Beyond Hubble

Though our orbiting 2 Telescope is stillgiving us an unprecedented view of the heavens, within a decade or so, accumulated glitches will finally make THE HUBBLE go dark. But don't worry: NASA has a fleet of new telescopes aimed at a universe we can now only imagine.

By David H. Freedman, Dana Berry|Sunday, February 01, 1998
RELATED TAGS: TELESCOPES
 

The Space Infrared Telescope Facility

In March 2002 the space shuttle will pay its fourth and final maintenance visit to the Hubble Space Telescope. When the work is complete, the shuttle’s robotic arm will affectionately nudge the cylindrical, bus-size observatory into the twilight of its illustrious career. From then on, whatever breaks stays broken. With luck, the Hubble could beam back breathtaking pictures for another ten years or so. But sooner or later entropy will claim its due and the Hubble will go black.

This event, however, will not mark the end of NASA’s foray into space-based telescopy. No fewer than four big-science telescopes are slated to leap out of NASA-sponsored labs into outer space over the next few decades. Their goal isn’t to replace the Hubble—it’s to better it, by a long shot.

The Hubble will be a tough act to follow. Its 94.5-inch mirror has captured startling views of everything from Pluto to stellar clusters to galaxies near the fringes of the observable universe. But astronomers are junkies when it comes to good views—the more they get, the more they seem to want. Even now, the Hubble is so overbooked that during its remaining expected lifetime it will likely be able to look at only about one-third of the celestial hot spots that astronomers have requested. Its capabilities are even beginning to seem a little limited: its mirror is too small, its focus too narrow, its heat too obscuring, its near-Earth neighborhood too crowded.

The coming telescopes will use new technologies and approaches to overcome such obstacles, enabling them to see farther, wider, and more clearly. In some cases they will take up investigations from where the Hubble left off; in others they will be able to answer questions that the Hubble couldn’t touch. Perhaps the most exciting of these, and one that has increasingly captured the attention of scientists as well as the public, is whether life exists elsewhere in the universe.

“In a sense, the primary goal of NASA is to answer the question, Are we alone?” says Harley Thronson, a project scientist at NASA’s Office of Space Science. “These missions will have thrilling implications not just for scientists but also for the person on the street.”

The Space Interferometry Mission

If any single image could represent the triumph of the Hubble, it would probably be that of the luminously multicolored Eagle nebula and its three “pillars” of dust, through which peeks the veiled light of a handful of newly formed stars. But we may not have seen anything yet. If all goes as planned, in 2001 a newcomer with the ungainly name of SIRFT will take off and, to some degree, humble the mighty Hubble. “The Hubble peters out where the dust starts,” notes Jet Propulsion Laboratory project scientist Michael Werner. “SIRTF will look through the depths of the dust and find the rest of the stars.”

The Hubble, like most telescopes, is blind to about half the light in the sky. For the most part, its detectors are tuned to see the same type of light that we do: that of the optical, or visible, spectrum. Visible light is great for avoiding furniture and looking at mature stars, but many of the most provocative objects in the universe spill their secrets in invisible handwriting, in forms including X-rays, gamma rays, ultraviolet light, and radio waves. Of particular interest to astronomers these days is the light that reaches us as infrared radiation—the radiation given off by warm objects.

But infrared observation is strictly a space-based game. “Doing infrared astronomy from the ground is like doing optical astronomy at high noon,” sniffs Werner. “The atmosphere’s heat emissions make the sky bright in the infrared range.” And that, of course, obscures the view into space. Above the atmosphere, the view is literally a million times better. As long as you keep cool, that is. The Hubble can’t be equipped to see more than the near-visible end of the infrared spectrum, because basking in the sun and the heat of Earth keeps it at nearly room temperature, making it the equivalent of a fluorescent bulb, as far as infrared light is concerned.

That won’t be a problem for SIRTF—the Space Infrared Telescope Facility—because it’s going to be packing enough liquid helium to keep it at a crisp 450 degrees below zero Fahrenheit, or just 10 degrees above absolute zero. SIRTF will also enjoy a better neighborhood than the Hubble’s. “Besides constraining the view,” Werner explains, “being in Earth orbit exposes a telescope to Earth’s thermal impact and heats up the outer shell, so that the only way to keep the telescope cool is to put it inside a thermos bottle. But we’re not going to orbit Earth. We’re going to push away from Earth’s gravity and fall into an orbit of the sun, trailing behind Earth and gradually increasing our distance from it.” SIRFT will fall back about 10 million miles a year. This arm’s-length approach not only minimizes Earth’s infrared glare, it also allows NASA engineers to get away with a single-layer protective shell rather than a hefty thermoslike shell of two layers separated by a vacuum.

