The New Horizons spacecraft, which will fly past Pluto on July 14th, went into safe mode on Saturday. This happens occasionally with most spacecraft. It is like a reboot on a computer. Something unexpected happened, for example a memory bit flipping its value due to a cosmic ray hitting a RAM chip, and the control system is programmed to shut everything down and run only the most essential components including the communications gear.
The ground system team then re-establishes contact with the spacecraft and downloads data regarding why the system went into safe mode. After diagnosing what happened, the team will carefully restart one subsystem after another and will try to either fix or route around any component, e.g. a bad section of memory, that has a problem that could result in another safe mode incident.
Here is a statement on Saturday from the New Horizons team about the incident:
The New Horizons spacecraft experienced an anomaly this afternoon that led to a loss of communication with Earth. Communication has since been reestablished and the spacecraft is healthy.
The mission operations center at the Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, lost contact with the unmanned spacecraft — now 10 days from arrival at Pluto — at 1:54 p.m. EDT, and regained communications with New Horizons at 3:15 p.m. EDT, through NASA’s Deep Space Network.
During that time the autonomous autopilot on board the spacecraft recognized a problem and – as it’s programmed to do in such a situation – switched from the main to the backup computer. The autopilot placed the spacecraft in “safe mode,” and commanded the backup computer to reinitiate communication with Earth. New Horizons then began to transmit telemetry to help engineers diagnose the problem.
A New Horizons Anomaly Review Board was convened at 4 p.m. EDT to gather information on the problem and initiate a recovery plan. The team is now working to return New Horizons to its original flight plan. Due to the 9-hour, round trip communication delay that results from operating a spacecraft almost 3 billion miles (4.9 billion kilometers) from Earth, full recovery is expected to take from one to several days; New Horizons will be temporarily unable to collect science data during that time.
Status updates will be issued as new information is available.