Adaring mission offering great scientific payoff, Deep Impact is the first spacecraft ever to attempt impact with a cometary nucleus in an effort to probe and discover the secrets that lie beneath its surface. The mission is a tremendous technological challenge -- the equivalent of hitting a bullet with a bullet, while taking a picture from a third bullet flying by. Launched 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 structure and 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 turnaround 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.

Launch
Deep Impact was launched Jan. 12, 2005, from Space Launch Complex 17B at Cape Canaveral Air Station. The launch vehicle was a variant of the Delta II 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 was topped by a Star 48 solid-fuel upper-stage booster that provided the final kick to send Deep Impact on its way toward comet Tempel 1.

Commissioning phase
The first weeks after launch was a time of initial operation, checkout and calibration for the spacecraft and payload. During this phase, the mission team verified the basic state of health of all subsystems and tested the operation of science instruments. The spacecraft's autonomous navigation system was activated and tested using the moon and Jupiter as targets. Atrajectory correction maneuver was performed, refining the spacecraft's flight path to comet Tempel 1.

Cruise phase
The cruise phase began 30 days after launch and ended 60 days before the cometary encounter. As the spacecraft flies toward the comet, the mission team conducted scientific calibrations, an encounter demonstration test and readiness tests of ground systems.

Approach phase
The approach phase extends from 60 days before encounter until five days before encounter. Sixty days out roughly coincides with the earliest time that the team expected the spacecraft to be able to detect comet Tempel 1 in its medium-resolution camera. This milestone marked the beginning of an intensive period of observations to refine knowledge of the comet's orbit. Regular scientific observations are being 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 intensive period includes two final targeting maneuvers, leading up to release of the impactor and its dramatic collision with the comet's nucleus. The encounter phase also includes imaging conducted by the flyby spacecraft during its close flyby of the nucleus, and transmission to Earth of all the flyby's collected data.

Both the Deep Impact spacecraft and comet Tempel 1 are in their own unique orbits around the Sun. However, the comet is traveling substantially faster (29.9 kilometers per second (66,880 miles per hour)) than the spacecraft (21.9 kilometers per second (48,990 miles per hour)). Just as the goal of a wide receiver in football is to arrive at the same location and time as the faster traveling ball thrown by the quarterback, Deep Impact's impactor needs to arrive at a certain point in space at the right time so that the comet actually runs over it at a relative velocity of 10.3 kilometers per second (about 23,000 miles per hour).

Trajectory Correction Maneuvers
Beginning three weeks before the encounter, the mission team will focus on final targeting of the impactor. During this time, optical navigation images will be collected nearly continuously. These will be used to shape the final two trajectory correction maneuvers before the impactor is released. These two final thruster firings by the flyby spacecraft are designed to deliver the impactor precisely to the comet.

The first of these maneuvers will be conducted June 23, and will target the impactor at a window in space about 100 kilometers (62 miles) wide.

The last targeting maneuver is scheduled July 2, only 30 hours before encounter, or 6 hours before the impactor is released from the flyby spacecraft. This maneuver will shrink the target window to about 15 kilometers (9 miles) wide.

Impactor release
One day out from the comet, the flyby spacecraft will deploy the impactor. Before impactor release, the flyby spacecraft will adjust its orientation in space and configure itself for separation, including turning on heaters, arming separation actuators and enabling the impactor's battery. At the time of release, electrical disconnect actuators and separation pyros will fire, causing a spring to separate the two spacecraft at a speed of 34.8 centimeters per second (0.78 mph). Broken wires will verify that electrical disconnect took place.

Flyby deflection maneuver
Shortly after separation, the flyby spacecraft will slew to a new orientation in space and fire its thrusters in what is called a deflection maneuver. This is designed to steer the flyby spacecraft away from the impactor just enough that it does not also collide with the comet or pass too close to potentially hazardous cometary particles surrounding the nucleus.

The maneuver begins 12 minutes after release of the impactor spacecraft, and lasts about 14 minutes. It will be the longest burn of the flyby spacecraft's thrusters up to that time. The burn will slow the flyby spacecraft's speed by 102 meters per second (about 227 miles per hour). As a result, the flyby spacecraft will be 8,606 kilometers (5,348 miles) away from the impactor and comet at the time of impact. Because it is now traveling slower than the impactor, the flyby spacecraft will not make its closest approach to the comet nucleus until 14 minutes after the impact, passing within about 500 kilometers (310 miles).

Autonomous navigation
Due to the rapid pace of mission events leading up to the comet impact and the distance from Earth, both the flyby and impactor spacecraft were designed to perform automated onboard navigation, firing thrusters to change their orientation and flight path as necessary. Beginning two hours before impact, the "autonav" software on both flyby and impactor spacecraft will begin taking images of the comet nucleus at 15-second intervals. Autonav then performs image processing, orbit determination and maneuver computations. The autonav capability is based on heritage from the Deep Space 1 mission, which flew past comet Borrelly in 2001. Some of this capability was also used on the Stardust spacecraft to control pointing of observations of comet Wild 2 in January 2004.

