Deep Impact Impactor (DII) Vehicle

Description

The Deep Impact Impactor (DII) vehicle was designed to collide with Comet 9P/Tempel 1 at over 10 km/s, producing a crater and ejecting material which could be observed by the Deep Impact flyby bus. 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. 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. This project was selected as a Discovery class mission in July, 1999.

Spacecraft and Subsystems

The DII 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. Communication with the flyby spacecraft is via S-band. The impactor was mechanically and electrically connected to the flyby spacecraft until 24 hours prior to encounter. After separation it ran on internal battery power.

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 rendezvoused 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. The comet and spacecraft were about 0.89 AU from Earth and 1.5 AU from the Sun during the encounter. Selected impactor images and flyby images and spectra were returned to Earth in real time during the encounter. Primary data return will be over the first day after encounter, with a 28 day supplemental data return period. Earth-based observatories also studied the impact. The total budget for the mission was $240 million.

For information on the Deep Impact Flyby Bus, see:

http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=2005-001A