The Space Infrared Telescope Facility |
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 |
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.