With a mere 33-inch mirror, SIRTF won’t be able to match the sheer light-gathering horsepower of the Hubble’s 94.5-inch reflector. But its infrared-sensitive detectors will open up worlds the Hubble couldn’t approach. It should, for example, be able to spot “superplanets” and brown dwarfs—would-be stars too small for their gravitational pressure to ignite fusion and give off lots of visible light. They might account for a chunk of the “missing mass” that scientists believe must pervade the universe. SIRTF will also be able to examine the birthplaces and cemeteries of stars—the clusters and nebulas that are created from and ultimately explode into clouds of gas and dust. These clouds are like blackout shades to the Hubble, but SIRTF can see right through them to the stars behind. Or it can focus on the radiation emitted by the warmth of the dust itself. “Anything that’s cool or buried in dust will be a natural for SIRTF,” Werner says.

In the astronomy business, distance is time. Because light travels at a finite speed, the images we see of distant galaxies are made of light that left the galaxies long ago; the farther the galaxy, the farther back in time we’re peering. When the Hubble revealed young galaxies from about 80 percent of the way back to the Big Bang, the very beginning of time, the images caught astronomers by surprise. Many had expected to see big, fuzzy clouds of stars, which presumably contracted to form big galaxies such as the Milky Way. Instead the Hubble revealed smaller versions of mature galaxies. Most alternative theories now have galaxies starting small and then growing larger through successive collisions. To verify this rugby-scrimmage view of the early universe, astronomers need to see even younger, tinier proto-galaxies, at about 90 percent of the way back to the Big Bang. The expanding universe is taking these extremely distant galaxies away from us so fast that the light waves they emit are being stretched out—or Doppler-shifted—into the infrared part of the spectrum. That makes them invisible to the Hubble but right up SIRTF’s valley. “SIRTF will be a time machine for us,” Werner says. “We should know once and for all how the galaxies were formed.”

SIRTF won’t be powerful enough to spot Earth-size planets, but over its five-year operating life it may be able to find the disks of dust around stars that astronomers believe are associated with planet formation. And SIRTF may be able to pick up signs of the carbon and water vapor in these disks that would further enhance the prospects of their harboring life-supporting planets. Those kinds of findings would undoubtedly guarantee its popular appeal. “People relate to the discovery of water,” says Werner. “It has that anthropomorphic ring to it.”  

Over the past few years, ground-based telescopes have discovered a dozen stars that might be accompanied by Jupiter-size planets, some of which are broiling in orbits tighter than Mercury’s. That seems odd, but it doesn’t mean that such big, hot planets are the rule; it may simply be that ground-based telescopes aren’t well-suited to finding other types of star-planet systems. These telescopes rely on detecting any Doppler shifting of the parent star caused by an orbiting planet tugging it this way and that, but this method is vulnerable to interference from eruptions on the star’s surface and other distractions. A more certain method would be to observe the star’s side-to-side wobble directly. Detecting the wobble caused by an Earth-size planet orbiting at a more temperate distance from its sun is out of the question for a ground-based telescope. It would require a space-based telescope mirror with a diameter of 30 feet, nearly four times the size of Hubble’s.

Or maybe not—it turns out there’s a loophole. The resolution of a big mirror can be nearly duplicated by several smaller mirrors separated by a distance equal to the diameter of the big one. The trick is to combine the light waves received by each of the smaller mirrors so that the waves line up, creating an interference pattern—a bright spot where the peaks of the separate waves overlap, and darkness where the troughs overlap. Any change in the viewing angle will throw off the alignment and the pattern of light and dark. Since even the wavelength of infrared light is so small, a change of millionths of an inch is enough to cause the peaks and troughs of light received by the mirrors to fail to line up. This sensitivity increases the farther the mirrors are from one another.

NASA will be launching such an arrangement into orbit in March 2005: the Space Interferometry Mission, or SIM. Its express purpose is to detect minute star wobbles and the potentially life-supporting planets those wobbles imply. “By combining seven smaller telescopes to synthesize the accuracy of one large one,” says Michael Shao, the scientist at the Jet Propulsion Laboratory who heads the SIM team, “we’re going to be able to search the nearest 40 or so stars to find planets that are from one to two times the mass of Earth and that are in a habitable zone around their stars.”