Impactor navigation
During its 24 hours of free flight, the impactor will travel over half a million miles and maneuver itself directly into the path of the comet. The impactor will execute up to three thruster firings to fine-tune its flight path as it closes in on the comet nucleus. The first is scheduled 90 minutes before impact, followed by a second one 35 minutes before impact and a final firing 12.5 minutes before impact. The maneuvers will use four 22-newton thrusters firing in pulses varying in length from .015 to 0.5 second each.

The goal of the thruster firings is to aim the impactor to hit the comet in an unshadowed area at a location easily observable by the flyby spacecraft.

Impactor imaging
The impactor begins science imaging 22 hours before impact with a pair of full-frame images -- one exposed for the nucleus, and one exposed for the coma, the dimmer cloud that surrounds the nucleus. Similar image pairs will then be obtained every two hours until 12 hours before impact. At that time, the impactor will spend two minutes taking the same pictures and other data that it will collect during the final two minutes before impact. This demonstration is designed to verify that it will execute this critical data-taking correctly during the final and most critical segment of its mission.

Beginning 10 hours before impact, images will be taken every two hours until 8 hours before impact; every hour from 7 to 4 hours before impact; and every 30 minutes from 3 to 1 hour before impact. At that time, the pace of imaging will increase until it reaches a maximum of one picture every 0.7 second at about 12 seconds before impact. Engineers say that odds are at least 50-50 that dust hitting the impactor will end transmission of its images during the final 10 seconds before impact. The final potential image that could be transmitted in its entirety is one scheduled at about 2 seconds before impact, with a scale of about 20 centimeters (approximately 8 inches) per pixel.

Encounter timing
The impact with the comet on July 4, 2005 has been scheduled at 05:52 Universal Time -- late evening in the eastern Pacific region. This time has an uncertainty of 2 to 3 minutes. The time allows NASA's Deep Space Network complexes in both California and Australia to 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 as well as at Palomar Observatory in California, Kitt Peak National Observatory in Arizona and other observatories in the southwestern continental United States and Baja California. The time also allows approximately 10 to 15 minutes of observation by Hubble Space Telescope before it moves behind Earth in its 90-minute orbit. Observations by other satellites will also be carried out during and/or shortly after the impact.

Impact
Impact is scheduled to occur at 10:52 p.m. Pacific Daylight Time on July 3, 2005 (1:50 a.m. Eastern Daylight Time on July 4). There is a 2- to 3-minute uncertainty in the actual impact time, due to imperfect knowledge of the comet's path.

The kinetic energy released as the impactor smashes into the comet nucleus is expected to be 19 gigajoules -- similar to detonating 4.5 tons of TNT. The time it takes for the crater to form could vary, depending on the properties of the comet nucleus material, but is expected to be on the order of four minutes.

During the impact, the flyby spacecraft will train its high- and medium-resolution instruments on the predicted impact point. They will take pictures at a high rate beginning a few seconds before impact.

Shields Up! Shield Mode
During most of the encounter phase before closest approach, the flyby spacecraft instruments will be pointed at the comet. 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 passage through the inner coma. Although comet imaging is not possible in the shield mode, pointing of the flyby spacecraft's high-gain antenna toward Earth is maintained during this time, allowing the downlink of critical science data at the highest supportable data rate. The high-resolution instrument's infrared sensor will make a scan across the coma near the nucleus in the first few seconds after entering shield mode. After entering shield mode, the spacecraft remains in this orientation until about 22 minutes after closest approach, when the dust-impact hazard zone has been safely passed.

Flyby closest approach
The "miss distance" between flyby spacecraft and comet nucleus is planned for about 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 flyby spacecraft will make its closest approach to the comet nucleus 14 minutes and 10 seconds after the impact event.

Twenty-one minutes after the flyby spacecraft's closest approach to the comet nucleus, it will begin a maneuver to point its instruments back toward the nucleus. This maneuver takes about 9 minutes to complete. At that time, the first "look back" science data of the nucleus and its surroundings begins.

Post-impact flyby data
As the flyby spacecraft pulls away from the comet nucleus, it will use all of its instruments to monitor any new activity at the crater site. The rate of observations will gradually decrease as the comet becomes smaller in the instruments' fields of view. Ground-based telescopes will continue to monitor the comet for any later changes following the impact.

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.

Telecommunications
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 that determines the restrictions applied in implementing the Outer Space Treaty is maintained by the International Council for Science's Committee on Space Research, which is headquartered in Paris. NASA's planetary protection policy adheres to the committee's policy, and 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, 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. Amini-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.