Achievements
The ion propulsion engine initially failed after 4.5 minutes of operation. However, it was later restored to action and performed excellently. Early in the mission, material ejected during launch vehicle separation caused the closely spaced ion extraction grids to short-circuit. The contamination was eventually cleared, as the material was eroded by electrical arcing, sublimed by outgassing, or simply allowed to drift out. This was achieved by repeatedly restarting the engine in an engine repair mode, arcing across trapped material.
It was thought that the ion exhaust might interfere with other spacecraft systems, such as radio communications or the science instruments. The PEPE detectors had a secondary function to monitor such effects from the engine. No interference was found.
Another failure was the loss of the star tracker. The star tracker determines spacecraft orientation by comparing the star field to its internal charts. The mission was saved when the MICAS camera was reprogrammed to substitute for the star tracker. Although MICAS is more sensitive, its field-of-view is an order of magnitude smaller, creating a greater information processing burden. Ironically, the star tracker was an off-the-shelf component, expected to be highly reliable.
Without a working star tracker, ion thrusting was temporarily suspended. The loss of thrust time forced the cancellation of a flyby past Comet Wilson-Harrington.
The Autonav system required occasional manual corrections. Most problems were in identifying objects that were too dim, or were difficult to identify because of brighter objects causing diffraction spikes and reflections in the camera, causing Autonav to misidentify targets.
The Remote Agent system was presented with three simulated failures on the spacecraft and correctly handled each event.
- a failed electronics unit, which Remote Agent fixed by reactivating the unit.
- a failed sensor providing false information, which Remote Agent recognized as unreliable and therefore correctly ignored.
- an attitude control thruster (a small engine for controlling the spacecraft's orientation) stuck in the "off" position, which Remote Agent detected and compensated for by switching to a mode that did not rely on that thruster.
Overall this constituted a successful demonstration of fully autonomous planning, diagnosis, and recovery.
The MICAS instrument was a design success, but the ultraviolet channel failed due to an electrical fault. Later in the mission, after the star tracker failure, MICAS assumed this duty as well. This caused continual interruptions in its scientific use during the remaining mission, including the Comet Borrelly encounter.
The flyby of the asteroid 9969 Braille was only a partial success. Deep Space 1 was intended to perform the flyby at 56,000 km/h (34,797 mph) at only 240 m (787 ft) from the asteroid. Due to technical difficulties, including a software crash shortly before approach, the craft instead passed Braille at a distance of 26 km (16 mi). This, plus Braille's lower albedo, meant that the asteroid was not bright enough for the autonav to focus the camera in the right direction, and the picture shoot was delayed by almost an hour. The resulting pictures were disappointingly indistinct.
However, the flyby of Comet Borrelly was a great success and returned extremely detailed images of the comet's surface. Such images were of higher resolution than the only previous pictures, of Halley's Comet taken by the Giotto spacecraft. The PEPE instrument reported that the comet's fields were offset from the nucleus. This is believed to be due to emission of jets, which were not distributed evenly across the comet's surface.
Despite having no debris shields, the spacecraft survived the comet passage intact. Once again, the sparse comet jets did not appear to point towards the spacecraft. Deep Space 1 then entered its second extended mission phase, focused on retesting the spacecraft's hardware technologies. The focus of this mission phase was on the ion engine systems. The spacecraft eventually ran out of hydrazine fuel for its attitude control thrusters. The highly efficient ion thruster had a sufficient amount of propellant left to perform attitude control in addition to main propulsion, thus allowing the mission to continue.
Read more about this topic: Deep Space 1
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