After an intense annual checkout - more like a deep-space workout - New Horizons is getting some well-deserved rest.

New Horizons operators at the Johns Hopkins University Applied Physics Laboratory (APL) eased the spacecraft into electronic hibernation on Dec. 16, wrapping up nearly four months of tests, data collection and software upgrades. The spacecraft's Earth-bound crew has now turned its attention to detailed Pluto-encounter sequencing.

"I'm in awe of all the team accomplished during this checkout - multiple software uploads, full spacecraft and payload checkouts, instrument calibrations and new capability tests, star-tracker imaging, trajectory tracking refinement, science measurements and more, and all of it went well," says New Horizons Principal Investigator Alan Stern. "What a great team and spacecraft we have!"

New Horizons is spending most of its 9� -year cruise to Pluto in hibernation, with its major systems and most instruments turned off, save for an annual period when the team wakes it up for system checks and other activities.

Mission managers expect each annual checkout to last about 10 weeks, but they also have to be flexible. The first annual checkout (ACO) in 2007 took about three months as operators took extra time to update the spacecraft's autonomy software and finished commissioning the science instruments.

New Horizons Mission Operations Manager Alice Bowman of APL says this year's checkout was even more complicated, mainly because the team loaded new software to all of the spacecraft control systems - Command and Data Handling, Guidance and Control, and Autonomy.

"Loading any one is delicate in itself, but loading three requires a carefully scripted dance," she says.

The team first decides the safest order for loading software to each processor, then tests the load sequences in that order, while practicing contingency scenarios for anomalies. An issue with any code sequence during transmission means the team has to stop the upload and either fix the problem or completely remove the code, before moving on to the next system.

"Ground tests verified that the new code would work with specific codes in the other processors, so you have to make sure the tested configuration is maintained onboard during the loading process in order to ensure the safety of the spacecraft," Bowman says.

"It's a testament to the software development engineers, to the testers and to the mission ops team that we did not have any issues following the loads on the spacecraft. Because of this, we were able to hold to our schedule and complete ACO-2 in four months."

New Horizons will remain in electronic slumber until July 2009, when it's awakened for the next annual checkout, expected to last four to six weeks. Until then, the team, working with NASA's Deep Space Network of receiving stations, will listen weekly for the radio beacon tones and bimonthly for the 10 bit-per-second telemetry contacts that indicate the probe's health.

New Horizons checked in with a 10-bps telemetry contact on Dec. 18, indicating all systems were normal as it continued to cruise through the outer solar system. The spacecraft is currently 1.2 billion miles from Earth, speeding away from the Sun at nearly 11 miles per second - covering more than 950,000 miles a day.

"Now, it gets a few hundred-million miles of quiet cruise," Stern says.