 

SIM will have a set of seven one-foot-wide mirrors strung along a 30-foot-long boom that will fold up for launching. All seven mirrors combined gather less than one-hundredth as much light as one 30-foot-wide mirror, meaning they won’t pull in the faintest objects in the sky. And since interferometers look only at points of light rather than entire objects, SIM also won’t produce the sorts of spellbinding pictures that the Hubble spoiled us with. But when it comes to locating the position of an object precisely, SIM will be the boss—about 1,000 times better than the Hubble, which is already twice as accurate as any ground-based telescope.

With such precision, star wobbles shouldn’t be a problem. SIM would easily be able to spot the wobble of our sun caused by Jupiter from a distance of 30 light-years—and there are about 400 stars within that distance of Earth. Although SIM won’t be able to spot the meager wobble caused by a planet as scrawny as Earth at that distance, it should be able to comb the nearest 30 to 50 stars for Earth-size planets.


NGST’s large mirror will gather nine times more light than the Hubble’s. That will bring into view stars and galaxies never before spotted by any telescope anywhere at any wavelength.


SIM will have plenty of other duties over its five-year expected life span. It will pin down the exact positions of thousands of stars, including many at the edge of our galaxy. It will provide the first opportunity to make out individual stars in the tightly packed conglomerations known as globular clusters. “From a ground-based telescope everything looks blurred in the middle of these clusters because of atmospheric distortion,” says Shao. “Even the Hubble sees mostly a blur. But with SIM we should be able to see all the individual stars that make up the cluster center.” SIM will also measure the motions of stars near the center of many galaxies, which should tell us whether they harbor enormous black holes at their core.

The Next Generation Space Telescope

Building such a precise instrument and launching it into space should prove to be a challenge. “To keep an interferometer stable on the ground, you just build it on 250 tons of concrete,” Shao says. “But in space it has to be extremely lightweight, which means we can’t make it rigid. It’s going to be so flexible that if we erected it on the ground it would collapse under its own weight.” In space the main problem is keeping the elements aligned and vibration-free. The motors and some of the electronics will be isolated on a computer-controlled shock absorber that measures the vibrations and cancels them out with counter-vibrations. This setup should keep the mirrors in their proper positions to within a few ten-millionths of an inch.

But that’s not nearly good enough for the sorts of delicate observations SIM will be making. “We need to be able to control the location of the mirrors to within twice the size of an atom,” says Shao. A series of laser beams will run alongside all of the telescope’s elements and into detectors, which will sense any vibrations the shock absorbers let through. Then an onboard computer will compensate for the movement by fiddling with the light waves coming from each of the seven mirrors.

Shao says most of the individual technologies needed to keep SIM on target have been demonstrated in the lab—it’s putting them all together that keeps him awake at night. “The software is going to be amazingly complex,” he says. “And we won’t know if it all works until we try it.” By 2000 Shao expects to have a full-scale model hanging from the lab’s ceiling to demonstrate the mechanics, as well as a one-fifth-scale model operating in a vacuum to try out the laser-guided alignment technique. Just last summer the shuttle hung an instrument-lined boom outside its cargo bay to record the sorts of vibrations that SIM will be coping with. Shao and his colleagues have seven years before the launch date to work out the glitches, but given the aspirations for SIM, they will probably be grateful for every minute of it.

By the time NASA launches its next Generation Space Telescope in 2007 or thereabouts, we will already have grown bored with SIRFT’s dust-piercing infrared images. Fortunately they are only the appetizers before NGST’s main course. NGST, like SIRFT, will pick up infrared light, but its large mirror—plans call for one as big as 25 feet—will give it the ability to gather nine times more light than the Hubble. That will bring into view stars and galaxies never before spotted by any telescope anywhere at any wavelength. It’s an enormous design challenge, admits Goddard Space Flight Center scientist and NGST team leader John Mather. “NGST is going to be much bigger, much more powerful, and much colder than the Hubble,” he boasts. “But we’ve got to build it for $500 million, which is one-quarter the Hubble’s construction budget. Plenty of people have said we can’t do it.”

One reason for the incredulity is that a conventional rigid glass mirror 25 feet across would weigh a couple of tons, and even if NASA were willing to pay for a rocket powerful and spacious enough to carry it, it would almost certainly distort or crack during the voyage into space. But for NGST’s purposes, the surface of the mirror can’t deviate by so much as a millionth of an inch across its entire width.

