NASA is making a fundamental change to how the Hubble Space Telescope orientates itself to focus in on its targets.
The change affects Hubble's gyroscopes, which are the instruments that enable Hubble to swing up and down, left and right to focus in on a particular planet, nebula, galaxy or star cluster.
Gyros contain an internal wheel that spins at 19,200 revolutions per minute, enabling Hubble to change the direction in which it is pointing (known as 'slewing' in astronomy).
The space telescope would normally operate with 6 gyros, which were newly installed during the final Hubble servicing mission in 2009.
Out of the original six, just three gyros remain active, and the Hubble Space Telescope will now operate with only one.
Why Hubble is moving down to one gyro
Over the past six months, one of Hubble's gyros has consistently given the NASA ground team on Earth faulty readings.
This has caused Hubble to enter 'safe mode' and led to a suspension of science observations.
The faulty gyro is experiencing something called 'saturation', NASA says, which means it's giving the Hubble science team readings that indicate the 'maximum slew rate' possible, regardless of how quickly the telescope is actually slewing.
And while Hubble engineers have been able to reset the gyro's electronics and return it to normal operations, this measure has proved only temporarily, with the gyro quickly returning to giving out false readings.
So, NASA has made the decision to operate Hubble with just one gyro for the rest of its operational life.
How will one-gyro mode affect Hubble?
This one-gyro mode is not completely out of the blue: NASA had developed such a backup plan over 20 years ago as a means to extend the Hubble Space Telescope's lifespan.
NASA says "Hubble uses three gyros to maximise efficiency but can continue to make science observations with only one gyro," stating it's the "best operational mode to prolong Hubble’s life and allow it to successfully provide consistent science with fewer than three working gyros."
Naturally, however, there will be limitations.
In one-gyro mode, Hubble will need more time to slew and locate a target.
It won't have as much observing flexibility, meaning there will be be limitations as to where it can point at any given time.
And it will not be able to track moving objects closer than Mars.
What next?
Hubble engineers now need to work on reconfiguring the spacecraft and ground system, but also to consider whether this new operational mode will affect planned future observing projects.
Astronomers around the world apply for time using the Hubble Space Telescope to complete their observing projects, so the question is whether any of these projects might be affected by the telescope's new one-gyro mode.
Science operations are expected to resume by mid-June, says NASA.
"Once in one-gyro mode, NASA anticipates Hubble will continue making new cosmic discoveries alongside other observatories, such as the agency’s James Webb Space Telescope and future Nancy Grace Roman Space Telescope, for years to come."
