sci18007 — Announcement

Hubble is exploring the Universe once again

17 December 2018

In early October 2018, Hubble operations ceased and Hubble entered safe mode following the failure of Gyro 2. Backup Gyro 3 was powered on shortly thereafter but its non-nominal performance prolonged Hubble’s stay in safe mode for about 3 weeks while an anomaly review board investigated the behaviour and established a path forward to recover full science operations. Thanks to the outstanding efforts of all involved in this recovery, Hubble has resumed nominal science operations since 27 October 2018 using the 3-gyro combination 3-4-6.

In order to observe astronomical targets, Hubble needs to be able to position itself in space while orbiting the Earth and then steady itself in order to stare at or “lock onto” science objects for long periods of time. Such a feat is achieved by Hubble’s Pointing Control System (PCS) which is made up of three subsystems: the Fine Guidance Sensors (FGS) which are Hubble’s targeting devices and which aim Hubble at the desired astronomical fields and objects; the gyroscopes (gyro) which sense spacecraft angular motions and indicate when Hubble needs to be repointed; and the reaction wheels which steer Hubble in the desired direction.

The gyros essentially measure the speed at which the spacecraft is turning, which is necessary to help Hubble turn and lock onto new targets or stay pointed at targets for extended periods of time. Six gyros are on board Hubble and at any one time, 3 gyros are used to ensure maximum sky coverage and optimum pointing accuracy. Following the failure of gyro 1 in late April 2018, Hubble has been using gyros 2, 4 and 6 to perform its routine science operations.

However, on 5 October 2018 Gyro 2 suffered a failure too, causing the spacecraft to enter safe mode. The Gyro 2 failure was not unexpected since this gyro had shown an anomalous behaviour that was actively monitored since late 2017 and successfully mitigated until its failure. Immediately after the failure, Gyro 3 was powered on but showed non-nominal performance as the rate bias — essentially a systematic error in the rate reported by Gyro 3 — was well outside the operational range needed for fine pointing control.

Hubble remained in safe mode while the Anomaly Review Board that was formed shortly after the failure analysed the problem and the historical data. A number of tests were performed, including toggling between operational states, small vehicle manoeuvres, and a running restart of Gyro 3 (i.e., power off and then power on again one second later). The manoeuvres soon demonstrated that Gyro 3 was sensing telescope motion. Additional manoeuvres resulted in a dramatic reduction in the Gyro 3 rate bias, bringing Gyro 3 within specifications for nominal operations. The Hubble mission then characterised this behaviour and developed safeguards against sudden bias shifts to allow Hubble to safely return to 3-gyro operation.

Starting on 26 October, the science instruments were restored to their science ready state following successful execution and analysis of a series of calibration programs and procedures provided by the Hubble instrument teams.

On 27 October, Hubble officially resumed science operations having completed its first science observations with the WFC3 using gyro 3-4-6 combination. As of 29 October, all Hubble science instruments have successfully resumed science operations and Hubble enjoys a scheduling efficiency on a par with efficiencies routinely achieved before Hubble entered the prolonged safe mode period in early October 2018. The 1-gyro mode configuration, which the mission has an operation plan for and direct experience of, does not need to be implemented at this time.

Like the phoenix rising from its ashes, Hubble is back in science operation again, as powerful as ever, relentlessly exploring every corner of the Universe accessible to its eager eye today and for years to come. A heartfelt thank you to all involved in bringing Hubble back to its “old but amazing self” again!



Paule Sonnentrucker
ESA Acting Hubble Mission Manager, STScI

Antonella Nota
ESA HST Project Scientist, STScI

About the Announcement



SM2: Hubble in Free Orbit
SM2: Hubble in Free Orbit

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