Mather’s group is undaunted. They plan to build the mirror out of very thin, lightweight, flexible glass, ship it folded into segments, and then deploy it in outer space. Three separate efforts are already under way to develop prototypes. In one, the mirror will consist of seven thin glass segments, each with several hundred small, spring-loaded screw adjusters attached to the back. A computer will measure the distortion in each segment and direct each of the screw adjusters to smooth out the bumps and valleys. In another prototype, each mirror segment will be glued to a stiff but lightweight graphite fiber backing, which should hold the mirrors rigidly in place. In case it doesn’t, the design includes a handful of screw adjusters mounted on the back, to be used only as a last resort. “Right now both ideas look workable,” Mather notes. “We’ll see which one turns out to be cheaper and more practical.”

 NGST’s mission will be to a large extent much the same as SIRFT’s: to see through the dust surrounding star creation, and to see farther back in time to the creation of galaxies. The large mirror will gather 80 times more light than SIRFT’s, revealing a vast range of even dimmer star clouds and galaxies. Getting breathtaking Hubble-like pictures with an infrared telescope would normally be out the question because, compared with visible light, infrared light’s longer wavelength makes for blurrier images—it’s a little like measuring with a ruler marked only in inches versus one marked down to eighths of an inch. Fortunately, NGST’s size makes up for this deficiency: sharpness of image increases with larger mirror size. “The Hubble changed astronomers’ view of the world,” says Mather, and his team expects no less of NGST.

Conceivably, NGST could provide the first evidence of extraterrestrial life. Since different elements and compounds absorb light at characteristic wavelengths, astronomers can determine what chemicals make up a planet’s atmosphere by measuring the relative amounts of light that come in at particular wavelengths. NGST will be powerful enough to zoom in on the light of Jupiter-size planets in orbit around distant stars and look for molecules associated with life—water, oxygen, and carbon dioxide. The unique wavelength signatures of methane and ozone would be particularly exciting—especially if these two molecules were found side by side. Ozone, the triplet form of oxygen, indicates that life-giving oxygen is also in abundance, and methane, a complex hydrocarbon, is a by-product of life processes.

The giant scope will also examine the composition of matter in distant young galaxies. Astronomers would particularly like to know when heavier elements like carbon and nitrogen first started appearing in large quantities. Since these elements, produced mainly in the explosive death of large stars, are crucial to life, pinning down their emergence would help determine when the earliest possible life-supporting planets might have appeared and give us a clue as to the likelihood of life’s existing elsewhere. And NGST will probably provide clues to astronomical mysteries as yet undreamed of. “Who knows what else people will want to see with it 20 years from now?” asks Mather. “We just want to make sure that whatever it is, NGST is good enough to see it.”

The Terrestrial Planet Finder

To achieve that sort of observational prowess, NGST will need every advantage it can grab. One could come from parking in a frigid orbit a million miles away—four times farther than the moon and clear of Earth’s infrared glare. Even better, though, would be a spot far away from the warmly glowing dust left over from asteroid collisions in the inner solar system. Surprisingly, this dust gives off the brightest infrared radiation in the solar system (except for the sun’s), some 300 times brighter than Earth’s. To escape it, NGST—which would be built on Earth and launched folded up like an enormous butterfly—would have to be rocketed out beyond Mars and the asteroid belt, halfway to Jupiter. There the only stray radiation the observatory would have to compensate for would come from stars, the trace debris of comets and asteroids, and the telescope’s own electronics (which would sit on a boom several yards away). In that 12-degrees-Fahrenheit-above-absolute-zero solitude, NGST might not need a cooling system or even a protective skin around its components. “The Hubble needs to be completely covered to protect it from the heat and light of both the sun and the Earth,” explains Mather. “But in our orbit all we need is a shade on one side that NGST can hold out at arm’s length to shield it from the sun. We don’t need to put the telescope in any sort of pipe—we’re planning on going naked.”

Despite the enormity of the task in front of him, Mather is already envisioning a yet more powerful and even cheaper telescope to follow NGST. “What we’d really like to do,” he says, “is make a mirror out of a Mylar-coated balloon, send it into outer space crumbled, and then blow it up.” And he’s not kidding, either; NASA is working on it.

If finding life on other planets is really NASA’s most important goal, then the Terrestrial Planet Finder is the big enchilada of the entire spaced-based telescope effort. When launched in 15 or so years, TPF will scope out our entire neighborhood, astronomically speaking, and tell us with little ambiguity who has the right stuff and who doesn’t. “TPF will look at each of the nearest few hundred stars for a few hours, and we’ll know for sure whether or not there’s an Earth-like planet around it,” says jpl scientist and senior project overseer Charles Beichman. “And if there are four planets around the star, it will find all four.”

