Completion of Primary Objectives
During the course of a mission, there may be inadvertent losses of data. In the case of an orbiter mission, it might be possible to recover the losses by repeating observations of areas where the loss was sustained. Such data recovery might require additional time be added to the portion of a mission during which its primary objectives are being achieved. However, major data losses and their recovery are usually planned for during mission design. One predictable data loss occurs during superior conjunction, when the sun interferes with spacecraft communications for a number of days. In the image above, missing Magellan radar data appears as swaths from pole to pole (arrows) which have been filled in with lower-resolution data from the Pioneer 12 mission. Magellan later recovered the missing high-resolution data.
Additional Science Data
Once a spacecraft has completed its primary objectives, it may still be in a healthy and operable state. Since it has already undergone all the efforts involved in conception, design and construction, launch, cruise and perhaps orbit insertion, it can be very economical to redirect an existing spacecraft toward accomplishing new objectives and to retrieve data over and above the initially planned objectives. This has been the case with several JPL spacecraft. It is common for a flight project to have goals in mind for extended missions to take advantage of a still-viable spacecraft in a unique location when the original funding expires.
Voyager was originally approved as a mission only to Jupiter and Saturn. But Voyager 2's original trajectory was selected with the hope that the spacecraft might be healthy after a successful Saturn flyby, and that it could take advantage of that good fortune. After Voyager 1 was successful in achieving its objective of reconnaissance of the Saturnian system, including a tricky solar occultation of Titan and associated observations, Voyager 2 was not required to be used solely as a backup spacecraft to duplicate these experiments. Voyager 2's trajectory to Uranus and Neptune was therefore preserved and successfully executed. Approval of additional funding enabled making some necessary modifications, both in the ground data system and in the spacecraft's onboard flight software, to continue on to encounter and observe the Uranus and Neptune systems.
By the time Voyager 2 reached Uranus after a five-year cruise from Saturn, it had many new capabilities, such as increased three-axis stability, extended imaging exposure modes, image motion compensation, data compression, and new error-correction coding.
In 1993, after 15 years of flight, Voyagers 1 and 2 both observed the first direct evidence of the long-sought-after heliopause. They identified a low frequency signature of solar flare material interacting with the heliopause at an estimated distance of 40 to 70 AU ahead of Voyager 1's location, which was 52 AU from the sun at the time.
After fulfilling its goal of mapping at least 70% of the surface of Venus, the Magellan mission went on with more than one mission extensions, eventually to accomplish special stereo imaging tests, and interferometric observation tests. Mapping coverage reached over 99% of the surface.
Rather than abandon the spacecraft in orbit, the Magellan Project applied funding which had been saved up over the course of the primary mission to begin an adventurous transition experiment, pioneering the use of aerobraking to attain a nearly circular Venusian orbit, and a low-latitude gravity survey was completed. All of these accomplishments far exceeded the mission's original objectives.
Orbiting Relay Operations
As mentioned in the last chapter, some Mars orbiting spacecraft are equipped with radio relay capability intended to receive uplink from surface or airborne craft. Typically such relay equipment operates at UHF frequencies.
In order to serve as a relay, at least some of the orbiter's own science data gathering activities have to be reduced or interrupted while its data handling and storage subsystems process the relay data. This may or may not present an undesirable impact to the mission's ability to meet its primary objectives. Relay service, then, is a good candidate for extended mission operations. Since relay service entails neither keeping optical instruments pointed, nor flying a precise ground track, the demand on the attitude control and propulsion systems is minimal, and a little propellant can go a long way. The demand on other subsystems, such as electrical supply, can also be reduced in the absence of other science instruments to operate.
The black and white image above shows the nadir-facing deck of the Mars Global Surveyor spacecraft currently orbiting Mars. The MR (Mars relay) antenna is visible on the right.
End of Mission
Resources give out eventually. Due to the age of their RTGs in 2000, the Pioneer 10 and 11 spacecraft, plying the solar system's outer reaches, faced the need to turn off electrical heaters for the propellant lines in order to conserve electrical power for continued operation of science instruments. Doing so allowed the propellant to freeze, making it impossible to re-thaw for use in additional spacecraft maneuvers. The spacecraft were still downlinking science data while Earth eventually drifted away from their view, and over the following months contact was lost forever.
Voyagers 1 and 2 continue to make extraordinary use of their extended mission. They are expected to survive until the sunlight they observe is too weak to register on their sun sensors, causing a loss of attitude reference. This is forecast to happen near the year 2015, which may or may not be after they have crossed the heliopause. Electrical energy from their RTGs may fall below a useable level about the same time or shortly thereafter. The spacecraft's supply of hydrazine may become depleted sometime after that, making continued three-axis stabilization impossible.
Pioneer 12 ran out of hydrazine propellant in 1993, and was unable to further resist the slow decay of its orbit about Venus, resulting from friction with the tenuous upper atmosphere. It entered the atmosphere and burned up like a meteor after fourteen years of service.
Components wear out and fail. The Hubble Space Telescope has been fitted with many new components, including new attitude-reference gyroscopes, to replace failed and failing units. Two of Magellan's attitude-reference gyroscopes had failed prior to the start of the transition experiment, but of course no replacement was possible. To date, a JPL mission has not been turned off because of lack of funding.
Once a mission has ended, the flight team personnel are disbanded, and the ground hardware is returned to the loan pool or sent into long-term storage. Sometimes it is possible to donate excess computers to schools. Oversubscribed DSN resources are freed of contention from the terminated mission, and the additional tracking time allocations can be made available to missions currently in their prime.
While layoffs are not uncommon, many personnel from a disbanded flight team are assigned by their JPL section management to new flight projects to take advantage of valuable experience gained. Interim work is often available within the Section itself. Many Viking team members joined the Voyager mission after Viking achieved its success at Mars in the late 1970s. Many of the Voyager flight team members joined the Magellan project after Voyager's last planetary encounter ended in October 1989. Other ex-Voyager people joined the Galileo and Topex/ Poseidon missions. Some ex-Magellan people have worked on Cassini, Mars Global Surveyor, Mars Pathfinder, SIRTF, and Mars Exploration Rover. Mission's end also provides a convenient time for some employees to begin their retirement, and for new employees to be hired and begin building careers in interplanetary exploration.
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1 The Solar System
2 Reference Systems
3 Gravity & Mechanics
5 Planetary Orbits
7 Mission Inception
9 S/C Classification
11 Onboard Systems
12 Science Instruments
17 Extended Operations
18 Deep Space Network