Why isn't there anything else comparable to Hubble in LEO?

[gomipile]

A telescope can only point in one direction at a time. There is always use for multiple telescopes of a particular specification. So, since putting spacecraft in LEO is cheaper than ever, why doesn't Hubble have any companions yet?

Whenever I try to research this topic, all that I see is discussions of JWST. JWST is great and all, but it is a different beast, IMO. It has much larger aperture, etc. but it has a lot less flexibility in where in the sky it can look at a given time of year. And, as I understand things, it is expected to have a much shorter operational lifetime than a Hubble-like spacecraft is capable of.

It seems to me that we would benefit greatly from several Hubble-type orbital observatories in addition to our current capabilities. I'm sure there are plenty of ways that something in a similar form factor to Hubble could be made both cheaper and better(than Hubble) given the decades of advancements in electronics, sensors, other hardware, and general improvement in spacecraft engineering and operational knowledge.

If feel like at least 3 updated Hubble-type observatories would give us a significant boost in the flexibility of observations we could do. Maybe start a rolling schedule of launching overlapping update/replacement/companion missions like we've done with the Landsat series.

[Peelee]

Fun fact! When making the mirror for Hubble, all work was done over three miles away from public roads, and at night, because a single car driving on the road would be enough to disrupt the mirrors construction. A paint chip that measured fifty times less than the width of a human hair was enough to cause catastrophic failure in image resolution, resulting in a multi-billion-dollar repair program to fix the issue (and, keep in mind, this was the cheaper option), which took three years of planning.

Putting it in space is the easy and cheap part of a space telescope.

[sihnfahl]

Fun fact! When making the mirror for Hubble, all work was done over three miles away from public roads, and at night, because a single car driving on the road would be enough to disrupt the mirrors construction. A paint chip fifty times less than the width of a human hair was enough to cause catastrophic failure in image resolution, resulting in a multi-billion-dollar repair program to fix the issue (and, keep in mind, this was the cheaper option), which took three years of planning.

Fun fact! At current, we have no means of performing maintenance on anything we put into space.

[IthilanorStPete]

As others have mentioned, cost is the big issue. There's many more competing proposals for space telescopes than there's budget to build, launch, and operate them, especially bigger observatories like Hubble. There's also not really any opportunity for economies of scale, especially when it comes to the difficult, time-consuming, expensive process of manufacturing the big mirrors.

That said, there are probably more than you might expect. Looking through Wikipedia's list, NASA's currently operating:

• Swift (gamma-ray)
• Fermi (gamma-ray)
• Chandra (X-ray)
• NuSTAR (X-ray)
• IXPE (X-ray)
• Hubble (visible light)
• TESS (visible light)
• JWST (infrared)
• WISE (infrared)

ESA's also operating a good few different telescopes, and there's a grab bag of missions from other agencies.

Fun fact! At current, we have no means of performing maintenance on anything we put into space.

I mean, we can generally perform maintenance on stuff on the ISS. Not many astronomical instruments there, though there are a couple of small ones, IIRC.

[Peelee]

Fun fact! At current, we have no means of performing maintenance on anything we put into space.

Fun fact! The James Webb mirrors are so flatsmooth that if you scaled them up to the size of the United States, there would only be a ~3 inch variance in elevation.

[gomipile]

As others have mentioned, cost is the big issue. There's many more competing proposals for space telescopes than there's budget to build, launch, and operate them, especially bigger observatories like Hubble. There's also not really any opportunity for economies of scale, especially when it comes to the difficult, time-consuming, expensive process of manufacturing the big mirrors.

That said, there are probably more than you might expect. Looking through Wikipedia's list, NASA's currently operating:

• Swift (gamma-ray)
• Fermi (gamma-ray)
• Chandra (X-ray)
• NuSTAR (X-ray)
• IXPE (X-ray)
• Hubble (visible light)
• TESS (visible light)
• JWST (infrared)
• WISE (infrared)

ESA's also operating a good few different telescopes, and there's a grab bag of missions from other agencies.

TESS is the closest on there to being what I mean, but it's wide-field, not a long focal length telescope.

Fun fact! When making the mirror for Hubble, all work was done over three miles away from public roads, and at night, because a single car driving on the road would be enough to disrupt the mirrors construction. A paint chip that measured fifty times less than the width of a human hair was enough to cause catastrophic failure in image resolution, resulting in a multi-billion-dollar repair program to fix the issue (and, keep in mind, this was the cheaper option), which took three years of planning.

Putting it in space is the easy and cheap part of a space telescope.

I referred to more types of construction and operation cost reduction than I did launch cost reduction in the OP.

