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DART Seeks its Target


As far as technology goes NASA's DART spacecraft is about as sophisticated as they come. Its mission is designed to test technologies needed for a spacecraft to locate, communicate with and maneuver close to other craft in space, completely without human intervention.

NASA has recognized the need for autonomous rendezvous capability for some time. In the late 1980s and early 1990s NASA's Marshall Space Flight Center (MFSC) in Huntsville, Ala. established the Automated Rendezvous and Capture Program to demonstrate automated capabilities for autonomous rendezvous hardware and software that could rendezvous and dock with a target vehicle.

The MSFC-developed Video Guidance Sensor flew successfully onboard two Space Shuttle missions (STS 87 and STS 95) as experiments and led to the current work on the DART project.

NASA and Orbital Sciences Corp., Dulles, Va., worked together to develop the new generation sensor, the Advanced Video Guidance Sensor (AVGS). This will be the primary sensor for the close-proximity operation in the DART mission.

The DART technologies took several years of research to complete -- progressing from a concept tested in a laboratory, to the actual development of a prototype system.

Artistic concept of DART and MUBLCOM Image Left: MUBLCOM, an experimental communications satellite, was outfitted with optical retro-reflectors designed for future use with a video guidance system. The new AVGS sensor uses laser signals returned from retro-reflectors on MUBLCOM to calculate measurements. Image credit: NASA

The mission evolved by integrating the AVGS and Autonomous Rendezvous and Proximity Operations (ARPO) algorithms into the fourth stage of a Pegasus launch vehicle. The fourth stage of the Pegasus is an integral part of the DART spacecraft, sharing avionics and propulsion components while in orbit.

The new-generation AVGS incorporates advanced optics and electronics allowing DART to communicate with and track a target the MUBLCOM or MUltiple Paths, Beyond-Line-of-Site COMmunications, built by Orbital Sciences and launched in May 1999.

Software on DART will test and execute collision avoidance maneuvers in addition to navigating around the satellite. Navigation at distances greater than several hundred meters is provided by a Global Positioning System.

The spacecraft, which measures 6 feet long and 3 feet in diameter, is mated to a Pegasus vehicle that is air-launched from under an Orbital Sciences Stargazer L-1011 aircraft at about 40,000 feet altitude.

The Pegasus fourth stage includes the avionics assembly and the Hydrazine Auxiliary Propulsion System (HAPS). The HAPS assembly also includes a set of six nitrogen thrusters, providing three-axis attitude control during orbital drifts and roll control during HAPS burns.

About 10 minutes after launch, the Pegasus will deliver the DART vehicle to a polar orbit that is designed to match that of the MUBLCOM satellite. With the Pegasus launch mission complete the DART space mission begins.

DART, which weighs only 800 pounds including fuel, will be controlled using a combination of three propulsion subsystems including 16 nitrogen-fueled thrusters for proximity operations.

The maneuver and rendezvous phase of the mission begins when the attitude control system transitions from the Pegasus Attitude Control Subsystem (ACS), to the 16-thruster DART Reaction Control System (RCS). After the second approach, DART will fly away from MUBLCOM and the mission will be successfully completed.

Although NASA has performed several rendezvous and docking missions, all have been piloted by astronauts. The objective of the 24-hour DART mission is to demonstrate, in space, the ability of computers and cameras to determine the spacecraft's functions and direction.

Mission operations will be conducted from the DART Mission Operations Center located at Vandenberg Air Force Base, Calif. and spacecraft communications will be operated through NASA's Space Network (TDRSS).

NASA's DART mission is the next step in advancing the Vision for Space Exploration for safe and reliable autonomous rendezvous capabilities for the U.S. Space Program. The use of this technology and others to be developed in the future will assist with our mission to return astronauts to the lunar surface and go beyond.

For further information, visit:
Marshall Space Flight Center's DART site

Elaine M. Marconi
NASA's John F. Kennedy Space Center and Marshall Space Flight Center

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