A week ago, April 23rd, I was in Washington to see my first NASA press conference, held at the Newseum for the 25th anniversary of the launch of the Hubble Space Telescope (HST). Afterward, I and the other attendees at the NASA Social were taken out to NASA’s Goddard Space Flight Center. We first learned about the James Webb Space Telescope (JWST), then got some Hubble Space Telescope (HST) history and saw the Satellite Servicing Capabilities Office (SSCO).
We got a peek in through the window into the HST mission control room. Commands are sent up to HST after being carefully checked and double checked to make sure they don’t accidentally instruct the spacecraft to do something stupid and/or fatal.
For many years all of the command consoles were staffed 24/7/365. With the upgrades both on HST and on the ground, many of the operations no longer require constant monitoring. There is an extensive system in place to alert Goddard staff when anything goes “off nominal.” Minor issue will result in a text message or email, more critical problems are met with more aggressive alerts.
In the HST mission operations center we saw where the data from Hubble is coming down and the astronomical observations were monitored. There was data everywhere on dozens of big monitors — heaven of the little kid inside me who still likes to see buttons pushed and lights flashing.
In particular, I asked about the “pickle” diagram, visible on the center monitor closest to us. It shows how Hubble is using its three Wide Field Detectors (WFDs) to track stars in the current field of view. With two detectors tracking stars, Hubble can maintain tracking accuracy to less than 7 milliarcseconds per day.
How good is that? A circle around the sky is 360 degrees. Each of those degrees is split into 60 arcseconds. Now split each of those arcseconds into 1,000 bits. That’s a milliarcsecond. In the real world, 7 milliarcseconds is the size of a dime on the Washington Monument in DC as seen from the Empire State Building in New York City.
The HST is the size of a school bus, weighs twelve tons, and is floating weightless in space. How do they keep pointing it that accurately? (Assuming it’s not some serious black magic.)
These little guys are key. Some of the most accurate gyroscopes ever made are at the heart of these rate sensor arrays (RSAs). Hubble has six of these assemblies, two each on each of the three axes. Usually they run just three at a time, one from each set.
These gyros and RSAs are among the hardest working systems on Hubble and were replaced on three of the five servicing missions. In fact, the failures of gyros were the reason that the third servicing mission was broken into two separate Space Shuttle missions. Four of the six RSAs had failed by November 1999, putting Hubble into a hibernation or safe mode. With less than three working gyros, Hubble could have started drifting and tumbling, making it difficult or impossible to capture by the astronauts on the next shuttle servicing mission. So the planned June 2000 mission was split, with STS-103 going up to replace the RSAs (and other things, such as the primary onboard computer) in December 1999, while STS-109 went up in March 2002.
Hubble doesn’t use thrusters or jets to control its movements in space. For one thing, the gasses used (primarily hydrazine) are very nasty to have around delicate optical instruments. In addition, once the fuel is gone, so is your ability to control your attitude. (Remember the bit yesterday about RRM?) So Hubble is controlled by four massive Reaction Control Wheels (RCWs) which move the spacecraft by gyroscopic action. (For a lot of detail and technical minutia on the Hubble guidance system, see this NASA technical paper.) In high school, did you ever do the experiment where you sit on a bar stool and hold a spinning bicycle wheel, tilting the wheel to make you spin around? That’s how Hubble moves, but with wheels that weigh hundreds of pounds each and are precision machined.
In addition to the gyroscopes in the RSAs, Hubble also senses its position in space using large, sensitive Charge Coupling Devices (CCDs), the same kind of sensors that are at the heart of your digital cameras. Two CCDs are built into each WFD and there are three WFDs on Hubble. (Remember the “pickle” diagram above?) As long as two of the WFDs have a star to track in their field of view, the WFDs and RSAs combined can give Hubble that 7 milliaarcsecond per day guidance.
That’s some seriously awesome engineering there!
Originally HST was launched with three Engineering & Science Tape Recorders (ESTRs) for recording and playing back data. Designed in the 1980s, the ESTRs were reel-to-reel tape assemblies. Data was written and retrieved sequentially and when the tape broke or jammed (as this one did) the ESTR was useless. These ESTR units held 1.2 gigabites of data, state of the art at the time.
The ESTRs were one of the components of HST which were designed from the beginning to be upgraded as technology advanced. They were replaced on the second and third servicing missions with solid state recording units. The upgraded units are like huge, radiation hardened memory sticks. Not only do they hold over ten times as much data as an ESTR, but they can also be read instantaneously instead of sequentially, and sections of memory which become damaged (perhaps by a radiation hit) can be bypassed, leaving the rest of the unit still functioning.
For those interested, I believe these three pieces of hardware were all flight-flown, coming back down from HST after being removed and replaced during a servicing mission.
