Very few experiments have changed the way we perceive our Universe, but the Kepler exoplanet survey telescope is one such example. Simply by monitoring a single patch of the sky continuously, it provided a new understanding of how many planets exist in the galaxy. Since its launch in 2009, Kepler identified 115 exoplanets with over 2,700 other potential planet candidates—including a number that are comparable in size to Earth or orbiting within the habitable zone where liquid water might exist.

However, Kepler is an orbiting telescope, unreachable by spacecraft for repairs. Today, NASA announced that a reaction wheel—required to keep the telescope pointed steadily in one direction—ceased functioning. This is the second reaction wheel failure, meaning Kepler can't continue to monitor the same stars and their exoplanets it has watched since 2009.

The Kepler engineering team had been anticipating this problem for some months, so this news was not unexpected. NASA associate administrator John Grunsfeld mentioned hope that engineers could restore communication with the control system managing the reaction wheel, but the fact that this state of failure has persisted for some days indicates how faint the hope is.

The telescope can continue to operate in "drift mode," where different portions of the sky could come into view, but it's uncertain how much science could be done in that configuration. Kepler has amassed a great deal of data still to be analyzed, though, so we'll continue to see new papers and announcements from its archives. Kepler will supply our exoplanet needs for some time to come.

Grunsfeld called the mission a resounding success, and few would disagree. Whether engineers get lucky and manage to restart the reaction wheel, Kepler stands as one of the most successful and inspiring of all astronomical endeavors. Kepler's successor, the Transiting Exoplanet Survey Satellite (TESS), is scheduled for a 2017 launch.