A major step in the plan for a manned landing on Mars was realized on October 24, 1998, when NASA's Jet Propulsion Laboratory successfully launched Deep Space 1, a technology-validation test vehicle with the primary goal of confirming and refining first-ever, operational ion-propulsion technology.
The spacecraft's experimental, concentrating solar-power array was deployed, and 36 years after NASA established its first electronic-propulsion research program, the first operational ion-propulsion spacecraft engine system came to life at 2:53 p.m. P.S.T., Tuesday, November 24, and following an initial shut-down problem common to this type of engine, it continued running smoothly. After running overnight in 500-watt mode, engineers stepped-up the engine to 1300-watt operation and left the engine running over the Thanksgiving weekend. At full power, the engine expends 2500 watts, generating thrust equivalent to the weight of a sheet of paper at sea level.
The engine was later restarted, and on Tuesday, January 5, 1999, the autonomous navigation system turned off the ion engine, completing the first part of the Deep Space 1 mission thrust profile, during which time the engine logged more than 850 hours and 59 short-duration shutdowns, which are recycle operations done to prevent damage from particles it encounters.
The new solar array technology, combined with the battery and new power electronics system are critical components of the ion propulsion, which depends upon them for a reliable flow of about 2,000 watts of electricity for engine power.
Ion propulsion, ten times more fuel-to-thrust efficient than chemical-based propulsion systems, expels stripped xenon electrons at 70,000 mph, as compared with chemical-engine thrust velocities of about 10,000 mph. The spacecraft will "burn" 99 pounds of xenon gas during the mission, which will slowly accelerate the craft more than 4,000 mph above the orbital velocity achieved by DS-1's Delta II launch vehicle. Ion propulsion will provide major weight, speed and cost advantages in the cruise phase of future Mars landing and other planetary missions, and to station-keeping satellites in geosynchronous Earth orbit.
The DS-1 test of the new autonomous optical navigation system has worked flawlessly, allowing the spacecraft to determine its position, adjust attitude to control trajectory and operate the propulsion system. The integrated camera and imaging spectrometer was utilized to join with the Saturn-bound Cassini spacecraft in making observations of the solar wind, and a new, light-weight transmitter to link with the Deep Space Communication Network at a new, higher frequency was tested and is being used to align and fine-tune Deep Space instruments for reliable operation at the new frequency (Ka-band) which, at four-times higher than the current band, carries more data with less power consumption from the spacecraft. As of May 1, seven of DS-1's payload of 12 experimental/confirmation technologies had accomplished 100% of their basic testing procedures with the others on schedule and having completed 75% of the experiments.
On January 12, 1999 the spacecraft was more than 10 million miles from Earth and receding from us at more than 1.1 miles per second. On March 15, 1999, DS-1 completed a more than two-month coasting phase when its ion propulsion system was reactivated, gradually propelling the spacecraft to greater speed until April 27 when it was shut down.
The six-week burn accelerated DS-1 by more than 650 miles per hour, and on Wednesday, July 28, 1999, at 9:46 p.m. Pacific time, DS-1, the first mission under NASA's New Millennium Program, ended what scientists call its "amazingly successful mission" by completing its planned close fly-by of asteroid 9969 Braille. The maneuver, which occurred 117 million miles from Earth and brought the spacecraft within 16 miles of the asteroid, was executed by the experimental on-board autopilot system, validating the autopilot's technology and marking the successful completion of testing for all 12 of the spacecraft's new technology systems.
In the early morning before completing the asteroid fly-by, DS-1 experienced what is called a "safing" event, lasting about six hours, during which time a small software glitch was detected by DS-1's fault-detection software, triggering the execution of a program of protective measures: the spacecraft halted non-critical activity, oriented its solar panels toward the Sun, pointed its light- and heat-sensitive instruments away from the Sun and switched to its low-gain antenna while awaiting commands from mission controllers. The mission team diagnosed the software glitch, recovered the spacecraft from the safing event and executed a trajectory correction maneuver to fine tune DS-1's 35,000 mph approach to the asteroid.
