New Horizons enters safe mode 10 days before Pluto flyby [UPDATED]

[UPDATE] Normal operations are planned to resume July 7; see the end of this post for more.

Welp. New Horizons decided to put on a little 4th of July drama for the mission's fans. It's currently in safe mode, and it will likely be a day or two before it recovers and returns to science, but it remains on course for the July 14 flyby. Here's the mission update in its entirety.

New Horizons Team Responds to Spacecraft Anomaly

The New Horizons spacecraft experienced an anomaly the afternoon of July 4 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 (ARB) 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.

Okay. This is scary. It's not what the team wanted to be dealing with right now. However, the spacecraft is healthy. There was some brief confusion on Twitter over the phrase "return New Horizons to its original flight plan," but don't fear: the spacecraft is on course. Even if (heaven forbid) the spacecraft never recovered from safe mode, it would still fly past Pluto at the planned distance, speed, and time; no further trajectory correction maneuvers are planned from now until after the flyby. The "original plan" refers to the science plan.

So, if contact was lost at 1:54 p.m. EDT, what are we now missing? That is about 18:00 UT, and those signals left New Horizons about 4.5 hours previously, or 13:30 UT. I don't have detailed information on the science plans being performed now, but I do have a table of optical navigation images that were planned for this period.

The good news: no images were planned at all for July 4. There were a few planned for July 5, some of which will probably be missed. And only one planned for July 6. None of these is critical for navigation; images taken on days before and after will give the mission the information that they need to target future images precisely. Assuming they can get the spacecraft back into normal operating mode relatively fast, the only result of today's safe mode will be an annoying -- but educational -- gap in our approach animations, and a less educational (but not catastrophic) gap in our light curves for Nix and Hydra.

How long will it take? Well, two-way light time between New Horizons and Earth is nine hours. It sounds like they want more information from the spacecraft before determining the best course of action; they had to command that some time after they regained contact at 3:15 pm EDT / 19:15 UT, so they won't get the information until the wee hours of their morning. In the meantime, they'll probably have developed a list of possible explanations for the anomaly. If the further information that they get tomorrow morning matches one of their explanations, and it's a benign thing, they could conceivably return the spacecraft to science with a command sent later tomorrow morning ET -- leaving a science gap of slightly more than one day. If they still aren't sure they understand the spacecraft's condition, it could take at least one more nine-hour round of communication followed by another meeting, resulting in most of another day.

Safe modes are scary and annoying but not uncommon and not, at this moment, anything that New Horizons fans should be freaking out about. I'll willingly admit that I freaked out, just a little bit, when I first heard this news; but I have confidence that the team will handle it and will return the mission to normal operations with no serious loss to science.

I'll post an update when I have any news to report, but with that nine-hour delay, it's going to be a while.

UPDATE: NASA issued a statement at about 19:30 PT / 22:30 ET July 5 / 02:30 UT July 6 saying that the cause of the safe mode is understood, and that New Horizons will resume science operations on July 7:

NASA’s New Horizons mission is returning to normal science operations after a July 4 anomaly and remains on track for its July 14 flyby of Pluto.

The investigation into the anomaly that caused New Horizons to enter “safe mode” on July 4 has concluded that no hardware or software fault occurred on the spacecraft. The underlying cause of the incident was a hard-to-detect timing flaw in the spacecraft command sequence that occurred during an operation to prepare for the close flyby. No similar operations are planned for the remainder of the Pluto encounter.

“I’m pleased that our mission team quickly identified the problem and assured the health of the spacecraft,” said Jim Green, NASA’s Director of Planetary Science. “Now – with Pluto in our sights – we’re on the verge of returning to normal operations and going for the gold.”

Preparations are ongoing to resume the originally planned science operations on July 7 and to conduct the entire close flyby sequence as planned. The mission science team and principal investigator have concluded that the science observations lost during the anomaly recovery do not affect any primary objectives of the mission, with a minimal effect on lesser objectives. “In terms of science, it won’t change an A-plus even into an A,” said New Horizons Principal Investigator Alan Stern of the Southwest Research Institute, Boulder.

Adding to the challenge of recovery is the spacecraft’s extreme distance from Earth. New Horizons is almost 3 billion miles away, where radio signals, even traveling at light speed, need 4.5 hours to reach home. Two-way communication between the spacecraft and its operators requires a nine-hour round trip.

Status updates will be issued as new information is available.