[DavidSh]

Fun fact! The James Webb mirrors are so flat that if you scaled them up to the size of the United States, there would only be a ~3 inch variance in elevation.

I think you mean "smooth", not "flat". If they were that flat, their focal length would be a lot longer.

[Peelee]

I think you mean "smooth", not "flat". If they were that flat, their focal length would be a lot longer.

Yeah, that was my fault. The surface has an average roughness of 20 billionths of a meter, which is smoothness and not flatness.

[sihnfahl]

I mean, we can generally perform maintenance on stuff on the ISS. Not many astronomical instruments there, though there are a couple of small ones, IIRC.

Point, but everything else? Better pray nothing goes wrong, otherwise it's to the satellite graveyard ... or you had thrusters to push it back to Earth, so it'd land someplace remote. Like the middle of the Pacific.

[halfeye]

In my opinion a super-Hubble would be a very fine thing indeed. There are so many options, it could be bigger, it could be sharper, it could be higher resolution, it could see more colours like a mantis shrimp.

[Jimorian]

The JWST does have a port to be robotically refuelled to extend its life, there just isn't currently a plan or a craft to do so. Just in case money/will comes along to do so.

[Willie the Duck]

I think the fundamental reason is interest (amongst those people who might foot the bill). Sure, a second Hubble would be cheaper than the first, but a lot more expensive than not doing a second Hubble. Having two of them means you can have them pointed at two things at once, but there's no one specific new thing that we are doing with the second. Thus no new and amazing full-color pictures (the likes of which had rarely been seen) in all the newspapers and magazines (and now web articles) wowing the populace and saying, in effect, "here, this is why we did this and why you should support us having done so."

[Kriegspiel]

A telescope can only point in one direction at a time. There is always use for multiple telescopes of a particular specification. So, since putting spacecraft in LEO is cheaper than ever, why doesn't Hubble have any companions yet.

Hubble does have companions in orbit, at least 5 currently. Your first sentence is the reason why we don't see Hubble-like images from them as they are pointed at the earth rather than towards space.

If you look through past news stories you'll find that in 2012 the NRO essentially donated two nearly completed Hubble class telescopes to NASA. One of those is being repurposed as the Nancy Grace Roman Space Telescope (originally the Wide-Field Infrared Survey Telescope) & scheduled to launch in between 2025 & 2027.

The bottom line is that as others have said it essentially comes down to money (and spending priorities).

[gomipile]

Hubble does have companions in orbit, at least 5 currently. Your first sentence is the reason why we don't see Hubble-like images from them as they are pointed at the earth rather than towards space.

If you look through past news stories you'll find that in 2012 the NRO essentially donated two nearly completed Hubble class telescopes to NASA. One of those is being repurposed as the Nancy Grace Roman Space Telescope (originally the Wide-Field Infrared Survey Telescope) & scheduled to launch in between 2025 & 2027.

The bottom line is that as others have said it essentially comes down to money (and spending priorities).

The NRO telescopes don't have the focal length to match Hubble's observational capabilities. You couldn't just turn one to the stars and drop it into the same niche Hubble fills.

[Mechalich]

One of the reasons there hasn't been a move to replace and/or copy the Hubble is the development of a variety of methods in ground-based astronomy to bypass atmospheric distortion, such as adaptive optics. These techniques became practical reality in the 1990s, shortly after the Hubble was launched and allowed to ground-based astronomy to largely match Hubble-level capabilities without the immense expense of being launched into space. Space still has a number of advantages, such as no light pollution, but they aren't as insurmountable as they were when the Hubble was produced and launched.

There's also the fact that the retirement of the Shuttle program has made maintenance of any space-based asset far more difficult and politically involved than it was during the Shuttle era. This has changed, due to the actions of SpaceX and other private companies capable of manned missions, but that development is very recent (SpaceX's second crewed mission returned two months ago).

[Rockphed]

Fun fact! When making the mirror for Hubble, all work was done over three miles away from public roads, and at night, because a single car driving on the road would be enough to disrupt the mirrors construction. A paint chip that measured fifty times less than the width of a human hair was enough to cause catastrophic failure in image resolution, resulting in a multi-billion-dollar repair program to fix the issue (and, keep in mind, this was the cheaper option), which took three years of planning.

Putting it in space is the easy and cheap part of a space telescope.

Ahem, the problem was that the mirror of Hubble was ground to the wrong curvature, not that there was a minuscule paint chip. Also, the backup mirror was ground to the correct curvature.

While a paint chip would interfere with crisp optics, it would be the same as either copying the whole image offset by the size of the paint chip compared to the whole aperture or as chopping out the center of the central pixel. I don't feel like doing the math to figure out which of the two options is more accurate.

