Overview of NASA's Deep Impact comet mission
FROM NASA PRESS KIT
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.
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.
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.
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.
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.
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
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