Safe mode (spacecraft)

Safe mode is an operating mode of a modern spacecraft during which all non-essential systems are shut down and only essential functions such as thermal management, radio reception and attitude control are active.^[1]

Triggering events

Safe mode is entered automatically upon the detection of a predefined operating condition or event that may indicate loss of control or damage to the spacecraft. Usually the trigger event is a system failure or detection of operating conditions considered dangerously out of the normal range. Cosmic rays penetrating spacecraft electrical systems can create false signals or commands and thus cause a trigger event. The central processor electronics are especially prone to such events.^[2] Another trigger is the lack of a received command within a given time window. Lack of received command can come from hardware failures or missprogramming the spacecraft, the Viking_1 lander for example.

Entry

The process of entering safe mode, sometimes referred to as safing,^[3] involves a number of immediate physical actions taken to prevent damage or complete loss. Power is removed from non-essential subsystems. Regaining attitude control, if lost, is the highest priority because it is necessary to maintain thermal balance and proper illumination of the solar panels.^[1] A tumbling or cartwheeling spacecraft can quickly roast, freeze or exhaust its battery power and be lost forever.^[4]

In safe mode

While in safe mode the preservation of the spacecraft is the highest priority. Typically all non-essential systems, such as science instruments, are shut down. The spacecraft attempts to maintain orientation with respect to the Sun for illumination of solar panels and for thermal management. The spacecraft then awaits radio commands from its mission control center monitoring for signals on its low-gain omnidirectional antenna. Exactly what happens while in safe mode is dependent on the spacecraft design and its mission.^[2]

Recovery

Recovery from safe mode involves reestablishing communication between the spacecraft and mission control, downloading any diagnostic data and sequencing power back on to the various subsystems to resume the mission. The recovery time can be anywhere from a few hours to days or weeks depending on the difficulty in reestablishing communications, conditions found on the spacecraft, distance to the spacecraft and the nature of the mission.^[5]

Suppression

A spacecraft's ability to enter safe mode may be suppressed during crucial spacecraft operations (such as the orbit insertion maneuver of the Cassini spacecraft at Saturn), during which – if a critical failure were to occur – most, if not all, of the mission objectives would be lost anyway.^[3]

Manual entry

On occasion, a spacecraft is placed in safe mode deliberately by mission control, as the Spirit rover was on sol 451.^[6]

Modern incidents

2005

• Spirit rover has had several incidents with safe mode.^[6]

2007

• Cassini–Huygens download of the Iapetus flyby data was interrupted by a safing event September 10, 2007.^[3]
• New Horizons entered safe mode March 19, 2007 due to an uncorrectable memory error in the primary Command and Data Handling (C&DH) computer.^[5]
• Odyssey has broken the communications link between the Mars rovers and Earth during several sudden safing events.^[7][8]

2009

• MRO entered safe mode on August 26, 2009 for the second incident in a month, the fourth in 2009 and the eighth since launch in 2005.^[9][10] The spacecraft was kept in safe mode until December 8, 2009.^[11]
• Kepler entered safe mode on June 15 and again on July 3, 2009. Both cases were triggered by an on-board processor reset.^[12]
• Dawn entered safe mode due to a programming error during its February 17, 2009 Mars flyby.^[13]
• MESSENGER entered safe mode during its third flyby of Mercury on September 29, 2009.^[14]

2015

• New Horizons entered safe mode on July 4, 2015, ten days before its closest approach to Pluto, after a timing problem in a command sequence. Some scientific data was lost, but with only minimal impact on mission objectives.^[15]

Incidents resulting in spacecraft loss or near loss

• SOHO entered safe mode and was nearly lost on June 25, 1998. Normal operations were eventually restored after a gap of four months.^[4][16]
• NEAR entered safe mode, tumbled out of control and was nearly lost during the first attempt of Eros orbit insertion on December 20, 1998.^[17]
• Mars Global Surveyor entered safe mode and was lost when its batteries were overheated and destroyed by incorrect solar orientation on November 2, 2006.^[18]
• ISEE-3 was lost on September 16th, 2014, during a civilian reboot effort.^[19] It is believed that the 36-year-old spacecraft entered safe mode due to a drop in power from its solar panels.^[20] The crowdfunded project has been unable to re-establish contact.

Notes

The term safing is also used to describe the process of rendering a weapon inactive (safe).^[21]

References