PASADENA, Calif. Engineers at NASA's Jet Propulsion Laboratory are cobbling together a navigational fix for a pioneering spacecraft that lost its bearings in space last month due to a broken instrument, leaving the probe without a way to orient itself amid the stars.
The work involves writing computer code on Earth and sending it to the Deep Space 1 spacecraft, instructing it to press its science camera into a new role as a navigational instrument.
"Were going to build a star tracker from 150 million miles away," said Marc Rayman, Deep Space 1s chief mission engineer.
The fix should be ready this spring, but the delay means Deep Space 1 will miss one of two cometary flybys JPL had hoped to pull off in 2001.
Deep Space 1 literally lost its bearings on Nov. 11 when its navigational camera stopped working.
Since then, the $152-million spacecraft has remained in a near-constant slumber, pointed toward the sun with most of its instruments switched off.
Protective software aboard Deep Space 1 shut the spacecraft down after making several attempts to fix the spacecrafts star tracker. The navigational device uses a camera to image its surroundings, which it then compares to its on-board catalog of thousands of stars to determine where it is pointed in space.
NASA now assumes the star tracker is damaged beyond repair, possibly due to one or more short circuits aboard the spacecraft.
Stan Dubyn, chief operating officer of Spectrum Astro Inc., the Arizona-based builder of the spacecraft, said there were a handful of possible reasons why the star tracker stopped working. Divining why could take weeks more.
"We're kind of taking our time, because there's no mission criticality," said Dubyn of Deep Space 1, which finished its primary mission in September.
The loss of the star tracker has left mission members at NASAs Jet Propulsion Laboratory with two choices: One would be to write the spacecraft off, leaving it to drift in orbit around the sun. The other is to cobble together a fix that will allow Deep Space 1 to fire up its ion propulsion engine once again and speed off to a possible comet rendezvous in 2001.
Predictably, engineers have scrambled for the latter.
Making do with onboard parts
Although Deep Space 1s primary mission, including the testing of 12 new technologies, has been over for three months, the spacecraft still has plenty of life in it. More than three-quarters of its 180 pounds (82 kilograms) of xenon propellant remain in its tanks.
But without spare parts to work with, the mission team will have to make do with what is already aboard Deep Space 1 if that propellant is to thrust the spacecraft in any kind of meaningful direction.
Rayman said he was eyeing the spacecrafts science camera, pressing it into service in an unscripted navigational role with the aid of new software sometime this spring.
First, however, the spacecraft has to regain knowledge of its attitude, or how it is oriented.
Navigating blind
Over the past two weeks, engineers at JPL have been working on new ways of turning and pointing the spacecraft without the benefit of the star trackers assistance.
Engineers have coaxed the spacecraft out of standby mode and commanded it to both rotate and stop with extreme precision.
By plotting the signal strength of Deep Space 1s low-gain antenna during the rotations, engineers on the ground can infer when the spacecraft is pointed directly at Earth. Since the antenna is mounted off center on Deep Space 1, its signal is naturally strongest when it aimed closest to Earth during each rotation.
Since Deep Space 1 can already find the sun on its own, adding the ability to locate Earth will allow the spacecraft to recalculate its bearings that vital bit of knowledge it lost in early November. From that point which could come as early as the spring the mission can resume its course.
Lost gravy: a comet
The delay will cut into some of the gravy engineers hoped the extended Deep Space 1 mission would deliver, however.
Initial plans called for flybys of the comet Wilson-Harrington in March 2001 and the comet Borrelly in September later that same year.
The spacecraft would have to start thrusting in January to make both encounters, a prospect that is now all but impossible until a navigational fix is made.
"Its unlikely were going to be thrusting in time to get to both extended mission targets," Rayman said. If thrusting can resume by April, Deep Space 1 will be able to make one of its two bonus science dates.
Deep Space 1 was launched Oct. 24, 1998. It is now 153 million miles (246 million kilometers) from Earth.
Deep Space 1 is the first mission under NASAs New Millennium Program, which aims to test new technologies in space. Deep Space 2, the twin martian microprobes lost at Mars in early December, was the programs second mission.
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