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Overview of NASA's Deep Impact comet mission
Posted: January 9, 2005

Deep Impact is the first mission ever to attempt impact with a cometary nucleus in an effort to probe and discover the secrets that lie beneath its surface. Scheduled for launch in January 2005, Deep Impact will fly directly to its encounter with comet Tempel 1, making no planetary flybys along the way. The voyage will take about six months.

The mission has been designed as the most expedient way to accomplish the project's primary scientific objective - to observe close-up the internal composition of a comet. The mission is part of NASA's Discovery program, aimed at launching many small, relatively low-cost missions that perform focused science with fast turn-around times, and are joint efforts with industry, small business and universities.

Mission phases
Six mission phases have been defined to simplify descriptions of the different periods of activity during the mission. These are the launch, commissioning, cruise, approach, encounter and playback phases.

Deep Impact will be launched from Space Launch Complex 17B at Cape Canaveral Air Station, Florida. The launch period continues through Jan. 28. Two instantaneous launch windows occur each day.

The spacecraft will be launched on a variant of the Delta II launch vehicle known as a Delta 7925. This version of the Delta II uses a first-stage rocket with nine solid-fuel boosters and a second-stage rocket with a restartable engine. It is topped by a Star 48 solid-fuel upper-stage booster.

Launch events
At the moment of liftoff, the Delta II's first-stage main engine ignites, along with six of its nine solid-fuel boosters. The remaining three solids are ignited in flight following the burnout of the first six. The spent booster casings are then jettisoned in sets of three. The first-stage main engine continues to burn for 4.4 minutes, when it shuts down.

Seconds later, the Delta's first and second stages are separated, and approximately 5 seconds later the second stage is ignited. The Delta's payload fairing, or nose cone, is jettisoned approximately 5 minutes into flight. The rocket's second stage continues to burn until a 167-kilometer-high (90-nautical-mile) circular parking orbit is achieved. The second stage shuts down just under 10 minutes after liftoff.

After achieving this parking orbit, the Delta rocket and Deep Impact spacecraft will coast for approximately 17 minutes before reaching the proper position to depart from Earth orbit. At this point the Delta's second-stage engine is restarted and burns for almost 2 minutes. After a brief coast lasting 50 seconds, the Star 48 upper stage with attached Deep Impact spacecraft is spun up to about 60 rpm to stabilize the vehicle for the third-stage burn. Three seconds later, the second stage separates from the upper stage. Thirty-seven seconds after separation of the second and third stages, the Star 48 spin-stabilized third stage is ignited. The burn lasts for approximately 87 seconds.

Approximately 4-1/2 minutes after burnout of the third stage, a yo-yo despin system is used to decrease the spin rate of the third-stage/spacecraft stack from about 60 rpm to nearly 0 rpm. A few seconds later, the spacecraft is separated from the spent thirdstage motor. Pyrotechnic actuators and push-off springs on the launch vehicle release the Deep Impact spacecraft on its trajectory to comet Tempel 1.

About one minute after third-stage separation, the spacecraft's solar array will be deployed, and the spacecraft will rotate to point it at the Sun in about 5 minutes.

In order to assess the health of the spacecraft and respond to any anomalies, mission controllers plan to establish communications with the spacecraft as soon as possible after separation from the third stage. The Delta's upper stage sends the spacecraft out of Earth orbit over southern Africa, so the spacecraft is headed east over the Indian Ocean when it separates from the launch vehicle. The first opportunity for contact with NASA's Deep Space Network is via the tracking complex near Canberra, Australia. The first downlink from the spacecraft is expected 11 to 15 minutes after separation depending on the launch date and time.

Commissioning phase
The phrase "commissioning phase" is used to describe the period after the spacecraft is stabilized in flight until 30 days after launch. This is a time of initial operation, checkout and calibration for the spacecraft and payload. Thrusters will be fired in one initial trajectory maneuver to correct for any errors in the flight path remaining from the launch.

During this phase, the spacecraft's scientific instruments will be tested using the Moon as a calibration target. The spacecraft's autonomous navigation system will be tested using the Moon and Jupiter as practice targets.

Cruise phase
The cruise phase begins 30 days after launch and ends 60 days before the cometary encounter. As the spacecraft flies toward the comet, the mission team will conduct scientific calibrations, an encounter demonstration test, ground operational readiness tests and a second trajectory correction maneuver. In addition, some initial observations of comet Tempel 1 will be attempted.

Approach phase
The approach phase extends from 60 days before to five days before encounter. Sixty days out roughly coincides with the earliest time that the team expects the spacecraft to be able to detect comet Tempel 1 in its high-resolution camera. This milestone marks the beginning of an intensive period of observations to refine knowledge of the comet's orbit. Regular scientific observations will be used to study the comet's rotation, activity and dust environment.