TPF will build on SIM’s design concept of combining the light from a line of several smaller mirrors so that the light waves add up or cancel out, providing the resolution of one giant mirror. But where SIM’s foldable arms will spread to 30 feet, TPF’s line of mirrors will stretch out to the length of a football field. That will provide such a high resolution that the telescope won’t need to bother with star wobbles. If there are any planets to be seen—even planets slightly smaller than Earth—TPF will eyeball them directly.

That’s a trickier job than it may sound. Of all the countless photons of light that leave an Earth-size planet each second, only one or two would make it to a three-foot-wide mirror 40 light-years, or about 250 trillion miles, away. It’s not that TPF will have trouble spotting those few photons, or that it will lack the resolution needed to identify where in the sky they came from. The challenge is to do all that while being bombarded with millions of photons from the parent star sitting some 100 million miles away—far less than a hairbreadth, from our point of view. “It’s like spotting a firefly next to a searchlight,” says Beichman.

The sly solution is to employ an interferometry sleight of hand known as nulling. Instead of aiming its mirrors straight at a planet and then combining the light waves, TPF will aim its mirrors at the host star and then let the light waves from the different mirrors cancel each other out by combining the peaks of one mirror’s waves with the troughs of another’s. Because any planet in the telescope’s field of vision would be off to the side a bit, its light waves wouldn’t line up neatly and thus wouldn’t cancel out. As a result, the glaringly bright star would be blanked out, but the dim planet would shine through. Not only will TPF spot planets, it will be able to analyze their atmospheres for life-friendliness.

Spending a few hours per star, TPF will be able to find every Earth-size or larger planet within habitable distance of its sun—50 million to 200 million miles for an average-size star—for each of the nearest few hundred stars. “It will be a large enough census so that if TPF doesn’t find any Earth-like planets, that will say something about the uniqueness of Earth,” says Beichman. TPF’s budget, construction details, and potential space locations are still up in the air, so to speak. But it seems clear that if Earth has anything like a twin in our corner of the galaxy, TPF will be the best hope by far for locating it.

By then, of course, entirely new generations of space telescopes will be taking shape in the labs. What will they be trained on? No one knows, but if history is any guide, a shortage of interesting targets won’t be the problem. Says Mather with a sigh, “We astronomers tend to crank out new questions a lot faster than we crank out new telescopes.”

 

The New Millennium Interferometer


The big four space telescopes—SIRTF, SIM, NGST, AND TPF—will no doubt satisfy even the most hard-core space junkies for at least a little while, but there’s no need to wait.

NASA, as part of its Origins program, is planning no fewer than four so-called precursor missions by 2001. Each will capture interesting tidbits of data and images.

WIRE. The first off the block will probably be the Wide-Field Infrared Explorer, a four-month low-Earth orbiter scheduled for launch in September 1998. With its 12-inch mirror, it is small enough to fit in the backseat of a Ford Taurus. Its job will be to find galaxies that are hatching new stars at faster-than-normal rates so that astronomers can learn about how galaxies form.

FUSE. The Far Ultraviolet Spectroscopic Explorer, scheduled for launch this fall into a circular 500-mile orbit and expected to last three years, will look at a part of the spectrum the Hubble can’t see. Its ultraviolet light detectors will reveal the composition of interstellar gas, the cores of galaxies and quasars, the outer atmospheres of cool stars and planets, planetary nebulas, and supernovas. Astronomers are hoping it will give them a clue as to how much normal (as opposed to dark) matter there is in the universe. Although its four-segment mirror will be used mainly for spectroscopy, a separate detector should give visible-light images.

SOFIA. Not so much an orbiting observatory as just a very high one, the Stratospheric Observatory for Infrared Astronomy is being built into a Boeing 747. When it takes to the skies in 2001, it will train an infrared eye on interstellar clouds, the center of the Milky Way, planets in the solar system and distant galaxies—many of the same things that sirtf will look at a few years hence.

NMI. As the first space-based interferometer, the New Millennium Interferometer is intended mainly to test the laser-guided system for keeping several telescopes separated from one another by precise distances—in this case, three visible-light telescopes as much as half a mile apart. nmi will be launched into orbit around the sun in 2001.

And in your ripe old age, you’ll have PI to look forward to. That’s Planet Imager, and it seeks to answer the following question: If you can string several telescopes together to make an interferometer, why not string several interferometers together to make a super-interferometer? As envisioned, pi would be a parabolic-shaped network of five Planet Finders spanning 3,600 miles. With this monster, if there are any extraterrestrials out there, you should be able to see the whites of their eyes. Right now, though, it’s just an idea—nasa is wisely concentrating first on making a Planet Finder. It won’t happen for at least a couple of decades.


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