12 January 2005

Deep Impact launches from Cape Canaveral.

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, 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. Communication was unexpectedly lost in September 2013 while the craft was heading for another asteroid flyby.

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.

The spacecraft consists of two main sections, the 372-kilogram copper-core “Smart Impactor” that impacted the comet, and the 601 kg “Flyby” section, which imaged the comet from a safe distance during the encounter with Tempel 1.

The Flyby spacecraft is about 3.3 meters long, 1.7 meters wide and 2.3 meters high. It includes two solar panels, 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, and the Medium Resolution Imager. 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, 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 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, with a weight of 100 kg. Including this cratering mass, copper formed 49% of total mass of the Impactor; 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 metric tons of TNT, considerably more than its actual mass of only 372 kg.

The mission coincidentally shared its name with the 1998 film, Deep Impact, in which a comet strikes the Earth.

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 429 million km in 174 days to reach comet Tempel 1 at a cruising speed of 28.6 km/s. 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 10.3 km/s. The Impactor 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 wide, larger than the bowl of the Roman Colosseum. The size of the crater was still not known one year after the impact. The 2007 Stardust spacecraft’s NExT mission determined the crater’s diameter to be 150 meters.

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 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 80 million km 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.