<P> Similarly, the Advanced Camera for Surveys (ACS) main camera primary electronics failed in June 2006, and the power supply for the backup electronics failed on January 27, 2007 . Only the instrument's Solar Blind Channel (SBC) was operable using the side - 1 electronics . A new power supply for the wide angle channel was added during SM 4, but quick tests revealed this did not help the high resolution channel . The Wide Field Channel (WFC) was returned to service by STS - 125 in May 2009 but the High Resolution Channel (HRC) remains offline . </P> <P> HST uses gyroscopes to detect and measure any rotations so it can stabilize itself in orbit and point accurately and steadily at astronomical targets . Normally, three gyroscopes are required for operation; observations are still possible with two, but the area of sky that can be viewed would be somewhat restricted, and observations requiring very accurate pointing are more difficult . There are further contingency plans for observations with just one gyro, but if all gyros fail, continued scientific observations will not be possible . In 2005, it was decided to switch to two - gyroscope mode for regular telescope operations as a means of extending the lifetime of the mission . The switch to this mode was made in August 2005, leaving Hubble with two gyroscopes in use, two on backup, and two inoperable . One more gyro failed in 2007 . By the time of the final repair mission in May 2009, during which all six gyros were replaced (with two new pairs and one refurbished pair), only three gyros were still working . Engineers determined that the gyro failures were caused by corrosion of electric wires powering the motor that was initiated by oxygen - pressurized air used to deliver the thick suspending fluid . The new gyro models were assembled using pressurized nitrogen and they should be much more reliable . </P> <P> Hubble orbits the Earth in the extremely tenuous upper atmosphere, and over time its orbit decays due to drag . If it is not re-boosted, it will re-enter the Earth's atmosphere within some decades, with the exact date depending on how active the Sun is and its impact on the upper atmosphere . If Hubble were to descend in a completely uncontrolled re-entry, parts of the main mirror and its support structure would probably survive, leaving the potential for damage or even human fatalities . In 2013, deputy project manager James Jeletic projected that Hubble could survive into the 2020s . Based on solar activity and atmospheric drag, or lack thereof, a natural atmospheric reentry for Hubble will occur between 2028 and 2040 . In June 2016, NASA extended the service contract for Hubble until June 2021 . </P> <P> NASA's original plan for safely de-orbiting Hubble was to retrieve it using a space shuttle . Hubble would then have most likely been displayed in the Smithsonian Institution . This is no longer possible since the space shuttle fleet has been retired, and would have been unlikely in any case due to the cost of the mission and risk to the crew . Instead, NASA considered adding an external propulsion module to allow controlled re-entry . Ultimately NASA installed the Soft Capture and Rendezvous System, to enable deorbit by either a crewed or robotic mission . </P>

What layer of the atmosphere is the hubble telescope in