Deep Impact/EPOXI

Description

The goals of the Deep Impact mission were to rendezvous with comet 9P/Tempel 1 and launch a projectile into the comet nucleus. Observations were made of the ejecta, much of which represented pristine material from the interior of the comet, the crater formation process, the resulting crater, and outgassing from the nucleus, particularly the newly exposed surface. The scientific objectives of the mission are to: improve the knowledge of the physical characteristics of cometary nuclei and directly assess the interior of cometary nucleus; determine properties of the surface layers such as density, strength, porosity, and composition from the crater and its formation; study the relationship between the surface layers of a cometary nucleus and the possibly pristine materials of the interior by comparison of the interior of the crater with the surface before impact; and improve our understanding of the evolution of cometary nuclei, particularly their approach to dormancy, by comparing the interior and the surface. This project was selected as a Discovery class mission in July, 1999. After the primary mission, Deep Impact was selected for a two-part extended mission designated EPOXI.

Spacecraft and Subsystems

The spacecraft consists of a 370 kg cylindrical copper impactor attached to a 650 kg flyby bus. The spacecraft is a box-shaped honeycomb aluminum framework with a flat rectangular Whipple debris shield mounted on one side to protect components during comet close approach. Body mounted on the framework are one high- and one medium-resolution instrument, each of which consists of an imaging camera and an infrared spectrometer which will be used to observe the ejected ice and dust, much of which will be exposed to space for the first time in over 4 billion years. The medium resolution camera has a field of view (FOV) of 0.587 degrees and a resolution of 7 m/pixel at 700 km distance and is used for navigation and context images. The high resolution camera has a FOV of 0.118 degrees and a resolution of 1.4 m/pixel at 700 km. The infrared spectrometers cover the range from 1.05 to 4.8 micrometers with FOV of 0.29 degrees (hi-res) and 1.45 degrees (lo-res). The total flyby bus instrument payload has a mass of 90 kg and will use an average of 92 W during encounter.

The flyby spacecraft measures approximately 3.2 m x 1.7 m x 2.3 m, is three-axis stabilized and uses a blowdown hydrazine primary propulsion system with 5000 N-s RCS total impulse providing a total delta-V of 190 m/s. Communications with the ground from the flyby bus are via X-band (8.000 MHz) through a 1 meter diameter parabolic dish antenna mounted on a 2-axis gimbal or through a fixed low-gain antenna. Communication between the impactor and flyby spacecraft is in S-band. The uplink data rate will be 125 bps, downlink will be at 175 kbps. Power of 620 W at the encounter is provided by a 7.2 square meter solar array and stored in a small NiH2 battery. The spacecraft control system consists of four hemispherical resonator gyros, two star trackers, reaction wheels, and hydrazine thrusters. Pointing accuracy is 200 microradians with 65 microradian knowledge. Thermal control is achieved by insulating blankets, surface radiators, finishes, and heaters. The spacecraft has two redundant RAD750 computers with 309 MB each of memory for scientific data.

The Impactor

The impactor projectile is made of primarily copper (49%) and only 24% aluminum so it will be easily identifiable and minimize contamination in the spectra after the projectile is largely vaporized and mixed in with the comet ejecta on impact. The impactor is a short hexagonal cylinder built above the copper cratering mass. It has a small hydrazine propulsion system for targeting which can provide delta-V of 25 m/s. Targeting is accomplished using a high-precision star-tracker, auto-navigation algorithms, and the Impactor Targeting Sensor (ITS), a camera which provides images for autonomous control and targeting. The ITS will operate until impact, and images will be sent back to Earth via the flyby spacecraft. Damage to the instrument due to dust in the coma may make imaging impossible duing the last minute or so before impact. Communication with the flyby spacecraft is via S-band. The impactor is mechanically and electrically connected to the flyby spacecraft until 24 hours prior to encounter. After separation it runs on internal battery power.

For the Deep Impact Impactor (DII) spacecraft record, see:

http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=2005-001D

Mission Profile

Deep Impact launched on 12 January 2005 at 18:47:08.574 UT (1:47:08 p.m. EST) on a Delta II. The spacecraft transferred into a heliocentric orbit and will rendezvous with comet P/Tempel 1 in July 2005. Deep Impact was 880,000 km from the comet on 3 July 2005 moving at a velocity of 10.2 km/s relative to the comet. The projectile was released at this point and shortly after release the flyby spacecraft executed a maneuver to slow down relative to the impactor by 120 m/s and divert by 6 m/s. On 4 July the impactor struck the sunlit side of the comet nucleus 24 hours after release, at 5:52 UT (1:52 a.m. EDT). At 10.2 km/s velocity, the impactor had an impact energy of about 19 gigajoules, and hit at an oblique angle of approximately 25 degrees. Material from the nucleus was ejected into space and the impactor and much of the ejecta was vaporized.

The flyby spacecraft was approximately 10,000 km away at the time of impact and began imaging 60 seconds before impact. At 600 seconds after impact the spacecraft was about 4000 km from the nucleus and observations of the crater began and continued up to a range of about 700 km, about 50 seconds before closest approach. At this point (about 961 seconds after impact) imaging ended as the spacecraft reoriented itself by 45 degrees to optimize protection from dust damage as it flew by the nucleus. Closest approach to the nucleus was at a distance of about 500 km. At 1270 seconds the crossing of the inner coma was complete and the spacecraft oriented itself to look back at the comet and begin imaging again. At 3000 seconds the spacecraft began playback of data to Earth at 20 to 200 kilobits per second. The comet and spacecraft were about 0.89 AU from Earth and 1.5 AU from the Sun during the encounter. Real time return of selected impactor images and flyby images and spectra were returned to Earth during the encounter. Primary data return took place over the first day after encounter, with a 28 day supplemental data return period. Earth-based observatories also studied the impact. The spacecraft ranged over a distance of 0.93 to 1.56 AU from the Sun during the primary mission. End of primary mission was August 2005. The total budget for the primary mission was $240 million.

Deep Impact has been funded for an extended mission which has two parts. The Deep Impact Extended Investigation (DIXI) involves flying by comet 103P/Hartley 2 on 11 October 2010 at a closest approach of approximately 1000 km. (The original target was comet Boethin on 5 December 2008) and return images and data. The Extrasolar Planet Observation and Characterization (EPOCh) investigation will use the Deep Impact imaging system to observe nearby bright stars with known large planets to characterize the planets as they orbit behind and in front of the stars and to search other planets in those systems. This phase of the mission begins 26 January 2008 and continues until the end of July. The spacecraft made an Earth flyby on 31 December 2007 to target itself for Hartley 2. It will make two more Earth flybys before the encounter with Hartley 2. The two extended missions together are known as EPOXI. Total cost of the extended mission is $40 million.

Comet Tempel 1

Comet 9P/Tempel 1 is a periodic comet which orbits the Sun every 5.51 years. It has a semi-major axis of 3.12 astronomical units (AU, the distance from the Sun to the Earth) and a perihelion distance of 1.5 AU, which puts it between the orbits of Mars and Jupiter. Its orbit is inclined 10.5 degrees to the ecliptic. The orbit has changed in the past but the perihelion has been within 10 AU for at least 300,000 years. The nucleus is estimated to be roughly 14 km long and 4 km wide. The perihelion occurs on 5 July 2005, the day after the encounter. The comet was discovered on 3 April 1867 by Ernst Wilhelm Leberecht Tempel and was first recognized to be periodic in May of that year by C. Bruhns.