Hubble is BUSY!
The telescope is scheduled to the second, weeks in advance. If you are an astronomer who wants to use Hubble, you have to submit a proposal months before, explaining why the telescope should be used for your work rather than someone else’s. Hubble is massively overbooked. There are about twenty times as many serious requests for observations as there is time to make them.
And yet, Hubble is not collecting data 100% of the time, even if everything is going to plan.
To explain how the telescope is spending a normal day, let’s look at its schedule for the remainder of today (11 October 2019):
Image: Part of Hubble’s weekly schedule for 7–14 October 2019.
If you look at the observation begin and end times, there are sometimes a few minutes between the end of one observation and the beginning of the next one. This happens when the telescope needs to switch between instruments, or when it moves to point at another part of the sky, which takes time.
But there are also longer gaps. For example, there will be no data recorded between 16:39:43 and 19:00:00, and again between 21:34:51 and 23:00:00. There are several reasons why this happens. Sometimes there is maintenance to be done (remotely, of course). More interestingly, there are orbits when Hubble passes over the South Atlantic Anomaly (SAA for short), a region where Earth’s magnetic field is weaker than elsewhere. Cosmic radiation can penetrate into the higher layers of the atmosphere there. NASA has decided that it is too risky to operate the space telescope when it passes over this part of the Earth.
Image: The South Atlantic Anomaly is the red region, covering parts of South America, South Africa, and much of the southern Atlantic Ocean. The space telescope is routinely switched off when it flies over this part of the Earth. Image shows the SAA at an altitude of 560 km above the Earth.
As a space telescope, Hubble orbits the Earth once every 95 minutes, at an altitude of 540 km. It experiences 15 sunsets and 15 sunrises every day. The telescope is in sunlight half of the time. A telescope on the ground could not be used at those times (most telescopes only work at night), but Hubble is unaffected by this. It only needs to avoid pointing too close to the Sun, so sunlight does not get into the main body of the telescope, but apart from that, there are few constraints. To avoid damage from bright light, the telescope cannot point
- within 50 degrees of the Sun
- within 15.5 degrees of any illuminated portion of the Earth
- within 7.6 degrees of the dark limb of the Earth, or
- within 9 degrees of the Moon.
- 地球dark limb的7.6度内
There is always enough sky to find a target, 24/7 (well, apart from the down times I mentioned earlier).
There are two things I’d like to add:
First, even when Hubble is operational, it is not recording science data for much of the time. If you look at the schedule I posted, there is a column called “Target”. Many of those runs are dubbed “dark”, “bias” or something similar. The telescope needs to spend a lot of time recording calibration images. That’s something every astrophotographer has to do, and the space telescope (like every other professional telescope on the ground) needs to do it, too. Hubble actually spends about as much time (if not more) recording calibration images as it spends on science targets.
Second, Hubble is, unfortunately, showing signs of old age. The telescope has been in orbit since 1990. Some of its instruments have failed, the orbit is decaying (Hubble will inevitably burn up in the atmosphere some time in the 2020s or 2030s), and most importantly, its controls (the gyroscopes and the computers) have caused trouble in recent years. There have been times when Hubble was not operational for weeks because some system didn’t work correctly. The last time this happened was in October 2018. It can happen again at any moment. If you are a scientist and you’ve been waiting for years to get your two hours with Hubble, and the telescope has technical difficulties when your observations were scheduled, you are having a bad day. You don’t get your data.
You can follow what Hubble is currently doing on their Twitter account, Space Telescope Live.
Since its launch, Hubble has collected about 1.5 million scientific images, resulting in the publication of almost 20,000 scientific papers. Fingers crossed the telescope keeps going for many more years!