It will be a long two years for the science team. Margon can barely contain his enthusiasm when talking about what the two new science instruments will allow Hubble to do. The Wide Field Camera 3 (WFC3) and the Cosmic Origins Spectrograph (COS), both with built-in corrective optics to compensate for a flaw in Hubble's primary mirror, are expected to boost the observatory's data output 44 times above what it was 10 years ago.

WFC3 is the first panchromatic instrument built for Hubble, a wide-field camera with a wide spectral range that will open new windows on the universe and, at the same time, restore lost visual performance due to radiation damage in other detectors. In the near ultraviolet, WFC3 will boost discovery efficiency by 40 percent while the near infrared detector will allow much faster surveys.

The new camera will capture stunning views of planets in our solar system, distant Kuiper Belt objects and all the other usual deep space targets.

"The thing WFC3 has that's particularly exciting is sensitivity into the near infrared," Margon said. "The reason that's important is, once again, the red shift. If you want to look at the distant universe, it gets redder and redder as you look farther and farther away. Hubble is a general purpose telescope, it will look at everything, planets, stars, galaxies, all that. But the problem that probably excites people the most right now is this issue of the dark energy, which is accelerating the expansion of the universe. And that's a problem that didn't exist when Hubble was launched.

"The status of this dark energy now is, everybody agrees it's there, which is itself pretty astonishing, and that the dark energy that is responsible for accelerating the expansion is actually 75 percent of the matter/energy budget of the universe. So not only is it there, but it's the overwhelming form of stuff, even though five years ago we had not even a glimmer that it existed."

Hubble has played a major role in the ongoing search for answers, by finding distant Type 1A supernovas, stellar explosions thought to occur when a compact white dwarf in a binary star system accumulates enough mass from its companion to reach a critical density. At that point, the quantum mechanical property that had been resisting the inward crush of gravity is overwhelmed, triggering a catastrophic collapse and explosion.

Because the explosions occur at the same mathematical point - the moment the star's mass exceeds roughly 1.44 times that of the sun - astronomers believe their energy output is roughly the same. Thus, the light output of a Type 1A supernova can be used as a so-called "standard candle."

The apparent brightness of an object drops off with distance from the observer in a precise way and observations in the late 1990s showed Type 1A supernovas in remote galaxies were dimmer than expected. The most obvious explanation, assuming the supernovas really do behave like standard candles, was that the universe had expanded more - and that the supernovas were more distant - than would be expected if the cosmic expansion was slowing down.

Astronomers believe the dark energy driving that acceleration has been present since the big bang, but it was overshadowed by gravity through the first five billion years or so of the cosmic expansion. But as the universe thinned out and its density dropped, dark energy began reversing what to that point had been a gravity-driven deceleration. And so, the universe began accelerating and flying apart faster and fast.

Hubble has found the most distant Type 1A supernovas, helping scientists confirm the idea of dark energy. The problem is, Margon said, "nobody knows what it is, nobody has any clue as to why it's there, what its form is, it's just there. The next thing you want to ask is what the hell is it? Is it Einstein's cosmological constant, is it something else?"

"It turns out, an extremely sensitive test of what form the dark energy is in is to just ask how does this 'oomph' change with cosmic time? How does its importance change with cosmic time? And Einstein's cosmological constant, this repulsive gravity, it doesn't change at all with cosmic time. But if, for example, we're part of a multi-dimensional universe and there are other dimensions pushing on us and stuff like that, those things change with cosmic time.

"So the way you can actually probe that, of course, is to simply look backwards and look at distant objects, because then you're testing the geometry of the universe in the past. And if you ask how far back do I need to look to start to make a difference amongst the different ideas about what dark energy is, it turns out to be a red shift that corresponds very, very nicely to the reddest sensitivity of Wide Field Camera 3. Mostly by good luck, I've got to say!

"So if you can just continue to map out the deviations from the Hubble diagram (classical expansion) of very distant galaxies out in the reddest band where Wide Field 3 works, you should be able to differentiate between models of the dark energy. ... That I think is really exciting."

The Cosmic Origins Spectrograph, twice as sensitive as STIS and 10 to 20 times more sensitive than earlier instruments in medium and high resolution spectroscopy, offers equally exciting science.

COS was designed to study the large-scale structure of the universe, the intergalactic medium, the origin of the elements, the formation and evolution of galaxies, the interstellar medium and the formation of stars and planets.

"We now understand that the universe has sort of three slices of the pie," Margon said. "There's dark energy, which is about 75 percent. There's dark matter, which is about 20 percent. And then there are atoms (of normal matter), which is just about 5 percent. But something that there have been glimmers of for about 50 years and now we're finally quite certain of, is that in the atoms-we-know category, most of them are not contained in stars and galaxies, but are rather contained in a very dilute gas in between galaxies.

"The original naive picture of the way the universe was put together was that galaxies were the building blocks and in between galaxies there was essentially a perfect vacuum. Gradually, creeping up over 50 years, the picture is actually reversed. It turn out that probably more than 50 percent of all normal atoms are between galaxies, rather than inside them. Which, of course, continues to drive the Earth, sun and things we know to more of a footnote."

So how does one study the intergalactic medium, or IGM? By looking at distant objects like quasars and figuring out how that light was affected by its passage through the IGM on its way to Earth.

While COS is a general purpose instrument and will be used by astronomers to study a variety of targets, "sort of the motivating design problem was to look at very distant quasars, just as background targets, and your line of sight to them will have to traverse a huge number of these atoms in the intergalactic medium," Margon said.

"It turns out that given the conditions in the intergalactic medium, the only place they will interfere with light from those distant quasars is in the ultraviolet. ... The critical diagnostics cannot be reached from ground-based telescopes. And again, because you need to observe in the UV, there's no future clever technological development from ground-based telescopes that will overcome that. Nobody's going to invent some device to observe light that doesn't arrive.

"So characterizing the state of this intergalactic medium, where most atoms reside, is kind of the father problem for the Cosmic Origins Spectrograph. That's why it's called 'cosmic origins.' Because that dilute medium is the medium out of which galaxies and stars eventually collapsed. But it turns out what has been left behind is, in fact, the majority of the atoms in the universe. It's probably 90 percent hydrogen and 10 percent helium. Everything else, with the exception of just very trace amounts of lithium and deuterium have been built up later in stars."

It is not yet clear how uniform the IGM might be - the degree to which it is lumpy, filamentary or smoothly distributed - but COS may help find the answer.

"As we see absorptions, as we see interference in the spectra of background objects caused by the intergalactic medium, those pieces of matter will have characteristic red shifts depending on how far away they are," Margon said. "And so COS will take these ultraviolet spectra of very distant objects and will ask, are there discrete interruptions of the spectra that correspond to discrete red shifts, in which case it would be very lumpy. Or are there just kind of absorptions everywhere through the spectrum, in which case it might be more uniform. Nobody really knows."

But the answer, Margon said, "actually has very profound cosmological data in it."

"The lumpiness bears an imprint of conditions very early on in the big bang because there's essentially nothing to change it later," he said. "So aside from probing the majority of atoms in the universe, you also end up getting fundamental cosmological information about what were the conditions the instant after the big bang."

A lot has to go right between now and launch for Servicing Mission 4 to get off the ground and any major shuttle problem easily could bump the flight from the manifest. But just having a second shot at the mission after its termination in 2004 has buoyed spirits among the scientists and engineers who have dedicated their careers to keeping Hubble at the forefront of astronomical research.

"We're all pretty excited about it," Burch said. "It's been like a roller coaster, I'll tell you that. Boy, the ups and downs. Holy cow!