Why can't we use the hubble to get some great shots of Pluto?

Factual Questions
21

FalafelWaffle: The HST doesn’t use propellant to orient itself.

It uses the Earth’s magnetic field.

Neat trick, huh?

The problem with long-exposure shots of Pluto isn’t getting the telescope time. In fact, those pics you see with the albedo of various parts of Pluto took weeks of HST time (during the relatively-brief period when Charon was eclipsing Pluto) and then some pretty heavy super-computer analysis IIRC. The problem is that Pluto is rotating. Even if it’s only once every 6.4 days, it’s still enough to smear the picture. (I don’t know if they’ve corrected the problem yet, but it used to be that the HST couldn’t be used when it crossed from day to night and back again, because the expansion/contraction of various components caused the telescope to wobble. This would limit the practical duration of an HST exposure to about 50 minutes.)

If we could get better pictures of Pluto from the HST, you bet your bottom dollar we’d be doing it. Astronomers are just as curious about it as I am.

22

Actually, they used the rotation to help them build a sliightly higher resolution image than the actual snapshot images.

The names of the instruments on the HST often gives you a clue as to what the primary obstacle is to viewing different objects.

The Pluto pictures that Podkayne linked to were taken by the Faint Object Camera. Eh? Give you a clue?

The text on the page also describes some of the difficulties they overcame to make the images.

23

Actually, I met Pluto last April at the PNW Dopefest at Beppos. I think he’s very handsome.

oops. Wrong Pluto.

slinks away quietly

24

This is pretty much required. If they used a propellant, some of the exhaust would hang around the telescope and eventually get deposited on the mirror. Not a good thing.

So they have to use a non-propellant method of turning the scope. Even so, the shuttle uses rockets for manuevering and that is a source of mirror contaminant. They keep Hubble’s door closed when the shuttle comes calling which keeps most, but not all, of the stuff off the mirror.

25

Actually, if I remember right, there is no charge for observing on Hubble (getting a proposal through is rather tough, though). I think it’s something like 15% of time designated for scientists in European Space Agency member countries, and the rest is portioned out based purely on proposals with no regard to nationality.

In fact, American scientists can even request grant money from the Space Telescope Science Institute for the purposes of hiring a research assistant, buying computer equipment/time, and other things needed to analyze Hubble data along with their observing proposal.

2.4 m, actually, but pretty small in any case.

26

Huh? Neat trick indeed.

I’m an aerospace engineer, although I never worked with Hubble. I also read (twice, 'cause I’n a nerd who can’t get enough of this stuff) The Hubble Wars by Eric J. Chaisson, former media director for the Space Telescope Science Institute. Great book, although a wee bit slanted towards the astronomers and against those pesky engineers. It was an account of the mirror curvature error and other major boondoggles of the early Hubble program. Such as the vibration (probably of the solar arrays) as Hubble passed through the day-night terminator (thanks for reminding me, Mr Death). Anyway, the book goes into excruciating detail about the construction and operation of all the major instruments and systems on Hubble.

As I understand it, Hubble uses the torque from six large gyroscopes (a full set of three - one for each axis - and a backup set) to reorient itself. The gyros are kept running with boost from electric motors fed by its solar arrays.

Speaking of the gyros, they were all replaced on the last servicing mission. Three of the original set had failed, leaving them with no pointing ability if a fourth were to fail. Thankfully, it didn’t and operations were not interrupted. According to Chaisson, the original set of gyros were bench tested before installation by running them for their entire expected lifespan. They were then mounted on Hubble and sent into space. It’s no wonder that three failed.

There are no rockets on Hubble. I don’t believe the Earth’s magnetic field is used to help point Hubble. How could that be done?

27

I think it has to do with the apparant size of the objects. Hubble has taken some awesome photos of objects that are incredibly huge. And even though they are incredibly far away they still cover a larger piece of the sky then does pluto.

Let’s see when Pluto is closest to the sun it is 2,755.7 million miles away. When the Earth is at it’s furthest it is 94.5 million miles. Subtract the two and we get 2,661.1 million miles. Pluto is about 1486 miles in diameter. The Milky Way is about 100,000 lightyears in diameter. Let’s make a ratio

X / 100,000 ly = 2,661,200,000 mi / 1486 mi

Solve for X and we get 1,790,084,791,386 ly
Meaning that little bitty Pluto has the same apparant diameter as a Milky Way size galaxy 1.8 trillion lightyears away. Which is 150 times further then anything in the night sky. Meaning we’d see more detail in a galaxy at the edge of the universe then we would in Pluto because Pluto is so freaking small.

I’m right aren’t I?

28

Hubble is oriented by gyroscopes. It has 3. You ever go to the science museum and sit on a swivel seat and then pick up a bike wheel that’s spinning. When you tilt the bike wheel to the side you can cause you and the chair to spin. That’s that the Hubble and most other satellites use since then we don’t have to pack them full of propellant.

It’s a fun home science experiment too.

29

For some reason, I thought the post was about Mars. Man, I need to get my eyes checked.

30

I think so. The image of Pluto looks very pixelated. I would guess that the Hubble’s CCD is pretty much maxed out at that resolution. If you’re familiar with digital cameras it’s like taking a picture of a distant object and then trying to blow it up later in photoshop. It doesn’t work very well. You either need to get closer to the subject or use a longer lens.