Dr. Marc Kuchner wants your help in looking for targets for HST, and later for JWST. There’s a crowd sourced (Zooniverse) project called “Disk Detective” in which you can do real, honest-to-god science in your spare time.
The short version is that stars with planets are stars surrounded by dust due to the planets, asteroids, comets, and other assorted objects crashing into each other every so often. Because of celestial mechanics and conservation of angular momentum, the dust tends to flatten into a disc or ring. Conversely, we’re finding that when we see a star with a dust disc, we often find planets there.
It’s time consuming and inefficient to have have telescopes like Hubble look at every single star looking for planets, so we would like to improve our odds and find another way to narrow the search. A key tool here is the Wide-field Infrared Survey Explorer (WISE) mission, a space-based telescope which looks at big chunks of the sky at once, but at lower resolution than telescopes such as Hubble.
Dust is bright in the infrared part of the spectrum since it absorbs starlight and re-radiates the energy in the infrared. Therefore, if we look at a star in WISE images and find a dust disk, that’s a good candidate to look at more closely using Hubble or another big telescope.
But how do you look at all of the stars in the WISE images? Computers? Not really. It turns out that computers aren’t that good at examining images and “looking” for certain telltale characteristics. But the human eye and brain are pretty good at that. But there are billions of stars. So what if a whole lot of people each looked at a few dozen or a few thousand stars each?
That’s how Disk Detective works. If you go to the site it will show you a series of ten images for a single star, each image in a different wavelength. You can flip through the images as often as you want, then answer six simple questions about the images, such as if the object is moving. This only takes a minute or two, you submit your answers, and go on to the next image.
If multiple people independently judge a particular image to be a good candidate to have a dust disc and possibly planets, then the pros can take a look at it, possibly moving the observations up to a much bigger telescope, or even up to Hubble. It’s a piece of cake, and beats the hell out of playing Solitaire on your computer! Give it a try, maybe you’ll be the one who finds another new planet.
Finally, we got to talk to two of the engineers who helped to design and built the tools needed to perform the delicate tasks of the Hubble servicing missions. (Again, I apologize for not getting their names – if anyone can fill in the blanks for me in the comments, I would appreciate it.)
Here we see a panel that was designed to go over a series of over a hundred screws and bolts that had to be removed, without allowing any of them to get away or fly loose. A single nut, bolt, washer, or screw that escaped and drifted into the telescope could cause an electrical short or damage a lens or mirror, causing an incredible amount of damage. And remember, this work was being by astronauts wearing spacesuits with very limited mobility and dexterity, floating weightless, often with poor or little lighting. The astronauts described it as being like performing brain surgery while blindfolded and wearing oven mitts.
In order to safely open panels and instruments that were never designed to be opened while simultaneously preventing any loose bits from drifting away, some very complex tools were designed and built. For example, the blue panel above was attached in place with bolts screwed in using the big handles (easy to use with gloves). The clear holes lined up over the screws that needed to be removed, and the holes in the plastic were just big enough to allow a screwdriver tip or other tool to get through while still being small enough to not let the screw escape.
Another problem was dealing with sharp edges, knives, and cutting tools. In a pressure suit surrounded by hard vacuum, NASA doesn’t want astronauts handling a knife or anything sharp. When this top panel had to be removed with a whole series of cuts, this tool was built to screw on (again, big handles, easy to use in stiff spacesuit gloves), cut the top of the compartment off with blades that were recessed and not a danger to the spacesuit or gloves, then pull off with all of the loose bits captured inside.
Other new tools had be developed to make the Hubble repairs feasible. For example, the standard Pistol Grip Tool (PGT) which we saw in the SSCO is used to remove and install screws and bolts – but it’s really slow. For something like the job above with the blue panel, which had 100+ screws, that would take forever. So other faster, more lightweight tools were developed for the Hubble repairs. In addition, since they would be working in the dark a lot, let’s put some LED lights on there so we can see what we’re doing while looking out through that spacesuit helmet. Right?
Yeah, this is the one they developed (I believe it was flight-flown), and yes, they let me hold it.
Geek joy!!
FYI, it’s heavy and awkward to hold even there in the auditorium using just my bare hands. We can’t ever give enough credit to the astronauts who pulled off five astonishingly successful service missions, giving us one of the landmark scientific instruments of our generation.
Flip through the images that we’ve gotten from Hubble.
Go see one of the IMAX 3D movies about Hubble, or watch the PBS Nova special from last week.
Read about some of the discoveries that Hubble has allowed us to make in the last 25 years.
Study how the initial problems with the Hubble optics were overcome. It’s a classic study in recovery management, how an initial critical error, particularly a very public and very expensive one, can be faced head on and resolved, leading to one milestone achievement after another.
That’s why we’re celebrating twenty-five years of Hubble’s observations.
That’s why we’re looking forward to many more years of Hubble’s observations.
It was a great NASA Social!
We Love The Stars Too Fondly 