The Deep Impact spacecraft is en route to comet Hartley 2. The spacecraft must adjust its trajectory many times in order to intersect the comet's orbit. TCM 9, in September 2007, put the spacecraft on a trajectory to the comet. In mid-2008, trajectory planners used TCM 12 to put the spacecraft on a revised trajectory. Both paths had the spacecraft flying past Earth, but the revised trajectory modifies the details. The spacecraft will make a total of three Earth flybys (Dec '07, Dec '08, Jun '10.), two distant Earth flybys (Jun '09, Dec '09), and make three scientific Earth observations during its journey.
During near-Earth flybys, EPOXI scientists observe the Moon with the High Resolution instrument (HRI) and the Medium Resolution instrument (MRI) for calibration purposes. To make the calibrations, scientists compare the amount, or intensity, of light in the measurements of the Lunar surface and compare it with the expected intensity calculated from the well known reflectivity of the Moon. The following is brief overview of each of the Earth flybys and observations.
- Earth Flyby #1: (Dec 29, 2007)
Calibration data using the Moon as a target was obtained by the High Resolution Visible CCD (HRIV), High Resolution IR spectrometer (HRII), and the MRI. Additional observations were obtained on January 16, 2008. A scattered light test of the HRIV telescope system, in which the instrument obtained images with the Moon just outside its field of view, was performed on December 17, 2007. The spacecraft was closest to Earth on December 31, 2007 at a distance of 15,567.63 km above eastern Asia. Because of the close approach, several observers were able to image the spacecraft!
- Earth Flyby #2: (Dec 29, 2008)
The spacecraft was closest to the Earth on December 29th at a distance of 43,450 km above New Zealand and Tierra Del Fuego, Chile. On January 26, 2009 the HRII scanned the Moon at various speeds and images were taken with the HRIV. These observations were made to calibrate the IR spectrometer.
- Earth Flyby #3: (June 27, 2010)
The third and final Earth flyby is scheduled to occur on June 27, 2010. The spacecraft will be at a distance of 36,900 km from the Earth during the flyby.
- Distant Earth Flybys: (June 29 and Dec 28, 2009)
The first distant Earth flyby occurred on June 29th of 2009 and the second is scheduled for December 28, 2009. No observations are scheduled for the flybys.
- Earth Observations: (March 19, May 29, and June 5, 2008)
The DI spacecraft observed the Earth with the HRIV and HRII on three different days. The observations were made at various intervals for a full 24-hour period in order to observe a full rotation of the Earth. The data was obtained for the purpose of characterizing Earth as an extrasolar planet. This information will give astronomers a basis to compare future extrasolar planet observations once technology advances so they may obtain similar data of other solar systems.