After nine years, NASA's exoplanet-hunting Kepler mission came to a final end Thursday as the space agency sent the radio commands ordering the onboard computer to shut down the unmanned space telescope's systems. The "goodnight" commands were sent via the Deep Space Network from Kepler's operations center at the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder, and severed all communications with Earth.

Shutting down a spacecraft as complex as Kepler takes more than just sending an "off" command. According to NASA, the space telescope has a series of safety modes designed to turn it back on if it accidentally shuts down or loses communications with Earth, so each of these have to be disabled. Worse, the craft is slowly spinning, so commands sent to it must be timed for when the fixed radio antenna is pointed at the Earth.

The most important of these commands is to shut down Kepler's radio transmitters. Though it's in a safe orbit about 94 million mi (151 million km) from the Earth, it still poses a hazard to navigation – not in the sense that it could collide with another spacecraft, but because its radio beam could accidentally blind another probe or even the highly sensitive ground antennae of the Deep Space Network.

To make sure that Kepler has shut down, NASA says that it will continue to monitor the spacecraft to make certain that it has obeyed the commands.

Launched on March 6, 2009 at 7:49 pm PST, atop a United Launch Alliance Delta II rocket from Space Launch Complex 17B at Cape Canaveral Air Force station in Florida, the Kepler mission was originally slated to last only 3.5 years but was extended many times. During its career, it is responsible for confirming the existence of 2,723 planets orbiting other stars and has detected thousands of other potential candidates.

The decision to shut down Kepler was announced on October 30 when it was determined that the craft no longer had enough propellant aboard to operate its attitude thrusters. NASA says that, by coincidence, the shutdown commands were sent on the 388th anniversary of the death of its namesake, German astronomer Johannes Kepler.

The video below reflects on Kepler's career.

Source: NASA