Comet Tempel 1 was discovered in 1867 and orbits the Sun every 5.5 years. Planning and design for the Deep Impact mission ran from November 1999 through May 2001. This was followed by the building and testing of the two-part spacecraft. The larger flyby spacecraft was designed to carry a smaller impactor to Comet Tempel 1 and release it into the comet's path for a collision on July 4, 2005.
In January 2005, a Delta II rocket launched the combined Deep Impact spacecraft toward the comet. The combined spacecraft was to approach Tempel 1 and collect images of the comet before the impact. In early July 2005, 24 hours before impact, the flyby spacecraft would point its high-precision tracking telescopes at the comet and release the impactor on a course to hit the comet's sunlit side.
The impactor was a battery-powered spacecraft that would operate independently of the flyby spacecraft for just one day. After its release, it was to take over its own navigation and maneuvers into the path of the comet. A camera on the impactor would capture and relay images of the comet's nucleus just seconds before collision. After release of the impactor, the flyby spacecraft was to maneuver to a path with a closest approach of 500 km from the comet.
On July 4, 2005, the 370-kg impactor would hit the comet. On impact, a crater was produced expected to range in size from 10 to 100 m across. Ice and dust debris would be ejected, hopefully revealing fresh material beneath.
After release of the impactor, the flyby spacecraft was to maneuver to a path with a closest approach of 500 km from the comet. The flyby spacecraft would observe and record data on the impact, the ejected material blasted from the crater using cameras and a spectrometer. This should reveal the structure and composition of the crater's interior. A shield was to protect it as it passed through the comet's dust tail. Thereafter the flyby spacecraft would turn to look at the comet again, recording additional data from the other side of the nucleus and observing changes in the comet's activity.
The flyby spacecraft carried a set of instruments and the smart impactor. Two instruments on the flyby spacecraft observed the impact, crater and debris with optical imaging and infrared spectral mapping. The flyby spacecraft used an X-band radio antenna (transmission at about eight gigahertz) to communicate to Earth while monitoring the impactor on a different frequency. For most of the mission, the flyby spacecraft communicated through the 34-meter antennae of NASA's Deep Space Network. During the short period of encounter and impact, when there was an increase in volume of data, overlapping antennas around the world were used. Primary data would be transmitted immediately and other data stored and transmitted over the following week. The impactor spacecraft was composed mainly of copper, which was not expected to appear in data from a comet's composition. For its short period of operation, the impactor used simpler versions of the flyby spacecraft's hardware and software - and fewer backup systems.
The Deep Impact mission was a partnership between the University of Maryland (UMD), the California Institute of Technology's Jet Propulsion Laboratory (JPL) and Ball Aerospace and Technology Corp. The scientific leadership of the mission was based at UMD. Engineers at Ball Aerospace designed and built the spacecraft under JPL's management. Engineers at JPL controlled the spacecraft after launch and relayed data to scientists for analysis. The entire team consisted of more than 250 scientists, engineers, managers, and educators. Deep Impact was a NASA Discovery Mission, eighth in a series of low-cost, highly focused space science investigations.
The Flight System was about 3.3m long, 1.7m wide, and 2.3m high. It consisted of two spacecraft: the flyby spacecraft and the impactor. Each spacecraft had its own instruments and capabilities to receive and transmit data.
The flyby spacecraft carried the primary imaging instruments (the HRI and MRI telescopes) and the impactor (with an ITS telescope) to the vicinity of the comet nucleus. It released the impactor, received impactor data, supported the instruments as they imaged the impact and resulting crater, and then transmitted the science data back to Earth. The flyby spacecraft featured a high throughput RAD750 CPU with 1553 data bus-based avionics architecture, and a high stability pointing control system. The flyby spacecraft was three-axis stabilized and used a fixed solar array and a small NiH2 battery for its power system. The structure was aluminum and aluminum honeycomb construction. Blankets, surface radiators, finishes, and heaters passively controlled the temperature. The propulsion system employed a simple blowdown hydrazine design that provided 190 m/s of delta V. The flyby spacecraft mass was 650 kg.
The primary instruments on the flyby spacecraft were the High Resolution Instrument (HRI) and the Medium Resolution Instrument (MRI). The HRI, one of the largest space-based instruments built specifically for planetary science, was the main science camera for Deep Impact. It provided the highest resolution images via a combined visible camera, an infrared spectrometer, and a special imaging module. The HRI was optimally suited to observe the comet's nucleus. The HRI had a diameter of 30 cm, a focal length of 10.5 m, a field of view of 0.118 degrees, and an expected resolution of 1.4 m at 700 km from the comet in the visible spectrum. Its infrared imager would provide lower-resolution images in the 1.05 - 4.8 micrometer band.
The MRI served as the functional backup for the HRI, and was slightly better at navigation for the last 10 days of travel before impact due to its wider field of view, which allowed it to observe more stars around the comet. The difference between the two was the telescope, which sets the field of view (FOV) and the resolution of each. The MRI had a diameter of 12 cm, a focal length of 2.1 m, a field of view of 0.587 degrees, and a resolution of 7 m at 700 km range.
Flyby Spacecraft Technical Summary
The impactor guided itself to hit the comet nucleus on the sunlit side. The energy from the impact was to excavate a crater approximately 100 m wide and 28 m deep. The impactor separated from the flyby spacecraft 24 hours before it impacted the surface of Tempel 1. The impactor delivered 19 Gigajoules (equivalent to 4.8 tons of TNT) of kinetic energy to excavate the crater. This kinetic energy was generated by the combination of the mass of the impactor (370 kg) and its velocity when it impacted (10.2 km/s). Targeting and hitting the comet in a lit area was one of the mission's greatest challenges since the impactor would be traveling at 10 km per second and it must hit an area less than 6 km in diameter from about 864,000 km away. To accomplish this feat, the impactor used a high-precision star tracker, the Impactor Target Sensor (ITS), and Auto-Navigation algorithms (developed by Jet Propulsion Laboratory for the DS-1 mission) to guide it to the target. Minor trajectory corrections and attitude control were available by using the impactor's small hydrazine propulsion system. The impactor was made primarily of copper (49%) as opposed to aluminum (24%) because it minimized corruption of spectral emission lines that were used to analyze the nucleus.
The impactor was mechanically and electrically attached to the flyby spacecraft for all but the last 24 hours of the mission. Only during the last 24 hours would the impactor run on internal battery power. The propulsion system used hydrazine provided 25 m/s of delta-V for targeting.
The ITS on the impactor was nearly identical to the MRI on the flyby spacecraft. It differed only in that it lacked the filter wheel. The ITS had a diameter of 12 cm, a focal length of 2.1 m, and a field of view of 0.587 degrees. Expected resolution in the last image before impact was expected to be 20 cm at 20 km from the comet.
Impactor Technical Summary
Spacecraft delta v: 190 m/s (620 ft/sec). Electric System: 0.92 average kW.
Gross mass: 1,020 kg (2,240 lb).
First Launch: 2005.01.12.
Number: 1 .