Hubble's Sky Eye Finds Mysteries
Despite flawed mechanism, NASA's orbiting telescope is opening a door on galactic discovery
THE Hubble Space Telescope, crippled by its flawed mirror, may not see as clearly as its designers had planned. But its discoveries so far are challenging fundamental ideas about the universe, and what observations it can make it performs superbly well. "And that," says Space Telescope Science Institute director Riccardo Giacconi, "is quite a lot." Referring to findings made so far, he says it is clear that "the telescope is a unique instrument and is producing unique science even now."
Its discoveries include evidence for intergalactic hydrogen clouds where no such clouds were thought to exist. Their presence challenges cosmologists' notions of how the universe has evolved. Astronomer John Brandt of the University of Colorado at Boulder calls this "a bona fide discovery ... that could only be done by an instrument that operates above the Earth's atmosphere." It is a discovery that he expects to "have tremendous impact on our understanding of how the universe evolves - how galaxies wer e
formed, how parts of it didn't quite go into galaxies, how such structure is maintained."
Another challenging finding is detailed data on the peculiar chemical makeup of the star Chi Lupi. Trying to explain that chemistry pushes the frontier of atomic physics, Dr. Brandt says.
Then there is the intriguing star Beta Pictoris, whose surrounding disk of dust and gas may represent a nascent planetary system. Hubble observations have revealed such a wealth of new detail about that disk that investigator Albert Boggess of the NASA Goddard Space Flight Center in Greenbelt, Md., says astronomers now "have an extra set of mysteries on our hands."
Hubble investigators presented findings such as these during a science writers' workshop last Friday to illustrate what the space telescope can do in spite of its handicap.
The instrument's great advantage is that it orbits beyond the atmosphere that absorbs and distorts incoming light and other electromagnetic radiation. This is especially important for observations made by ultraviolet light, which the atmosphere substantially absorbs.
The mirror flaw has not totally undercut this advantage. It does restrict the telescope's ability to see extremely faint objects and, hence, limits its ability to study the age and early evolution of the cosmos. Astronomers had hoped to see objects as faint as magnitudes 28 to 30. That requires focusing 70 percent of a star's light into a spot only 0.1 arc-seconds across. Hubble's defect, called spherical aberration, smears something like 88 percent of the light into a fuzzy halo surrounding that tiny s p
ot. This limits sharp seeing to objects no dimmer than 25th magnitude.
What Hubble can image, it generally sees with 10 times the sharpness of ground-based telescopes, especially with the help of computer image-processing. Moreover, the imaging limitation does not apply to spectroscopic observations. These are studies in which an object's radiation is split into a range of wavelengths like a rainbow of colors. Astronomers use such spectra to tell the chemical composition and motion of dust, gas, and objects. "The quality of the data we're getting from the spectrographs i
s every bit as good now as we originally expected it to be," says Dr. Boggess, who is associate director for the Hubble Space Telescope at the Goddard Center.
As a result, two major Hubble instruments - the Goddard High Resolution Spectrograph and the Faint Object Spectrograph - can work at their full capability. This is especially valuable for ultraviolet spectrographs, which can't be taken very well through the atmosphere. For astronomers, "the entire world changed," Brandt says, when they compared the low-resolution ultraviolet spectra taken previously by satellites with the sharply detailed Hubble spectra. For this reason, he considers the Goddard instrum e
nt "at the present time ... the jewel of the Hubble Space Telescope."
It is this sharper spectroscopic view that has brought forward the challenge of the star Chi Lupi with its peculiarities of chemical makeup. For example, it has 100,000 times the amount of mercury in its outer visible layers as does the Earth's Sun. Moreover, as much as 99 percent of the mercury consists of a single isotope (mercury 204). Isotopes are atoms of an element that have the same chemical properties but have different atomic weights. Elements generally exist as a mixture of isotopes.
Brandt notes that these observations "are challenging the astrophysicist to understand how such a separation or concentration [of an isotope] might occur in this star." It is questions like these, he adds, that take scientists "beyond the current state of knowledge" in atomic physics.
Spectroscopic data have also brought out new mystery in the star Beta Pictoris and its dusty disk. Boggess explains that what had seemed a simple disk of dust and gas turns out to be very complex and unstable. It has several major components, including isolated clumps of gas that speed around the planet and spiral into it at speeds of around 200 kilometers (120 miles) a second.
Boggess says that "the whole system is active in a way that, I think, hadn't been anticipated." He adds: "It would be a clear stretch ... to say that there are planets here. But at least so far as we know now, this is the closest thing we've ever found to our own solar system." The Goddard team expects to start a search for possible planet-sized bodies in the disk next October.
Spectrographs from both the Goddard instrument and the Faint Object Spectrograph also challenge cosmologists. These spectrographs have found the puzzling intergalactic hydrogen clouds. Astronomers have known such clouds as they exist at great distances, where they see the universe as it was billions of years ago. But they hadn't expected to find them relatively close to Earth, in a part of the universe that is of relatively recent age.
Scientists study the clouds by detecting with a spectrometer how they absorb radiation from more distant objects, generally quasars. A quasar is an object so compact that its telescopic image appears no bigger than a star but whose energy output rivals that of an entire galaxy. In the present case, Hubble investigators used the ultraviolet-emitting quasar 3C273.
Cosmologists had thought such hydrogen clouds were relics of the early evolution of the universe. Finding them nearby means that either they form continuously, or they are able to persist for billions of years. Either way, theorists don't understand what is going on. "A new gauntlet has been thrown down," Brandt says, "It's time to get back to work."
Astronomers' delight at such findings does not mean they are complacent about the Hubble telescope's shortcomings. Institute director Giacconi recognizes that basic purposes "for which the telescope was built are not being met."
But, he adds, "There is hope on the horizon." A rescue mission is being planned for 1993 in which astronauts will install new equipment to restore Hubble's vision to its expected standard "Meanwhile", Dr. Giacconi says, "I think life is not so bad."