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This article is from the In-Depth Report Hubble at 20: An Astronomical Success

Last Dance with the Shuttle: What's in Store for the Final Hubble Servicing Mission

A Q&A with Hubble Space Telescope senior project scientist David Leckrone



NASA

Last month marked the 19th anniversary of the launch of the Hubble Space Telescope, an orbiting observatory that has become a household name and a linchpin of astronomical science. The telescope has proved remarkably resilient, enduring numerous glitches over the years—from a flawed primary mirror at deployment to a serious electronic failure this past September. Each time, Hubble has held on until astronauts arrived to perform repairs, an operation that is about to take place for the final time by a shuttle crew.

On Monday space shuttle Atlantis is slated to lift off on the fifth and final servicing mission to Hubble (confusingly dubbed Servicing Mission 4—the nominal third mission was split into two parts, Missions 3A and 3B). Four mission specialists alternating in two-astronaut teams will attempt a total of five spacewalks from Atlantis to replace broken components, add new science instruments, and swap out the telescope's six 125-pound (57-kilogram) batteries, original parts that have powered Hubble's night-side operations for nearly two decades.

To find out what a refurbished Hubble will be capable of and how long the telescope will operate without further service, we spoke to astrophysicist David Leckrone, senior project scientist for the Hubble Space Telescope at the NASA Goddard Space Flight Center in Greenbelt, Md.

[An edited transcript of the conversation follows.]


From the Hubble team's perspective, what are the goals for this shuttle mission?
This is our final opportunity to service and upgrade Hubble. So we're replacing some items that are getting long in the tooth to give Hubble longevity, and then we'll try to take advantage of that five- to 10-year extra lifetime with the most powerful instrumental tools we've ever had on board.

We have to do maintenance on the spacecraft itself, like replacing the batteries. There are six batteries that were launched in 1990 and have never been replaced—I bet you couldn't do that with your flashlight. And we have gyroscopes that help keep Hubble pointing stably so it doesn't jitter and smear out our very high-resolution imagery. These things have known average lifetimes and wear-out mechanisms, so it's time to replace all six gyroscopes. We have to replace another sensor called a fine guidance sensor that is used both to help control the pointing of the telescope in the sky and also for the science of astrometry, which is very precisely measuring the positions of stars.

It's been seven years since we've serviced Hubble, and the normal servicing interval is three and a half years or so. It's as if you're supposed to service your car at 5,000 miles, but it's been 10,000 miles and things are starting to break down—particularly within our suite of scientific instruments.

In 2002, after the last time we serviced Hubble, we had 11 different channels operating among the six scientific instruments. A channel is like an individual camera within a box; for example, we put a new instrument on board in 2002, the Advanced Camera for Surveys, that has three separate cameras in it, each with unique capabilities, and each of these cameras we call a channel. So we had 11 channels active after the last servicing mission; we're now down to three. And among those three channels, only one was really heavily used prior to recent times. So there has been significant deterioration in the tools that we use for observing the sky.

After this mission is over, if everything goes perfectly—and this is an extraordinarily complex and ambitious mission, so nobody should be surprised if we don't get absolutely everything done—we should be up to 14 channels with the very highest technology that we've ever flown on Hubble. It will be more powerful as a scientific tool than it's ever been before.

Originally this mission was scheduled for October 2008, but with the problems in September with Hubble's data formatter, it was pushed back. How has that glitch changed the mission?
That was a scientific instrument command and data handling system (SI C&DH) and its subunit, known as a science data formatter, which is absolutely essential for doing observations and getting the data back home. Luckily we had two redundant electronic sides in what is essentially a computer system. It was one side that failed, so we were able to switch over to the other side. We had never done that before, but it worked fine.

The only problem is that we no longer have redundancy. And if we risk the lives of seven astronauts and go to all this trouble to get Hubble fully up to snuff for five to 10 more years, we don't want to have a single-point failure possibility, where if side B failed, suddenly all science would be over on Hubble. We didn't want to do that, and Mike Griffin, who was the NASA administrator at the time, didn't want to do that. So he called a halt to preparations for launching in October and we got our spare SI C&DH system ready to fly.

So you will replace the A side that failed?
We'll replace both the A side and the B side. We're going to replace the entire unit.

In terms of technical upgrades or longevity boosts, what do you hope to get out of this mission?
We're putting on two brand-new scientific instruments, and then the astronauts are going to attempt to repair two, including the Advanced Camera for Surveys and the spectrograph, which are quite modern instruments but had electronic failures.

