Mission Type: Flyby, Impact
Launch Vehicle: Delta II 7925 with Star 48 upper stage
Launch Site: Cape Canaveral Air Force Station, Florida
NASA Center: Jet Propulsion Laboratory
Spacecraft Mass: Flyby Spacecraft: 601 kilograms (1,325 pounds) at launch, consisting of 515 kilograms (1,135 pounds) spacecraft and 86 kg (190 lbs) fuel
Impactor: 364 kilograms (802 pounds)
Spacecraft Instruments: 1) High-resolution telescope and camera
2) Medium-resolution wide angle telescope and camera
Spacecraft Dimensions: Flyby Spacecraft: 3.3 meters (10.8 feet) long, 1.7 meters (5.6 feet) wide, and 2.3 meters (7.5 feet) high
Impactor: 1 meter (39 inches) long, 1 meter (39 inches) in diameter
Spacecraft Power: 2.8-meter-by-2.8-meter (9-foot-by-9 foot) solar panel providing up to 92 watts, depending on distance from Sun. Power storage via small 16-amp-hour rechargeable nickel hydrogen battery
Total Cost: Primary Mission: $267 million total (not including launch vehicle), consisting of $252 million
spacecraft development and $15 million mission operations
Deep Impact Launch Press Kit, January 2005
EPOXI Website, University of Maryland, http://epoxi.umd.edu/
Since the main Deep Impact mission, there are 9 time intervals of interest: Hibernation, Cruise 1, Earth Flyby, EPOCh Observations, Cruise 2, Approach, Hartley 2 Encounter, Playback and Analysis. They are described below in the order of their occurrence. Although an extensive search for comet Boethin was carried out, it could not be found and the spacecraft was directed to comet Hartley-2 instead. This made it necessary to plan four instances of Hibernation and three instances of both Cruise and Flyby to follow the impact event at Tempel-1 instead of those originally planned. The revised mission will take about two years longer than the original.
Hibernation-1: Once the impactor spacecraft met its fate - Independence Day 2005 - and subsequent lookback observations completed and all queued up data transmitted to Earth, the spacecraft executed a Trajectory Correction Maneuver, TCM-8. That maneuver directed the spacecraft to what was originally planned to be an Earth flyby en route to comet Boethin. While maintaining its sensors in a sun-safe position, the spacecraft went into hibernation for 25 months with only biannual health checks to disturb its sleep.
Cruise-1: On September 25, 2007, commands from earth woke the flyby spacecraft and the Cruise-1 phase began. Since comet Boethin could not be located in time, the spacecraft was commanded to perform the first of several more Trajectory Correction Maneuvers to set the spacecraft on course for the first of the three Earth Flybys. The correction maneuver was made on November 1, 2007. The modified trajectory will bring the spacecraft to Hartley-2 in October 2010. Since the instruments have been exposed to the ionizing radiation of the sun, their sensitivity may have changed. If so, users will need to compensate for the changes. The first of several observations to test and recalibrate the instruments took place in early November 2007. This included a photometry test to demonstrate that the EPOCh observations of stars will have the required sensitivity. Other Cruise-1 activities included: recomputing Hartley-2's orbit using tracking data; observing random points in sky to determine the background noise level of the optical and infrared detectors and checking the sensitivity of the optical detectors using an internal lamp.
Earth Flyby-1: In early December 2007, preparations began for the spacecraft's first flyby of Earth. It approached in such a way that the Earth stole some energy from the spacecraft, dropping it into a smaller orbit around the Sun. The amount of energy exchanged in this "gravity assist" is very large for the spacecraft, and essential to accomplishing its extended mission, but makes no measurable difference to the massive Earth. In doing this the spacecraft is set on course for an encounter with comet Hartley-2 at a time when DSN tracking stations in two different locations on Earth can "see" the spacecraft in order to receive data from it and send commands to it. In late December 2007, the HRI, High Resolution Instrument, including its infrared spectrometer, and the MRI, Medium Resolution Instrument were recalibrated using our Moon as a reference source. On the last day of 2007, the spacecraft achieved its closest approach to Earth - a mere 19,310 km above eastern Asia, three Earth-radii from Earth's surface. It will be closer to the Earth than the Moon, which is at 60 Earth-radii, but there is no danger of the spacecraft hitting the Earth.
