Hubble having a bit of a glitch

Milky-Way.Kiwi has fingers crossed that the current gyroscope problem on the Hubble Space Telescope will be resolved quickly. The massively successful space telescope has sent us thousands of breath taking images of the universe since it’s deployment on 25 April 1990 and collected nearly 150Tb of data. The telescope is currently in safe mode as NASA tries to figure out how to get one of the gyroscopes working. It’s not all bad news though as Hubble can still function with just one gyroscope working. They normally like to have three going to give the best performance of the telescope.

The HST has been a hugely successful programme by NASA since its launch back in 1990. In that time is has made over 1.3 million observations. It’s contribution to science can be seen by the astonishing number of scientific papers written with the benefit of the HST’s data, so far about 15,000. To be one of the most successful scientific instruments it has to perform with amazing accuracy. The reason that it’s in space is to avoid the problems of looking through the Earth’s atmosphere. But there’s no point in doing that if advantage is not taken of being able to point accurately and stay on target for along time while images are taken. The gyroscopes in Hubble allow it to point with a accuracy of 0.007 arcseconds, which is astonishing. Pluto takes up about 0.1 arcseconds and that’s nearly 5 billion km away. That accuracy allows Hubble to make out objects about 0.05 arc seconds. You can see from the below image taken by Hubble of Pluto how it can show up very coarse surface features.

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Pluto from Hubble in 2010 (Credit: NASA, ESA, M. Buie)

All six gyroscopes were replaced in 2009 during the last Space Shuttle mission (STS-125) to service the HST. The featured image at the start of this article was taken on STS-125 as the Space Shuttle approached the HST. The instruments are very important in keeping the telescope pointing in the right direction and to perform at the accuracy mentioned above. Each gyroscope has a wheel inside of it that spins at about 19,200 RPM and it’s this shell that gives the gyroscope it’s very high level of stability. All six were replaced in 2009 because some fine wires were corroding in the gyroscope assemblies reducing their reliability and ultimately leading to their failure. The type of gyroscopes in the HST are known as gas-bearing gyroscopes and they are the quietest and offer very high accuracy. Prior to this servicing mission to the HST it had also been operating on a reduced number of gyroscopes. From about August 2005 it relied on just two, and though it functioned quite well this reduction did limit the to where it could point.

Hubble Space Telescope rate gyro assemblies
HST Assembly showing where the gyroscopes go (Credit: ESA)

There’s no option to go back to the HST and do another repair job as NASA has moved it’s focus to the new James Webb Space Telescope that is currently being assembled. It’s an extraordinarily costly event going to the HST to do a repair mission and is simply not on the cards for NASA. The one gyro that had failed was not performing perfectly for about the last year and showing signs that it was nearing the end of its service life. Two other gyros had already failed so now HST is down to three. The problem was when they fired up the final remaining gyro, it too had some problems and was not performing as expected. NASA has formed a review board to get to the bottom of the problem and, hopefully, design a fix so the HST can get back to full 3 gyroscope operations. If that doesn’t happen then the HST will be operating on reduced gyroscope mode from now on. Fortunately this is still fine to make the most of it’s incredible capabilities so we are a long way from hearing the last from the Hubble Space Telescope.