One of the new instruments is called Wide Field Camera 3, and it's going to replace Wide Field Planetary Camera 2 (WFPC2)—the jargon is a little strange. We're taking out WFPC2, which has been in the observatory since 1993, and replacing it with a really golly-gee-whiz new camera that has two channels in it. One channel is optimized to observe light in the ultraviolet wavelengths, and the other channel is optimized for the near-infrared. We have a near-infrared instrument on board Hubble already, but its technology is very primitive, whereas the new infrared channel is superb. This thing is going to just clean up.

The most important program it's going to be doing in the year following the mission is another ultra-deep field. There was a Hubble Deep Field in 1995 and an Ultra-Deep Field in 2004 or so, and those were at visible wavelengths. Now we're going to another ultra-deep field in near-infrared wavelengths. Because the universe is expanding, the light emitted by very, very distant, far-back-in-time objects is shifted to red wavelengths. It may have been emitted in the visible or ultraviolet, but by the time the light gets to us, it's been shifted by the expansion of the universe to red and near-infrared wavelengths. So if you want to look really far back in time, as far back as you can, you really need to look in near-infrared or infrared wavelengths.

This near-infrared channel will probe further back in time than any image that humans have ever taken—with the exception of the microwave background explorers, which went all the way back to the big bang.

So this an ultra-ultra-ultra–deep field, essentially. Is there a name for it yet?
That's as good as any.

The same team that's going to be doing this "ultra-ultra-ultra–deep field" worked hard on the original Ultra-Deep Field to find the faintest protogalaxies or clumps of star formation that they could. And they now have identified seven or eight objects that emitted the light we see when the universe was about 700 million to 800 million years old. We think we will push back another 200 million years or so with this new camera.

What about the other new instrument?
The Cosmic Origins Spectrograph (COS) is the other box, as it were, and it has its own two channels. It's a spectrograph, not a camera, so it takes the light from a distant light source and spreads it out into its component colors. If you measure how the intensity of light changes as a function of color, that gives you a lot of information about the medium that emitted that light—its temperature, density, rotation, chemical composition, and so on.

This is the most sensitive spectrograph ever to fly in space, to the best of our knowledge. And the combination of the spectrograph behind our telescope will allow the observers to look at very distant light sources, such as quasars, and use them as background flashlight beams. A beam of light from a distant quasar will pass through the material between the galaxies, and that material is dark—it's not what we call dark matter, but it's not glowing, it doesn't emit its own light. So you have to look at the imprint of absorption that it leaves on light passing through it. And the idea in doing this is to analyze what's called the cosmic web—the large-scale, weblike structure within which galaxies are formed.

I like to say we're going to trace the story of galaxy formation and evolution from the nursery to advanced adulthood. And COS will play a huge role in that, complementing the Wide Field Camera 3 in the process—the two instruments can work together to put together this family album of galaxy history.

As you mentioned before, this is Hubble's last servicing mission. It appears that the shuttle program is now truly entering its planned obsolescence. Is there some chance that, if Hubble manages to hang on and NASA readies a replacement spaceflight system in time, there could be another mission to Hubble?
That is principally a policy question: Do you spend more money servicing Hubble, which will be 25 years old at the end of the life extension that we're trying to achieve here?

So the planned life extension from this mission is to get it to at least 2014 or so?
That's right. And of course it's going to be a remarkably refurbished observatory with lots of new things on board, so it wouldn't surprise anyone if it kept going much longer than that, but on paper that's the objective.

So now you have an observatory that may be working fine and that has upgraded technology on it, but it's 24 or 25 years old and is a rather small telescope in space. Would you rather spend money continuing its lifetime for another five or 10 years, or would you rather invest that money in building a similar telescope that is much bigger?

There are two camps: One camp says it's going to be a long time before we get the next big telescope after Hubble and the [infrared-only] James Webb Space Telescope, and we'll need…[Hubble's]…ultraviolet, visible and near-infrared capability. So in the interim, "Let's go ahead and plan another servicing mission using the Constellation vehicles that are being developed to replace the shuttle." The other school of thought says, "Let's use that money to go for the next big step." And right now the latter is the official policy of NASA.

I really do think we need to get on with what I like to call Daughter of Hubble, with an aperture of between nine and 16 meters, rather than Hubble's 2.4 meters. I can hardly imagine what we would see with that.

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