Comet encounter
The encounter phase begins five days before and ends one day after the impact with comet Tempel 1. This brief but very intense period includes two final targeting maneuvers, leading up to release of the impactor and its dramatic collision with the comet's nucleus. After releasing the impactor, the flyby spacecraft will execute a deflection maneuver so that it does not also collide with the comet; the maneuver will also slow it down enough to make observations after the impact and before flying past the nucleus. The flyby spacecraft then observes the impact event, the resulting crater and ejected material, before transmitting these data to Earth.

Both the Deep Impact spacecraft and comet Tempel 1 are in curved orbits around the Sun. However, the comet is traveling substantially faster in its orbit than is the spacecraft so the comet actually runs over the spacecraft at a relative velocity of 10.2 kilometers per second (about 22,820 miles per hour).

After releasing the impactor directly in the path of the oncoming comet, the flyby spacecraft fires its thrusters to change course, safely passing by the nucleus with adequate time to observe the impact and resulting crater. This deflection maneuver is designed to make the spacecraft miss the cometary nucleus by 500 kilometers (311 miles). This distance was chosen to provide a survivable path through the comet's inner coma dust environment while still allowing a sufficiently close view of the crater by the spacecraft's high-resolution camera. The spacecraft will be protected by dust shields and oriented in a way to allow its cameras to continue taking pictures throughout the approach until it comes to within about 700 kilometers (420 miles) of the comet's nucleus. At this point, the spacecraft will stop taking pictures and fix its orientation so that its dust shields protect it as much as possible during the closest pass by the comet.

The kinetic energy released by the collision event will be 19 gigajoules, which is about the equivalent of the amount of energy released by exploding 4.5 tons of TNT. This in turn is about the amount of energy used in an average American house in one month.

Encounter timing
The impact with the comet on July 4, 2005 has been scheduled during a 55-minute window in which Deep Space Network complexes in both California and Australia can track the spacecraft. Besides allowing for fully redundant coverage by these two ground stations, the timing also permits the event to be observed by the major observatories at Mauna Kea on the island of Hawaii (where it will still be the evening of July 3). Another consideration in the encounter timing was to provide an optimal opportunity for observations by two NASA spaceborne observatories, the Hubble Space Telescope and the Spitzer Space Telescope.

Playback phase
The playback phase begins one day after impact and continues until the end of mission 30 days after the cometary encounter -- or Aug. 3, 2005. Wrapping up the primary mission, data taken during the impact and subsequent crater formation will be transmitted to Earth. Backwards-looking observations of the departing comet will be continued for 60 hours after the impact to monitor changes in the comet's activity and to look for any large debris in temporary orbit around the nucleus.

Throughout the Deep Impact mission, tracking and telecommunications will be provided by NASA's Deep Space Network complexes in California's Mojave desert, near Madrid, Spain and near Canberra, Australia. Most data from the spacecraft will return through the Deep Space Network's 34-meter-diameter (110-foot) antennas, but the 70- meter (230-foot) antennas will be used during some critical telecommunications phases.

Planetary protection
The United States is a signatory to the United Nations' 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies. Also known as the "Outer Space Treaty," this document states in part that exploration of the Moon and other celestial bodies shall be conducted "so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter."

The policy used to determine restrictions that are applied in implementing the Outer Space Treaty is generated and maintained by the International Council for Science's Committee on Space Research, which is headquartered in Paris. NASA adheres to the committee's planetary protection policy, which provides for appropriate protections for solar system bodies such as comets.

For the Deep Impact mission, NASA's planetary protection officer has assigned a "Category II" status under the policy. This requires documentation of the mission and its encounter with Tempel 1, but places no additional operating restrictions on the mission. Comets are bodies that are of interest to the study of organic chemistry and the origin of life, but are not going to be contaminated by Earth-origin microorganisms.

It should also be noted that comets are exceedingly numerous in the solar system, and any particular comet has a finite lifetime. Indeed, Tempel 1 is a representative of a family of abundant comets. In this case, therefore, the benefits of the Deep Impact mission to cometary study far-outweigh any potential concerns about the fate of the comet itself.

Nom de plumes to help make cometary plume
Space fans worldwide may celebrate July 4, 2005, as the day their names reach a comet. The Deep Impact project sponsored a "Send Your Name to a Comet" campaign that invited people from around the world to submit their names via the Internet to fly onboard the Deep Impact impactor. A mini-compact disc bearing the names of more than half a million space enthusiasts is onboard Deep Impact. The mini-CD will melt, vaporize and essentially be obliterated -- along with everything else aboard the impactor -- when it collides with comet Tempel 1.