what is acting upon the hubble space telescope (hst) to keep it in orbit around the earth?
Happy 20th Ceremony, Hubble!
Photo by NASA
Tomorrow marks the 20th anniversary of the launch of the Hubble Infinite Telescope. I spent ten years of my life working on that magnificent machine, from using observations of a supernova for my PhD, all the way to helping examination, calibrate, and eventually use STIS, a camera put on Hubble in 1997.
Last year, I published X Things You Don't Know Most Hubble, and I don't recollect I can actually add much to it here. I also have a lot of new readers since then, and then I'll only repost it at present as my tip o' the dew shield to the world'due south nearly famous observatory.
Introduction
On April 24, 1990, the Space Shuttle Discovery roared into space, carrying on board a revolution: The Hubble Infinite Telescope. It was the largest and most sensitive optical-light telescope ever launched into space, and while it suffered initially from a focusing problem, it would before long render some of the most amazing and beautiful astronomical images anyone had ever seen.
Hubble was designed to be periodically upgraded, and even equally I write this, astronauts are in the Space Shuttle Atlantis installing two new cameras, fixing two others, and replacing a whole slew of Hubble's parts. This is the last planned mission, ever, to service the venerable 'scope, then what amend time to talk about it?
Plus, it's arguably the world'south most famous telescope (it'southward probably the only one people know past proper name), and nevertheless I suspect that there are lots of things about it that might surprise yous. Then I present to you Ten Things You Don't Know About the Hubble Space Telescope, part of my Ten Things series. I know, my readers are smart, savvy, uncommonly good-looking, and well-versed in things astronomical. Whenever I do a Ten Things post some goofball always claims they knew all ten. But I am extremely close to being 100% positive that no one who reads this blog will know all ten things hither (unless they've used Hubble themselves). I have one or two big surprises in this one, including some of my own personal interactions with the swell observatory!
Hubble took the deepest visible light image however made.
Photo by NASA, ESA, and T. Yard. Brownish (STScI)
In 2003, an astronomer (and friend with whom I worked on a Hubble projection) named Tom Chocolate-brown pointed Hubble at the outer fringes of the Andromeda Galaxy, a nearby large screw similar our own Milky way. Using the Advanced Camera for Surveys, he commanded the space telescope to basically sit down and stare at one spot for a total of iii and a half days. His goal to was to be able to get practiced information on very faint stars in Andromeda, to characterize the way stars grade in the milky way.
He certainly was able to do that (and constitute many stars younger than expected; in Andromeda's halo the stars were several billion years younger than in our ain halo), but what he also got was the deepest optical image of the Universe ever taken. Stars downward to 31st magnitude can exist seen in the information – those are stars one x-billionth as brilliant as what you can see with your unaided eye!
The image here shows different regions in that deep image. You can see faint groundwork galaxies, stars in both Andromeda and the Milky Way, a densely-packed globular cluster, and much more. If you dare, download a monster-sized version of the whole schmeer to encounter how powerful a infinite telescope tin exist.
The Moon is not too bright to come across with Hubble.
Photo past John Caldwell (York Academy, Ontario), Alex Storrs (STScI), and NASA
A lot of people merits that some objects are merely also bright to find with Hubble. For some express cases this is true – there's a camera on lath Hubble sensitive to ultraviolet light, and at a 2500 Volt potential too many UV photons can fry the musical instrument.
Merely that's not truthful for most of Hubble'south cameras. Actually, some of the brightest objects in the heaven have been observed… including the Moon! The paradigm shown hither is of Copernicus, a 90 kilometer wide impact crater on the Moon. Information technology wasn't actually Hubble's primary target; another photographic camera (the Space Telescope Imaging Spectrograph, or STIS, a camera I worked on for many years) was observing reflected sunlight off the Moon'south limb, and Hubble was rotated so that Wide Field/Planetary Camera 2 (WFPC2) would be able to accept snapshots of the crater.
So while the Moon is not besides bright to observe with Hubble, it is moving too apace across the sky for the 'scope to track it. So the observations were fabricated in what'south called "ambush style": Hubble is pointed at a spot in the sky where the Moon is going to be, and when the right moment arrives the images are taken. It's a very difficult operation, which is i the reasons why there are and then few observations of our nearest neighbor.
