EPOXI Mission Overview
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Overview - Since the main Deep Impact mission, there are intervals of interest: Hibernation, several Cruise segments, multiple Earth Flybys, EPOCh Observations, Approach, Hartley 2 Encounter, Playback and Analysis. They are described below. Although an extensive search for comet Boethin was carried out, it could not be found and the spacecraft has been directed to comet Hartley 2 instead. The revised mission will take about two years longer than the original.
- Hibernation - 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 26, 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 (TCM9) was made on November 1, 2007. The modified trajectory will bring the spacecraft to Hartley 2 in November 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 15,567.63 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 22, 2008, gathered EPOCh science data until the end of August, 2008. The main objective of EPOXI's EPOCh observations were to investigate planets known to orbit 8 distant stars, and to search those stars for previously undiscovered planets. EPOCh also searched 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 observed 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 - This phase started in January 2008 and lasts all the way up until the Hartley 2 Approach Phase in September 2010. This phase encompasses the EPOCH observations, spacecraft and instrumentation testing and calibration, Earth Flybys 2 and 3, Distant Earth Flybys 1 and 2, and the first Interplanetary Internet test.
Two Earth polar observations and a Mars observation In addition, two more observations of the Earth were performed during Cruise-2, taking advantage of the spacecraft's position and making up for two Earth observations that could not be performed during the original EPOCh phase. The Earth was observed in late March, 2009, from a direction looking 'down' at the North Pole. In late September, 2009, we observed the Earth from a direction looking 'up' at the South Pole.
A third EPOCh planetary observation has been added for late October 2009, when we will observe the planet Mars. This will provide us with data describing a terrestrial planet that is different from the Earth: different atmospheric chemistry, different surface rocks exposed, no surface plant life, and nearly (but not quite!) perfectly dry.
Interplanetary Internet Test - The first test of the deep space communication network, or Interplanetary Internet, was successfully tested from mid--October to mid-November 2008. During the test the DI flyby spacecraft transmitted dozens of images to and from Earth, which was 32 million kilometers away at the time.
- Earth Flybys - In mid-2008, TCM 12 was performed putting the spacecraft on an alternate (better!) trajectory to Hartley 2. The revised trajectory modified the remaining flybys and added two. The second was on 29 Dec 2008. A more distant flyby occurred 29 Jun 2009 and a second distant flyby is scheduled 28 Dec 2009. The final Earth Flyby occurs 27 June 2010. Each flyby may include a pair of Trajectory Correction Maneuvers.
- 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 to obtain 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 which begins in September 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 on November 4, 2010, a 21-day period for look back 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 - Meanwhile, back on Earth, raw data will continue to be converted into products suitable for analysis and archiving.