Telescopes are defined by 3 things: focal length, light collection, and resolution. Focal length is defined by the shape of the mirrors and lenses. Light collection is defined by how big the first aperture is and how good of photo detection equipment is at the back of it. Resolution is defined by the lesser of the diffraction limit or the atmospheric limit of the telescope. Hubble has a 2.4 meter first mirror and is in space, so its resolution is diffraction limited. A similar 2.4 meter telescope on Earth would be seeing limited and would only be able to see a very blurry version of what Hubble sees until adaptive optics came along.

[Peelee]

Ahem, the problem was that the mirror of Hubble was ground to the wrong curvature, not that there was a minuscule paint chip.

Yes, it was ground incorrectly due to a calibration error. Due to a paint chip. Bolding mine:

But, as everyone now knows, the mirror had a flaw called spherical aberration. It was slightly too flat, which meant that the light reflected from its edge, and light from its centre, were focussed in different places.

"It was shocking, I was upset by it," says Bud. "My association with the people that worked on that mirror, and that whole project, is very dear to me."

"There was an awful lot of time and energy put into this thing, a lot of personal commitment, a lot of technology, and to have it all be tarnished was very, very disappointing," adds Lou.

Unknown to anybody, a chip of paint had come off a measuring rod inside the null corrector. This exposed a chink of metal, and light hitting this chink distorted the measurements, producing an error that was copied onto the mirror.

"And that error was not picked up and resulted in the null lens being incorrectly built," says Chris Burrows from the Space Telescope Science Institute. "That error then got slavishly copied onto the primary mirror for the Hubble. And as a result the primary mirror was assumed to be correct because the null lens said it was, but in fact it was wrong."

[Yora]

Fun fact! The James Webb mirrors are so flatsmooth that if you scaled them up to the size of the United States, there would only be a ~3 inch variance in elevation.

The curvature of the mirror panels can also be adjusted by pushing or pulling on the backside. Forgot the exact description, but again with ridiculous degrees of precision.

[Strigon]

I think the fundamental reason is interest (amongst those people who might foot the bill). Sure, a second Hubble would be cheaper than the first, but a lot more expensive than not doing a second Hubble. Having two of them means you can have them pointed at two things at once, but there's no one specific new thing that we are doing with the second. Thus no new and amazing full-color pictures (the likes of which had rarely been seen) in all the newspapers and magazines (and now web articles) wowing the populace and saying, in effect, "here, this is why we did this and why you should support us having done so."

This is the biggest reason, I think. Sure, we could inspect objects at twice our current rate, but what's the rush? They're stars; they aren't going anywhere any time soon.

[Chronos]

If we had ten exact copies of the Hubble in orbit, astronomers would be able to make good use of all ten of them. If the question was just "have more Hubbles" vs. "not have more Hubbles", then it'd be a no-brainer.

But where would the funding come from? Money is a finite resource, especially money that astronomers can convince politicians to spend for them. And so instead of asking for another thing that can do the same thing that we can already do with Hubble (except quicker, because there would be two of them), astronomers ask for things that can do different things than Hubble. Because you get more science for your funding dollars that way.

[Maelstrom]

I think you mean "smooth", not "flat". If they were that flat, their focal length would be a lot longer.

Aren't each of the individual 18 hexagonal mirrors flat? That's my understanding and starting this week, each of them are being calibrated to focus precisely on the secondary mirror using servos with incredibly small steps?. If each was convex (even slightly) would that not make that task even more difficult and add distortion?

[Lord Torath]

Aren't each of the individual 18 hexagonal mirrors flat? That's my understanding and starting this week, each of them are being calibrated to focus precisely on the secondary mirror using servos with incredibly small steps?. If each was convex (even slightly) would that not make that task even more difficult and add distortion?

My understanding was that they were all slightly concave, so that when they were properly aligned, the disc they formed had the same smoothy curvature all the way across, rather than a series of flat plates approximating a curve. Think of a half-circle vs half an octagon. I've been trying to look back through the various articles I've read on it, but I'm not finding anything one way or the other.

Maybe a better example is a sliced pizza. The shape formed by the crust is a circle (well, approximately) if you properly align the pieces. If you cut the crusts off the pieces so they are triangles and arrange them the very same way, you get hard angles instead of a circle. It approximates a circle, the smaller you make the slices, but it's still not a circle.

[Maelstrom]

My understanding was that they were all slightly concave, so that when they were properly aligned, the disc they formed had the same smoothy curvature all the way across, rather than a series of flat plates approximating a curve. Think of a half-circle vs half an octagon. I've been trying to look back through the various articles I've read on it, but I'm not finding anything one way or the other.