The advantages of ion propulsion in the burn which concluded April 27 were dramatically demonstrated by the comparative fuel/thrust calculations. While the six-week, continuous, ion thrusting consumed less than 11 pounds of DS-1's xenon fuel, had the same amount of conventional, chemical propellant been burned instead, DS-1's 650-mile-per-hour acceleration would have been reduced to about 50 miles per hour.
The mission team had been working to solve continuing problems with DS-1's star tracker, which had still not recovered from its last safe-mode fault procedure, and they had been practicing with the aiming and exposure of DS-1's combination visible camera and imaging spectrometer (MICAS) in preparation for its extended-mission close approach to two comets of extremes in 2001: Comet Wilson-Harrington, a dormant comet, in January; and in September, Comet Borrelly, one of the solar system's most active. The spacecraft had been put on an initial trajectory for the intercepts, and NASA had approved the extended mission.
In mid December, 1999, engineers brought the spacecraft out of safe standby to practice orientation procedures without the onboard star tracker, which continued to malfunction, and then returned it to the standby mode, in which it remained until engineers were ready to proceed with setting the groundwork for a work-around to the failed star tracker. On January 14, 2000, they successfully determined the rotational velocity of the craft and executed the mathematics and commands required to point the spacecraft antenna at Earth for an extended period, which is significant because it made it possible to upload new software to control the craft without the star tracker, once the writing and testing of it had been completed. On June 8, 2000, the new software was uploaded to the spacecraft and configured successfully. Comprehensive testing of the software and spacecraft functions, which now allow the craft's camera to serve as a star tracker, was begun, and on June 12, the craft succeeded in locating and locking onto primary stars on the first try, which combined with its ability to locate the sun, provides the craft with two reference points and the ability to navigate itself again, without the star tracker. This accomplishment is significant and involved ground simulations and patient waiting for analysis and confirmation of each spacecraft system test and response.
The next testing phase, now successfully completed, was to determine if the spacecraft could maintain constant tracking while under ion-propulsion thrust. On June 28, 2000, the engine was powered to full thrust and after one week, engineers shut it down and ascertained the accuracy of the craft's tracking during the powered dequence. JPL's goal to resume long-term ion thrusting in July to set a course for a September 2001 flyby of comet Borrelly has now been met. The extended mission to flyby comet Wilson-Harrington has been scrubbed because of the time lost in developing and implementing the star tracker work-around. The new tracking software development and testing, called "...one of the most challenging yet one of the most successful and impressive robotic space rescues ever accomplished," has allowed NASA to reach its goal of a July restart of the engine on the extended mission. DS-1 will be under continuous ion thrust for many months to make the September 2001 interception of Borrelly. The extended mission goals are to obtain photos of the nucleus area, obtain charged particle measurments and infrared spectral data to determine the composition.
Deep Space 1 is now more than twice as far from Earth as is the Sun, more than 315 million kilometers (195 million miles) from Earth, so far away that the time to complete two-way radio communication, that is, to send a command and receive an acknowlegement, is more than half an hour. Almost everyone reading this has had to work with technical support on the phone to solve a computer or software problem. To appreciate what JPL has accomplished so far in working around the star tracker, imagine that you had to wait 15 minutes for the tech-support rep to hear your problem, and then another 15 minutes to get his reply, and so on for each and every step, and then imagine that the tech support rep is a robot that only speaks digital code.
- The number of spacecraft ion-propulsion-engine recycles (59 short-duration shutdowns) has shown that the environment of open space is more benign than a vacuum chamber, where a ground-test engine that operates in conjunction with the spacecraft engine has experienced more than four times as many shutdowns as the actual spacecraft engine.
- DS-1 pictures fell far short of expectations, but the spacecraft's infrared camera has returned valuable data which has determined the asteroid is composed of basalt and pyroxene. No visible-light pictures worth viewing were obtained, but by looking through the finger-print similarity of DS-1's imaging infrared spectrometer result, an idea of Braille's general appearance may be ascertained.
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