Back in 1999 I took part in a prepare of lunar observations using Hubble; we were hoping to get spectra of h2o ice splashed upward from the Moon's s pole when the Lunar Prospector probe impacted in that location at high speed. Unfortunately, the spectra were screwed up; the pointing was off by a bit and nosotros didn't see anything (information technology turns out no one saw anything using whatsoever telescope, so we didn't really miss much). Although it failed, that observation run was incredibly exciting, some of the well-nigh fun I've had using Hubble.
It observes the World… quite ofttimes!
Photo by Marking Clampin / NASA
If the Moon is not also brilliant to come across, what nigh the Earth? On average, it's much more cogitating and therefore much brighter. Well, it turns out Hubble non only has observed the Earth… it's done it thousands of times!
One trouble with using digital detectors is knowing exactly what y'all're seeing. If a star looks brighter than another, is the star really brighter, or is the electronic chip only a footling likewise sensitive correct there? You have to calibrate the chip to know exactly what it'due south doing. There are several steps in that process, merely one involves using a "flat field", observing a region of the sky that is perfectly evenly illuminated. That manner, if one pixel or another is too sensitive, you tin run across it in the observation.
With Hubble, though, every patch of heaven has some object in information technology, which would screw up the flat field. Some telescopes accept internal illumination; petty LEDs or some other method, simply using them is notoriously difficult to get an evenly illuminated field. So what can y'all do when using Hubble?
I method is to find the Globe! As Hubble orbits at eight km/sec, the out-of-focus Earth screams by. If you find for a while, objects will actually leave streaks in the prototype, and these can exist treated mathematically to produce a apartment field. The image shown here is but such a "streak apartment". That's a Hubble observation of our home planet, with objects flying past. Information technology's hard to say what they are, exactly. It depends on where Hubble was when the image was taken, and where it was pointed. They might exist trees, hills, valleys, mountains, or even houses!
But don't worry, it can't see people. If the Moon is as well fast to track, the World is certainly out of the question. But y'know, the visitor that made Hubble's mirror had an atrocious lot of those same sized mirrors lying around, and in that location are no other astronomical telescopes (you know, telescopes that point away from the Earth) with that same mirror. So what could those mirrors have been for?
Hmmm.
Hubble once observed… wait for it… wait for it… THE SUN.
Photo by Glenn Schneider
Oh, I got you with that ane, didn't I? Admit information technology: you had no idea that Hubble actually and for real one time observed the Sun, on purpose. I didn't know virtually it for a long time, until my friend and fellow astronomer Glenn Schneider clued me in. Glenn is a surprising guy in many ways – he chases solar eclipses all over the planet, for example – but this ane was a doozy.
He has the whole story on his website. The short version is that some kinds of electronic detectors get extra electrons trapped in them, kinda like plaque in your arteries. One way to flush out these extra electrons is to flood the detector with ultraviolet light. The fries used in the original Wide Field/Planetary Camera launched with Hubble suffered from this, and so they needed that UV flood. And information technology turns out there's a fairly vivid source of UV lite in infinite…
Perhaps you see where this is going.
So the engineers rigged WFPC with a niggling mirror that stuck outside the camera. This role of the photographic camera was actually mounted flush against Hubble's side, so the mirror stuck out from the 'scope similar a wee periscope (there'southward a picture on Glenn'due south site that'll aid). It faced backwards, towards Hubble's aft end. The great observatory was and so pointed in the opposite direction of the Sun so the rear-view mirror was facing the Sun, and the sunlight was channeled right into WFPC.
The consequence is the image above: a bona-fide 100% actual image (well, mosaic) of the Sun taken by the Hubble Space Telescope.
How freaking cool is that?
Hubble cannot encounter the Apollo artifacts on the Moon.
Photo by NASA / Phil Plait
This question is sent to me roughly in one case a month, and sometimes even more often: why don't nosotros shut up the people who call up the Apollo Moon landings were faked by pointing Hubble at the Moon and taking pictures of the Apollo sites?
Well, one reason is that, duh, NASA and astronomers accept amend things to practise than try to prove something blaringly obvious to people who would simply claim the resulting images are faked anyway.
But also, Hubble cannot see the artifacts on the Moon! They're mode too small.
This surprises a lot of folks, since they're used to seeing razor-sharp images of nebulae and galaxies. Notwithstanding, remember that while those objects may be far away, they are also very, very large. Light years across, possibly thousands of light years across. The remains of the lunar landers are simply 4 meters across. That'south a tad smaller.
Sure, you lot say, just the Moon's a lot closer, right? Yep, it is, but it turns out it's not close plenty.