EPOCh Observations: This phase, which began on January 26, 2008, will gather EPOCh science data until about the end of May, 2008. The main objective of EPOXI's EPOCh observations are to investigate planets known to orbit 3-6 distant stars, and to search those stars for previously undiscovered planets. EPOCh also will search for evidence of rings and moons associated with the known giant planets of the targeted stars. Some properties of interest include the reflectivity (albedo) of extrasolar planets. In addition to studying extrasolar planets, EPOXI will observe the Earth at both visible and infrared wavelengths. These data will help scientists to understand what an Earth-like planet might look like if it were orbiting a distant star. The data will help in making computer models of planetary images that are not so Earth-like, and will guide future efforts to detect and study extrasolar planets directly.
Cruise-2: In this phase, spacecraft and instrumentation is tested and calibrated. Most of this phase is concurrent with the EPOCh Observations phase. It begins in January 2008, after the first Earth Flyby, and continues to mid-2008.
Hibernations and Flybys, Two and Three: In mid-2008 the spacecraft begins its second hibernation, awakening in time for a second Earth Flyby near the end of December, 2008. Having completed the second Earth Flyby, the spacecraft hibernates for a third time and then awakes for its third and final Earth Flyby in late December 2009. Each flyby may include a pair of Trajectory Correction Maneuvers.
Hibernation-4: Hibernation-4, the final one, begins in 2010 after the third Earth Flyby and ends roughly in mid 2010.
Cruise-3: In Mid-2010 the spacecraft wakes up and the spacecraft and instruments are exercised and recalibrated. Final star tracking measurements are made in preparation for another Trajectory Correction maneuver that occurs before Cruise-3 ends.
Comet Approach Phase: The approach phase begins about sixty days before the encounter. The main activity will be to gather navigational data to plan Trajectory Correction Maneuvers as well as obtaining scientific observations. In this phase a search for possible outbursts of volatile material from the comet's surface will begin.
Encounter Phase: Most of the comet-science data is collected in the encounter phase. The encounter phase begins in October 2010. A final targeting maneuver may be executed shortly after the phase begins. The trajectory is chosen in such a way that there is always light on the solar panels. Unlike the approach trajectory chosen for the Tempel 1 encounter, the flyby spacecraft does not come close enough to need the protection of its meteoroid shields. After closest approach, a two-day period for lookback observations is planned.
Data gathering in the encounter phase supports the comet science goals in the following activities:
- Search for and, if found, produce spectral maps of outbursts of gas from the surface of comet Hartley 2. Track the outburst as the comet rotates. Correlate outbursts with surface features. Such outbursts were observed during the spacecraft's flyby of comet Tempel 1.
- Obtain infrared spectral maps of gasses in the innermost coma. Investigate the distribution of dust and gas in the coma.
- Search for frozen volatiles on the surface of the comet. Water ice, for example, was discovered when the flyby explored Tempel 1.
- Produce broad band images of the comet that will establish limits on the size of the nucleus. Produce a model of its shape.
- Map the brightness and color variations of the surface. Locate topographical features that disclose the processes by which the comet was formed. Compare the distribution of crater-sizes with the distribution of the size of craters on other comets, asteroids and planetary satellites.
- Map the temperature of the surface to assess the thermal conductivity of the interior and the migration of subsurface volatiles.
Playback: Beginning some weeks after closest approach and continuing for several days, long enough to ensure that all data have been accurately recovered from spacecraft memory, any data not yet downlinked to Earth will be transferred. The Deep Impact flight system is then decommissioned and the spacecraft will continue to follow its endless orbit of the Sun.
Analysis: Back on Earth, raw data will continue to be converted into products suitable for analysis and archiving.