31

The limitation is not telescope time, or amount of light, or the detector (CCD) resolution. The capability of the telescope is limited by the resolution of the optics, which is determined by the size of the aperture (mirror). Light is a wave, and when you pass it through a very small opening it spreads out, like the ocean waves coming into a harbor. This means if you look through a small opening, you can’t tell which way the original wave/light was coming from. The larger the opening, the better the resolution. With a 4-inch aperture, you can get 1 arcsecond resolution (1 arcsecond is 1/3600 of a degree.) The resolution is inversely proportional to aperture; with a 1-meter (3 feet) telescope you get 0.1 arcsecond resolution. The Hubble is about 2.5 meters, so you get about 0.03 arcsecond resolution.

Now, the apparent size of Pluto is only about 0.1 arcseconds, or only times the resolution. If the planet was separated into 3 parts (left and right ends are light and middle part is dark, for example) the Hubble can see that. But it won’t see any finer detail. The only way to get a better view of Pluto is to get closer, or build a telescope with a larger mirror.

However, it’s no use building larger telescopes on the ground, because the atmosphere causese all sorts of distortions. There are ways to use computer-controlled deformable mirrors to compensate, but the technology is not very mature yet, and the field of view is limited. So we have to launch a larger telescope, but you can’t put a larger telescope than the Hubble on the Space Shuttle. The only way out is to design a telescope with segmented mirrors that can be assembled in space, or unfolds automatically. As mentioned above, the design for such a telescope is proceeding, and hopefully will be launched before the end of the decade.

As for the pointing, it is indeed controlled by momentum wheels. (I think ‘gyro’ usually refers to devices that measure the angle, not control it) However, there are some external torques which are applied to the telescope - solar winds, photon pressure, effect of the residual atmosphere and so on. To compensate for these and keep the telescope stable, the momentum wheel sometimes needs to spin faster and faster, till the bearings break. So you need an additional method of applying external torque to the spacecraft. Attitude control jets (rocket engines) are sometimes used, and magnetic torquers that act on the magnetic field of the earth are also common.

Oh, by the way, the resolution/aperture relationship can also be thought of as a manifestation of Heisenberg’s uncertainty principle. If a particle (photon) passes through a small opening it means you measured the position of the particle to a high precision, and the uncertainty principle says you can’t know the momentum (direction) of the photon very accurately. If you use a larger aperture, you have a less precice measurement of the position, allowing a more precice measurement of its momentum.

32

Strictly speaking, the gyroscopes’ momentum is not used to orient the HST; the gyroscopes’ orientation provides a frame of reference that is used to orient the HST. The necessary momentum is provided by “reaction wheels”, which are basically glorified flywheels, only there’s no need to keep them “spun up”; you just change their angular momentum as the need arises. I coulda sworn I’d read that it uses Earth’s magnetic field to orient itself; at any rate, I retract that.

Question for astronomer types, one that I’ve thought about for a while now:

Say we put a telescope in Tsiolkovsky Crater, far side of the moon, and a second one like the HST in low Earth orbit (or say at the L2 point). Could we hook them together in a VLA-type configuration and have a telescope with an effective aperture of 400,000 km? I mean in theory it should be possible, but what are the practical problems?

How about if we used L2 and L3 instead so that the two 'scopes had a smaller relative velocity?

33

No, you were right the first time; the Hubble does have magnetic torquers to “unload” the reaction wheels. This obscure NASA page mentiones it, as well as many others. Reaction wheels are great for everyday pointing and stabilization needs, but to overcome the cumulative effects of various external forces, you need a device that acts on something outside to apply torque. Like throwing stuff off (i.e. rockets) or grabbing on to something (i.e. the earth, via its magnetic field).

Interferometry works by comparing the phase of the observed light/wave at various locations. Radio interferometry is relatively easy because radio waves are detected as waves - you have a complete record of the phase of the incoming wave. So you can just observe the same target at several points on (or off) the earth, FedEx the data tapes to a computer center and analyze the image. Optical interferometry is considerably harder because light is detected as particles - there is no record of phase. So instead of detecting the light at each telescope and comparing the data, you need to use mirrors and lenses to combine the actual light.

Such interferometer is essentially just a section of a huge telescope. And the mirror of a telescope needs to be accurate to something like 1/8 of the wavelength of light to yield a diffraction-limited image, if I remember correctly. So an interferometer must be accurate and stable to about the same precision. Think you can manage that between a telescope on (or near) Earth and one on the Moon? It’s difficult enough with two telescopes sitting side by side on the same mountain.

I believe a space-based interferometer has been proposed, but it’s just a big satellite with an expanding truss to connect two or three telescopes. The goal is to optically detect extrasolar planets (planets outside our solar system, i.e. around other stars).

34

There’s been a number of space-based interferometers considered, and I believe NASA is going to fly at least one in the next five years or so.

The ultimate interferometer I remember reading (theoretical, of course) has a ring of satellites in orbit around the sun, giving it an effective aperture big enough to resolve items the size of a football on planets orbiting other stars.

35

I think there have been several radio telescopes that have used interferometry from space I think they used a Japanese satellite so that the aperture was the width of the earth. As for other wavelengths I think there is one built on earth nearing the end of construction where there are a load of big high quality telescopes near each other that are gonna be linked. Doing that in space would be very hard but is possible.