A five-month investigation following the mishap found that DART had used up its pressurized nitrogen gas maneuvering fuel before it could complete the rendezvous. The investigation board determined that excessive thruster firings in response to incorrect navigational data caused the spacecraft to run out of thruster fuel during its approach, so it could not avoid the low-velocity collision with MUBLCOM.

"There were a lot of causes," Scott Croomes, who chaired the eight-member board, told reporters during a teleconference at NASA's Marshall Space Flight Center in Huntsville, Ala.

Croomes said the spacecraft's global-positioning receiver suffered from a "factory error," which caused DART to reset its position and speed continually, and thereby discard the real-time GPS data that could have kept it on a precise course for the rendezvous and avoided the collision.

Because of the error, DART's receiver consistently produced a velocity reading that was biased by about 0.6 meters per second from what it should have been. The spacecraft's onboard software could not reconcile the error with the real-time data, and hence kept firing the thrusters and using up its fuel.

The investigation board also found that although the DART team at Orbital Sciences Corp. � the spacecraft's builder - knew about the error, they never attempted to correct it. This proved to be a critical misstep, because the software model that simulated the receiver during preflight testing assumed the receiver measured velocity perfectly, and that assumption was transferred to the spacecraft's software.

Combined with other errors and complications, the miscalibration caused DART to collide with MUBLCOM, the NASA board found � although the collision was minor. DART missed its 6.3 meter target envelope by less than 2 meters.

DART's design did include a collision-avoidance mechanism, but the software was dependent on the same navigational data source as the guidance system, so it was ineffective.

"The reasons for this inadequately-designed logic include the unanticipated potential for navigational errors and a lack of adequate design review," the board concluded in the summary report.

"This almost worked," Croomes told reporters. "Had any one of those causes not been there, (the rendezvous) would have worked."

After its launch, during which DART was carried by a Pegasus XL rocket dropped from a lift aircraft at Vandenberg Air Force Base, Calif., NASA found that the spacecraft performed nominally during the first eight hours of its mission ? including the beginning of the rendezvous phase.

Only when DART began to approach MUBLCOM did ground controllers notice the anomalies in the spacecraft's navigation system. DART ended up nudging MUBLCOM, boosting its orbit by 1.2 nautical miles, but leaving the rendezvous satellite undamaged.

NASA's Exploration Systems Mission Directorate, which funded the project and produced the report and summary, said in a statement that both satellites remain in low-Earth orbits that do not threaten other spacecraft, and eventually they will burn up upon re-entry into Earth's atmosphere.

NASA released only a summary report because the full report contains information protected by U.S. International Traffic in Arms Regulations. That information contains details about the military's navigation and communications technology.

Related Links
DART mishap report
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