Maybe a better example is a sliced pizza. The shape formed by the crust is a circle (well, approximately) if you properly align the pieces. If you cut the crusts off the pieces so they are triangles and arrange them the very same way, you get hard angles instead of a circle. It approximates a circle, the smaller you make the slices, but it's still not a circle.

OK, found the answer (though the technical numbers behind these 'prescriptions' I'm not finding) :

Although there are 18 segments, there are three different optical prescriptions for the 18 segments: six segments of each prescription. The segment received is the first of the "A" prescription segments for which a total of 7 will be made - 6 flight and 1 spare. A prescription is similar to an eyeglass prescription and specifies a unique mirror curvature. Like eyeglasses, mirrors with the same prescription are interchangeable.



Source: https://www.nasa.gov/topics/technolo...b-mirrors.html

[Rockphed]

Based on that, I am certain that the mirrors are each partial paraboloids. The three antennas each occupy rotationally symmetric locations. They have three because they are three different distances from the center.

They might be partial spheres, but I think paraboloids makes more sense.

[DavidSh]

Based on that, I am certain that the mirrors are each partial paraboloids. The three antennas each occupy rotationally symmetric locations. They have three because they are three different distances from the center.

They might be partial spheres, but I think paraboloids makes more sense.

The official description of the Webb telescope is as of a "three-mirror anastigmat". The Wiki page on three-mirror anastigmat telescopes further explains that, for Webb, the primary is an ellipsoid, the secondary is a hyperboloid, and the tertiary is another ellipsoid. The citation for this is behind a paywall, for which I have not paid the toll, so I haven't verified.

[Rockphed]

The official description of the Webb telescope is as of a "three-mirror anastigmat". The Wiki page on three-mirror anastigmat telescopes further explains that, for Webb, the primary is an ellipsoid, the secondary is a hyperboloid, and the tertiary is another ellipsoid. The citation for this is behind a paywall, for which I have not paid the toll, so I haven't verified.

Well, looks like I was wrong both ways. I know " anastigmat " means there shouldn't be errors due to being off center. One day I will have to learn why mirrors for telescopes are elliptical while dishes for radio telescopes are parabolic.

[Darrell123]

There are a couple of additional satellites with different wavelength frequencies. For example, Chandra is a Hubble-class observatory that looks primarily for X-Rays.

And there was, briefly, a Japanese Hubble-class observatory named Hitomi.

Hitomi was launched in February 2016. But it had some issues as a result of its LEO orbit. Specifically, its star trackers suffered repeated single event upsets (SEUs) when it would go through the region known as the South Atlantic Anomaly (SAA). This is a distortion in the earthÂ’s magnetic field which causes LEO satellites to have a high exposure to highly energetic particles which cause such issues. When this happened, the engineers would have to reset the trackers.

37 days after it was launched, Hitomi was going through the SAA. Both trackers had gone offline when it suffered an SEU to its gyro rate bias estimate. Normally, the trackers would have allowed it to correct such an error, but not in this case.

This rate bias error made the satellite believe it had a spin rate, and it attempted to correct it. And so it induced an actual rate, but no matter how fast it went the false spin rate bias made it want to go faster. It transitioned into sunpoint on thrusters still using the bad gyro data, where it encountered an additional problem - an inertia matrix had not been updated properly following Solar array deployment. It fired thrusters to try to null the fake rate, and spun faster and faster.

A couple of hours later, ground radar sites were tracking multiple separate pieces of the satellite, including the two Solar array wings. It essentially dismantled itself.

This is a nasty failure; I know of two other other satellites that suffered similar issues; both were knocked out of operations for a while and burned a bunch of fuel but both survived.

The first images from Hitomi, taken a couple of days before the failure were amazing. May Hitomi’s soul find rest in silicon heaven.

There have been discussions of building a Hitomi 2, but I don’t think they have gone very far. Perhaps now that Webb has launched, NASA might be able to start looking at a Hubble 2 or Chandra 2 or work with Japan on a Hitomi 2.

[Gnoman]

Wiki says that the project was approved with NASA in 2017, with plans for a launch in the early 2020s. Was that cancelled?

[DavidSh]

Wiki says that the project was approved with NASA in 2017, with plans for a launch in the early 2020s. Was that cancelled?

As best as I can tell, the follow-on to Hitomi is now called XRISM, and is planned for launch "Fiscal Year 2022". See https://xrism.isas.jaxa.jp/en/about/index.html. I don't know how Japanese fiscal years are arranged.

Note that these are X-ray telescopes, and so more like Chandra than Hubble.