When a mission is complete
When does a mission end? Is it when the spacecraft is switched off, when it re-enters Earth's atmosphere or when all the data have been analysed?
It depends on your perspective. For a project manager, responsible for building the satellite, their mission ends at launch. For an operations manager, responsible for controlling the spacecraft during its operational lifetime, their mission ends when the spacecraft is switched off.
For the scientists involved in the mission, however, the mission often continues long after the spacecraft has ceased to collect data, and possibly long after the spacecraft has returned to Earth or disappeared beyond the Solar System.
When a science mission is approved, it is funded for a set period of time. A mission's operation usually lasts two years. In almost all cases, however, the engineers build the satellites with enough fuel and other consumables to allow their mission to be extended if necessary. The decision to extend a mission is taken on a case-by-case basis by the highest levels of management within ESA's Science programme.
If a mission is not extended – because the amount of scientific data to be gained do not warrant the additional cost, or because the satellite breaks down, the observational phase of the mission is brought to an end and scientists switch off the spacecraft.
In the case of ESA's Infrared Space Observatory (ISO), its sensitive detectors ran out of liquid helium, which was used to cool them to an operating temperature during the extended mission.
Although this signalled the end of the main science phase, an operations team then spent a month rigorously testing specific hardware and software systems. They did this to build up a better working knowledge of them, as identical systems were being built into two following ESA missions, XMM-Newton and Integral.
Once the tests were finished, the final commands were sent to the spacecraft to alter its orbit, so that in a decade or two, ISO will burn up in Earth's atmosphere. In this way, it will no longer be part of the space debris that orbits the Earth.
In some ways, scientists consider a mission to be complete when the work continues as normal for the science teams. All of the data collected by the spacecraft go into an archive that astronomers all over the world can access and use to further their own scientific research.
Another aspect of work following the completion of a mission is to either start or continue working on the study of the next, follow-on mission.
Astronomy often generates the next batch of questions at the same time as the previous generation has been answered. During the time a mission has been built and flown, things change. Technological advances usually allow engineers to build more complex spacecraft, with improved capabilities. In this way, families of spacecraft grow. However, each must be approved on its own merits.
For example, ESA is following up the work of its star-mapper, Hipparcos, and massively extending it with its Gaia mission, scheduled for launch around 2009/2010. ESA is also gearing up to build Herschel, the next generation of infrared satellite to follow ISO.
Last update: 28 September 2004