The end of a successful mission is an occasion to congratulate those who made it happen, along with the scientists who get to reap and make sense of the rich data harvest. In this case two such missions have reached or are close to the end: the Herschel infrared telescope built by the European Space Agency (ESA) and NASA’s Kepler planet-hunting spacecraft.

During its nearly four-year mission, Herschel returned stunning infrared images of some of the most distant and coldest objects in the universe, giving scientists new insights into the formation of stars, galaxies and even planetary systems. To date, some 600 peer-reviewed scientific papers based on Hershel observations have been published, with many more to come. 

Hershel was also a technical triumph, the largest and most powerful infrared telescope flown to date. After more than 35,000 observations, the mission’s end came April 29 when the observatory’s 2,300 liters of liquid helium coolant, which kept its sensors cold enough to observe previously unseen regions of the universe, finally ran out — as expected. 

In paying tribute to Herschel, it is worth noting the success of its companion mission, the Planck space telescope, which launched with Herschel in 2009. Designed to study remnant microwave radiation from the Big Bang, Planck continues to operate, although one of its two main sensors ceased operating in early 2012 when its coolant ran out — again, as expected.

Meanwhile, the end appears to be at hand for NASA’s planet-hunting Kepler Space Telescope, which recently lost the use of the second of four reaction wheel assemblies used to point its camera. The satellite needs three reaction wheels to operate, and while NASA continues to examine options for restoring one of those to operation, the outlook is not promising.

Launched in March 2009 on a three-and-a-half-year mission, Kepler was designed to find so-called exoplanets by observing the brightness dips they cause when passing in front of their host stars. The spacecraft in November began what was supposed to be a four-year mission extension, approved by NASA the previous April.

While the extended mission obviously will be cut well short — this is without question disappointing; the spacecraft was designed to operate for six years — there can be little disputing Kepler’s accomplishments. At last count, the spacecraft had detected more than 2,700 exoplanets, including several Earth-like bodies in what scientists call the habitable zone of their host stars — a distance from which conditions could be right to support life. Only a small number of the planets discovered by Kepler have been confirmed by other observations, but scientists are confident that up to 90 percent of them ultimately will be confirmed. 

Things didn’t always go smoothly on Herschel and Kepler. Like many, if not most, ground-breaking scientific missions, both experienced significant cost growth and delays during development. As is all too typical, their technical complexity was underestimated, and in the case of Herschel, there was the added challenge of launching with Planck as a dual payload.

Similarly, both experienced on-orbit difficulties that temporarily halted science data gathering.

In the end, however, both Herschel and Kepler delivered in spectacular fashion. These are the kinds of missions that not only enlighten but also inspire, while serving as testimonials to scientific creativity, technical know-how and perseverance. Successes like these restore the confidence of those who oversee government space agency spending, reminding them that it’s worth investing in ambitious missions even though many will go well over budget, and some will fail. This is a big part of the reason that NASA’s James Webb Space Telescope astronomy flagship, for example, continues to be funded despite its well-known difficulties.

The teams involved the Herschel, Planck and Kepler missions are to be heartily commended for a job well done, often in the face of adversity. May many, if not most, of them stick around for the next big thing.