You tin can calculate how small-scale an object a telescope tin can resolve (that is, see as more than merely a ane pixel wide dot) using really basic algebra. It depends by and large on the telescope's mirror size. When you exercise this for Hubble, y'all go an angular measure of nearly 0.i arcseconds, a tiny amount to exist sure. The Moon is 1800 arcseconds beyond, so 0.one arcseconds corresponds to nearly 200 meters on the Moon! In other words, something has to be bigger than a football stadium on the Moon before Hubble tin can see information technology.
It's surprising, I know, only that'south how the math works out. The lunar lander is about 0.002 arcseconds in size, well beyond the capabilities of any normal telescope (go to that link above for more info on ways this still might work).
So really, the but – and best – way to come across the Apollo artifacts is to go dorsum to the Moon. Of course, the Moon hoax believers will still deny it'due south real. Their refusal to see reality is cosmic in its proportions.
Hubble has observed every planet in the solar system merely ane: Mercury.
Photograph by L. Esposito (University of Colorado, Boulder), and NASA
And so Hubble has observed the Sun, but information technology did so literally bass-ackwards. That was to protect its mirror; raw UV from the Lord's day can photochemically damage sensitive parts inside the 'scope, and of course can rut them up to dangerous levels. Also, as I pointed out before, some of the cameras would in fact be damaged by direct sunlight.
Because of that, Hubble is not allowed to point anywhere virtually the Sun, just to make sure no devious light seeps in. This "solar avoidance zone" is a circle fifty degrees in radius around our star. Anything closer than that is forbidden.
This directive has been cleaved by Hubble precisely once: to observe Venus, which gets nearly 45 degrees from the Sun at maximum. These observations were made using WFPC2 (shown in the image in a higher place; it was taken in the about-UV to see structure in the Venusian clouds) and the Goddard High Resolution Spectrograph. Astronomers were looking for sulfur dioxide in the atmosphere of Venus, a chemical which had been detected past an earlier probe and might be emitted by volcanism on the planet. All kinds of overrides had to exist sent to the telescope to permit these observations, and it was so difficult that information technology hasn't been and probably won't ever be repeated.
Photo past NASA/APL/Carnegie Institution of Washington
But Mercury never gets fifty-fifty that far from the Lord's day; at most it is a mere 28 degrees from the Sun, far likewise shut to ever be seen past Hubble. Merely that's OK: nosotros accept the MESSENGER spacecraft. Information technology'due south zipped by Mercury twice already, and will wing by Mercury 1 more time in the coming months before falling into orbit around the innermost planet in 2011, where it volition map the planet with far higher detail than Hubble ever could.
Hubble doesn't use lenses. Sorta.
Photograph past NASA
Even today, xix years after Hubble's launch, it'due south not all that uncommon to hear a newscaster refer to "Hubble'due south lens". I one time heard it used by an announcer on a scientific discipline show produced past the Space Telescope Science Institute, the bureau that runs Hubble!
The thing is, Hubble doesn't use a lens. It has a mirror.
Galileo used a telescope with a lens, equally did everyone upwardly until Isaac Newton. He was the genius who figured out that a properly shaped mirror could focus light as well, and has advantages over a lens: mirrors need but exist ground on one side (lenses take ii), and mirrors can be made larger than lenses because they can be supported all across their back side, while lenses must be supported around their circumference, where the glass is thinnest and near vulnerable.
Over a certain size, lenses are simply impractical, so mirrors are used. Hubble'south chief mirror is 2.iv meters across, most eight feet. Although it's the biggest mirror for astronomy ever lofted into infinite, it's considered small by basis-based standards; many telescopes today take mirrors iv or more meters across. The mammoth twin Keck 'scopes in Hawaii take mirrors made of segments that total 10 meters across each!
It turns out the cameras on lath Hubble use mirrors too. Why? Drinking glass absorbs light. Not much, maybe 2% of the incident low-cal, but that adds upwards. A lens has two surfaces, each of which reflect a lilliputian bit of light, then you lot lose more through a lens than with a mirror. Also, mirrors can exist fabricated to reflect light of different colors about the same, only lenses bend light at unlike colors differently. Then all in all, doing it with mirrors makes a lot more than sense.
Still, at that place are lenses on board: they are used in the Fine Guidance Sensors, pocket-sized telescopes that track stars with incredible accuracy and help continue Hubble locked onto to its targets.
I could non notice whatever other lenses used on the entire observatory. I read the instrument guides and asked several people who work with Hubble, and no i knew of whatsoever either. It's possible they exist and I missed them, merely the bespeak is lenses are a very rare commodity on the 'telescopic.
