A mission's primary experimental objectives are spelled out well in advance of the spacecraft's launch. The efforts of all of the flight team members are concentrated during the life of the mission toward achieving those objectives. A measure of a mission's success is whether it has gathered enough data to complete or exceed its originally stated objectives. During the course of a mission, there may be inadvertent losses of data. In the case of an orbiter mission, the lost data can be recovered by making repeated observations of the areas of a planet where the loss was sustained when the planet rotates until the spacecraft's orbit coincides once again with areas of the surface which were missed. Such data recovery might add additional time to the portion of a mission when its primary objectives are being achieved. Major outages and their recovery may be planned for during the course of a mission, as in the case of the planet approaching superior conjunction, when the sun obstructs communications with the spacecraft for a number of days.
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 operate an existing spacecraft toward accomplishing new objectives, and 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 to Jupiter and Saturn. Voyager 2's original trajectory had been 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 preserved and executed. Approval of additional funding enabled making modifications which were necessary, both in the GDS and in the onboard flight software to continue on to Uranus and Neptune.
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 discovered 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.
The Magellan mission accomplished special stereo imaging tests, and interferometric observation tests after fulfilling its goal of mapping at least 70% of the surface. Once mapping had tallied 98%, and the low-latitude gravity survey was completed, all of its original objectives had been met and exceeded. Rather than abandon the spacecraft in orbit, the Project applied funding which had been saved over the course of the primary mission to begin the adventurous Transition Experiment, pioneering the use of aerobraking to attain a nearly circular orbit.
Resources give out. Due to the age of their RTGs, the Pioneers 10 and 11 spacecraft are facing a need in the near future to turn off electrical heaters for the propellant lines in order to conserve electrical power for continued operation of science instruments. Doing so would allow propellant to freeze, possibly making it impossible to re-thaw for use in additional spacecraft maneuvers. This will prevent them from being kept on Earth-point.
Voyagers 1 and 2 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 the spacecraft's supply of hydrazine may become depleted near the same time, 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 resulting from friction with the tenuous upper atmosphere of Venus. It entered the atmosphere and burned up like a meteor after fourteen years of service.
Components wear out and fail. The Hubble Space Telescope was fitted with many new components, including new attitude-reference gyroscopes, to replace failed and failing units in late 1993. 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. But this might not continue to be the case in the future.
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. DSN resources are freed of contention from the mission, and the additional tracking time allocations may be available to missions currently in their prime.
While layoffs are not uncommon, many personnel from a disbanded flight team are assigned by their section management to new flight projects or other interim work. Many Viking team members joined the Voyager mission after Viking had 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. Many other ex-Voyager people joined the Galileo and Topex/ Poseidon missions. Some ex-Magellan people are working on Cassini, Mars Global Surveyor, and Mars Pathfinder. Mission's end also provides a convenient time for some employees to begin their retirement.
Recap