An illustration of the Deep Impact spacecraft. Credit: NASA

The Deep Impact flight system is actually two spacecraft mated together. One part, an impactor, will fly into the nucleus of comet Tempel 1. The second part, a flyby spacecraft, acts as the mothership of the combo, carrying and powering the impactor until 24 hours before the comet impact. Each of these two spacecraft has its own instruments and capabilities to receive and transmit data.

Slightly less than half of the impactor spacecraft is composed of copper, a material chosen because it is not expected to appear in the natural chemical signature of the comet itself that will be studied by the mission's scientific instruments. For its short period of operation, the impactor uses simpler versions of the flyby spacecraft's hardware and software, and contains fewer backup systems.

Flyby spacecraft
The flyby spacecraft is about the size of an average mid-sized sport utility vehicle. It provides power, communications and maneuvering for both itself and the impactor while en route to the comet nucleus. It releases the impactor, receives impactor data, supports the instruments as they image the impact and resulting crater, and then transmits the scientific data back to Earth.

The flyby spacecraft is three-axis-stabilized, meaning that it does not spin as it flies through space. Its structure is constructed from aluminum and aluminum honeycomb. Blankets, surface radiators, finishes, and heaters passively control the temperature.

Most systems on the flyby spacecraft are redundant, meaning that there is a backup available if the main system encounters a problem. Automated onboard fault protection software will sense any unusual conditions and attempt to switch to backups. Both the flyby spacecraft and impactor will use onboard navigation software to find comet Tempel 1.

The spacecraft's main computer is based around a Rad 750 chip, a radiation-hardened version of a PowerPC processor used in various consumer computers. There are two redundant computers on the flyby spacecraft. Between them they have a total memory of 1,024 megabytes.

The flyby spacecraft uses an X-band radio to transmit to Earth at a frequency of about 8 gigahertz, and listens to the impactor on a different frequency. It is equipped with a single steerable, high-gain antenna and two fixed, low-gain antennas.

The spacecraft draws its power from a fixed solar array consisting of 7.5 square meters (about 80 square feet). A rechargeable 16-amp-hour nickel hydrogen battery provides power during one solar eclipse and while the solar array is directed away from the Sun.

To adjust its flight path through space, the flyby spacecraft has a propulsion system consisting of a group of thrusters. The fuel used by the thrusters is hydrazine.

Flyby scientific instruments
The scientific instruments on Deep Impact's flyby spacecraft have two main purposes. During the first part of the mission, they guide the flyby spacecraft and impactor onto a collision course with the cometary nucleus. Then, during the mission's climax, they collect scientific observations before, during and after the impact. This includes observing material thrown off by the collision event, called "ejecta," as well as the crater created by the event and the surrounding area on the comet's nucleus.

• The High-Resolution Instrumentis one of two imagers on the flyby spacecraft. It features a 30-centimeter-diameter (11.8-inch) telescope that delivers light simultaneously to both a multispectral camera and an infrared spectrometer. This camera is one of the largest instruments flown to date on a planetary mission. The instrument's camera will acquire images of the inside of the crater and obtain the best, most detailed pictures of a comet ever taken.

  Tests after launch indicated that the imager's focus was not as expected. Although the focus has improved after the instrument was heated, or "baked out," to remove normal residual moisture, spatial resolution remains a factor of 3 to 4 less than planned. A "tiger team" of engineers found that the most likely cause of the problem was a mirror that, while flat at room temperature, developed some curvature at the cryogenic temperatures of the thermal-vacuum tests. The team will compensate by using an image processing technique called deconvolution which was also used to improve images from the Hubble Space Telescope before its second camera instrument was installed. The science team is confident that the technique will allow them to recover resolution essentially the same as what was planned before launch.
  
  

• The Medium-Resolution Instrument is the other scientific instrument on the flyby spacecraft. It is a smaller telescope with a diameter of 12 centimeters (4.7 inches). Due to its wider field-of-view, it can observe more of the field of ejected material as well as the crater created by the impact event. It can also observe more stars around the comet and is therefore slightly better at navigation during the final 10 days of approach to the comet. When the flyby spacecraft comes within 700 kilometers (420 miles) of the comet's nucleus, this instrument can image the entire comet with a resolution of about 10 meters (about 33 feet) per pixel.

Impactor
The impactor spacecraft weighs a total of 372 kilograms (820 pounds), with 113 kilograms (249 pounds) of that being "cratering mass" -- dead weight designed to help the impactor make a substantial crater in the cometary nucleus. The cratering mass is made up of copper plates at the impact end of the impactor. The copper plates are machined to form a spherical shape.

The impactor is powered during its brief solo flight by a single 250-amp-hour battery. The computer and avionics interface box are similar to those on the flyby spacecraft; star trackers, inertial reference units and many propellant subsystem components are the same on both spacecraft. Like the flyby spacecraft, the impactor has a group of thrusters to refine its flight path. Because of its brief mission, the impactor does not have redundant backups as does the flyby spacecraft.

The impactor's single scientific instrument, called the impactor targeting sensor, is an imaging system identical to the medium-resolution instrument on the flyby spacecraft, but without a filter wheel. A 12-centimeter-diameter (4.7-inch) telescope provides navigation images as well as closeup scientific images of the comet just before impact.

The best resolution expected from this instrument is about 20 centimeters (approximately 8 inches) per pixel when the impactor is 20 kilometers (about 12 miles) away from the comet's nucleus -- although the dust surrounding the comet is likely to sandblast the mirror significantly in the last half minute or so. Dust impacts may also disturb the instrument's pointing in the final minute before impact.