Non everything information technology sees is on purpose.
Photo by Phil Plait
This one's a flake personal, so allow me to expound a bit here.
Hubble has several cameras on lath. They sit in the very bottom of Hubble, in the wider portion below the mirror (unlike a normal telescope, the mirror for Hubble is located a third of the mode upward from the aft terminate). Each sees a slightly different region of the sky, separated by a few arcminutes (the Moon is 30 arcminutes across for comparison). And then if one camera is being used to look at, say, the heart of the Andromeda galaxy, and then the others are looking well-nigh the galaxy's center but not correct at information technology.
Enter the Parallels Program. When a new solid country recorder was placed on board in 1997, information technology greatly enhanced Hubble'southward adequacy to tape data (which was done using record drives before then). The pipeline was fat enough to tape data from three cameras at the same time, and so when ane was observing as the primary photographic camera, the other two could take data every bit well.
Photo by NASA / STScI
Sometimes while observing some primary target Hubble would be rotated to point the other cameras at something interesting (similar was done with the lunar observations I mentioned earlier), simply sometimes they were simply allowed to tape whatever the heck they saw. This procedure was called the Parallels Program, because the other cameras were used in parallel with the primary one.
In Oct of 1997, Hubble was pointed at the Large Magellanic Cloud, a minor milky way that orbits the Milky Mode. WFPC2 was the primary musical instrument, but the camera I worked on, the Infinite Telescope Imaging Spectrograph, or STIS, was being used as a parallel instrument. That happened a lot, and at my part the offset thing I would do every morning time was get through the previous day'southward parallels using STIS and run across if at that place was anything interesting in them.
Yes, part of my job was to look at Hubble images of regions of space no one had ever seen before and check them out. And yeah, information technology was pretty damn cool.
The bulk of the time there wasn't much to see: faint fuzzy galaxies, or a wisp of nebulosity. Sometimes the primary camera would detect a nearby galaxy many times over the course of months, and after a while just by glancing at the STIS prototype I could tell yous what galaxy it was from the brightness and density of stars. Not a terribly marketable skill, but still. Absurd.
Anyway, 1 24-hour interval we got that LMC ascertainment – the one shown above – and I noticed the fuzzy circumvolve at the top. I knew right abroad it was a small planetary nebula, a blast of gas emitted from a dying star. Yous can see information technology in the image, and it's zoomed at the lesser left. To my disappointment it had been discovered before, so this wasn't new and I couldn't proper name it. But we did get good spectra, which allowed me to accept some basic diagnostics of the nebula that hadn't been washed before.
I was able to publish my results in a paper, which also was nice. My work on STIS was awesomely fun sometimes, only I rarely got to publish anything; my proper name was always manner down the list of people who contributed to the work. And so this was a squeamish perq.
The Parallels Program still continues. I don't know what it's constitute since I left the project. Mayhap anytime I'll poke around the archives and detect out.
Yous can see one of Hubble'due south cameras in the National Air and Infinite Museum.
Photograph by Phil Plait
Like I said, Hubble was designed to exist periodically updated. When new tech makes for better cameras, old ones tin exist taken out and replaced with new ones. When STIS and the infrared photographic camera NICMOS were inserted into Hubble in 1997, the Goddard Spectrograph and the Faint Object Spectrograph (FOS) were removed.
While I was still at Goddard Infinite Flying Center, I used to take a walk around the compound after tiffin. I'd sometimes slip through i building that had a massive warehouse, and usually there was something cool to meet in in that location. I saw satellites being constructed, the upper stage of a rocket (without fuel!) on a crane, and all sorts of odd and wonderful sights.
Ane day, from across the warehouse, I spot what looks like a big black phone booth sitting on a pallet. Could it be…? I walked over, and yeah! Information technology was the FOS! I couldn't believe it. It was but sitting at that place, this camera which cost tens of millions of dollars to build. Two sides of it had been removed, and 1 had been replaced with clear thick plastic. I realized information technology must be going to a museum; the plastic would permit people to run into inside it. But one panel was still removed, and then the guts of the camera were exposed. Hmmm…
So of class I reached in and poked around. I had used the FOS for my PhD, analyzing spectra it had taken of an exploding star on two different dates. We wound upwardly not using the data because we didn't know precisely where the telescope was pointed each time, and so I couldn't compare one spectrum with another. Even so, I spent months learning how the camera worked, and seeing it in front of me was likewise tempting. Information technology was amazing; I could encounter exactly how it worked, and all those diagrams I had pored over five years before suddenly came alive.
