[aejm]

“Once Hubble is on target, the steadiness of the telescope in one-gyro mode is almost comparable to that of a full three-gyro complement… Although one-gyro mode is an excellent way to keep Hubble science operations going, it does have limitations, which include a small decrease in efficiency (roughly 12 percent) due to the added time required to slew and lock the telescope onto a science target… If Earth or the moon block two of the fixed head star trackers’ fields of view, Hubble must move further along in its orbit until the star trackers can see the sky and its stars again. This process encroaches upon science observation time. Second, the additional time the fine guidance sensors take to further search for the guide stars adds to the total time the sensors use to complete the acquisition. Third, in one-gyro mode Hubble has some restrictions on the science it can do. For example, Hubble cannot track moving objects that are closer to Earth than the orbit of Mars. Their motion is too fast to track without the full complement of gyros. Additionally, the reduced area of sky that Hubble can point to at any given time also reduces its flexibility to see transient events or targets of opportunity like an exploding star or an impact on Jupiter. When combined, these factors may yield a decrease in productivity of roughly 20 to 25 percent from the typical observing program conducted in the past using all three gyros.“ [0]

[0] https://science.nasa.gov/mission/hubble/observatory/design/h...

[SoftTalker]

It was launched 34 years ago with a 15-year expected service life. So that it's still operating at all today is quite an achievement.

[dylan604]

The impact on Jupiter bit seems strange to me. Unless it's something that just pops up out of nowhere, it seems like we'd have been tracking the impactor for some time to be able to have plenty of lead time for Hubble to use its walker to slowly get into position.

[SiempreViernes]

Impactors are indeed not seen that far ahead in comparison to the Hubble time allocation time scale, so for scheduling purposes they do more or less pop up out of nowhere.

Since its not very likely that you happen to have a scheduled science target lying close to Jupiter around the time of impact, you would have to do a long slew which now takes such a long time you likely have to cancel some other science. But I think this is a general issue that will discard most targets of opportunity that Hubble could otherwise have observed.