising speed of . Once the spacecraft reached the vicinity of the comet on July 3, 2005, it separated into the Impactor and Flyby sections. The Impactor used its thrusters to move into the path of the comet, impacting 24 hours later at a relative speed of . The Impactor delivered 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 wide, larger than the bowl of the Roman Colosseum. The size of the crater was still not known one year after the impact. The Stardust spacecraft observed the impact site in 2011; it was determined that the crater's diameter is around .
Just minutes after the impact, the Flyby probe passed by the nucleus at a close distance of , taking pictures of the crater position, the ejecta plume, and the entire cometary nucleus. The entire event was also photographed by Earth-based telescopes and orbital observatories, including Hubble, Chandra, Spitzer, and XMM-Newton. The impact was also observed by cameras and spectroscopes on board Europe's Rosetta spacecraft, which was about 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 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 .
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 pm EST (1847 UTC) by a Delta II rocket.
Deep Impact 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.
On February 11, 2005, Deep Impact 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. 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.
Cruise phase
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 .
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 . 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, 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). 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. The spacecraft studied the images of various distant stars to determine its current trajectory and position. , 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." On June 23, 2005, the first of the two final trajectory correction maneuvers (targeting maneuver) was successfully executed. A velocity change was needed to adjust the flight path towards the comet and target the Impactor at a window in space about wide.
Impact phase
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). 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 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 Tempel 1 would collide with it. 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.
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.
Results
Mission control did not become aware of the Impactor's success until five minutes later at 05:57 UTC. 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."