Photo by Phil Plait
I convinced a friend to come up with me the next day to see information technology, and he took the motion-picture show above of me pretending (Aye! Pretending! That's it!) to snip the wires with a wire cutter. Haha!
Years later, I was visiting DC. I went to the National Air and Infinite Museum, having completely forgotten the incident at Goddard. I rounded a corner, and there was my one-time friend. I smiled; I knew it would terminate up here. The 2d exterior panel had been replaced with plastic, and you could see into the camera. If you compare the picture above with the one hither (click to embiggen) you lot tin can come across it'due south the same fauna.
It'due south the simply piece of Hubble I ever physically touched. Well, as well the insulating coating that flew on Hubble for years and was taken back to Earth later a servicing mission. Someone had draped the shiny silverish blanket over a chair in a room nosotros used to examination STIS. When I saw it, I… hmmmm. No. That'due south a whole 'nuther story.
You can look at all the images it has ever taken, equally long as they're over a twelvemonth one-time.
Photo by NASA / STScI
Since its launch in 1990, Hubble has orbited the Earth over 100,000 times and taken something like a half million separate observations. Those figures solitary are a fleck staggering. But did you know that you (potentially) take access to those images? Well, most of them, anyway.
All the data taken by Hubble that is more than one yr onetime is stored in an archive that the public can query. Want to know what Hubble was observing on your birthday two years ago, or at the moment your child was born? Just ask the database! In many cases, when you search the database, you can besides go a preview of the epitome; the higher up shot is of the spiral galaxy M51, also called the Whirlpool Galaxy. The preview shows the raw data correct off the 'telescopic; it'south not always particularly pretty. To adorn it you need to process it, which ways subtracting a night frame, a bias frame, dividing by the flat field, flagging bad pixels, combining multiple exposures to become rid of cosmic rays, performing a geometric correction… and if you want color, you lot accept to do that for the other filters used in the observation, and and so combining those using Photoshop or another software.
Obviously, non everyone tin do that (it's a lot harder even than it sounds). So not anybody is allowed to really call up the data; that would strain the annal servers. To do that you have to justify the need and get an business relationship. I used to have 1, but I lost my password a long time ago. Probably all for the best; I'd just download gigabytes of cool images and get everyone at the annal ticked off at me.
Oh, about that "… as long as they're over a year onetime" thing: data is proprietary to the person who took information technology for the menstruum of one yr, so the scientists involved have time to look it over. It does take some time to process the data, and a lot more time to analyze it; if everyone had instant access to all the information, someone more experienced than you could scoop you on your own observations! Yet, information technology'due south likewise not off-white to allow people accept the data forever. The compromise is the one year proprietary flow; that gives scientists time to look things over, only however motivates them to go things done. I call up this is a fine idea, and it even works in practice in the real world, amazingly. If a scientist wants, they tin can release the information early on, too, so everyone wins.
In fact, I used old data quite a bit dorsum in the day. If we found something interesting in our ain data, we could go await for older observations to come across if it had been seen earlier, or if there were related observations. And many times, even if the older data were all the same proprietary, the scientists involved were interested in collaborating. Funny matter almost scientists: in lots of cases they are open, friendly, and interested in seeing what everyone else is doing. There were exceptions, of course, but that'southward what I found for the most part.
Maybe that's the matter that'll surprise you lot most in this commodity. But it's truthful.
Photo by NASA
Conclusion
Choosing simply ten things for this commodity was, as usual, tough. I can think of lots more things to add together: JWST won't replace Hubble, information technology succeeds it; Hubble isn't really a telescope, it's a whole observatory; it has flown the finest UV photographic camera ever built, which was so sensitive that a massive and hot O-type star in the Andromeda Galaxy could have damaged it (and once I about blew it up); when there is a strong meteor shower, they point HST in the opposite direction.
There are tons of things about Hubble that I'm sure I don't know either; I worked on information technology for a decade, but in fact I haven't worked on information technology for nearly a decade since. It's a complicated and beautiful machine, and it changed the style we look at the Universe, peradventure forever. It certainly inverse the style scientists practise astronomy… and I know that the best thing it did, the all-time thing it could do, was to let people see only how phenomenally gorgeous our Universe is.
And for that, I'm very grateful. And that'due south one thing I do know.
byfordcomprignate.blogspot.com
Source: https://slate.com/technology/2010/04/happy-20th-anniversary-hubble.html
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