THE Hubble Space Telescope has entered its third year in space with a wealth of major accomplishments already to its credit.
Orbiting well clear of Earth's obscuring atmosphere, it is fulfilling its promise to give scientists the sharpest view of the cosmos they have ever had in spite of its flawed mirror.
This clearer vision has, for example, allowed scientists to make an accurate measurement of the cosmic abundance of two kinds of hydrogen, which gives new insight into the birth of our universe. It has revealed the hottest star yet known. It has provided the best evidence yet for the probable existence of massive black holes - objects that have collapsed to a density with a gravity too strong even for light to escape it.
Yet for cosmologist John N. Bahcall of the Institute for Advanced Study in Princeton, N.J., these and other impressive Hubble discoveries pale before the profound significance of one simple fact. However far across the universe the telescope looks, however far back in time are the scenes it shows, cosmic evolution appears to unfold more or less in accord with theories worked out by Einstein and others six decades ago.
Professor Bahcall explains that "we had no right to expect" that scientists in the 1930s could anticipate how big objects behaved far out in space and at very early times by using only pure reasoning and observations of our own small corner of the cosmos.
"It's awesome to me that the human mind has achieved that kind of synthesis," Bahcall says.
The scientists' delight at the telescope's achievements doesn't mean that they are blase about its handicap. They can use only about 25 percent of the light the telescope gathers because of the optical fault called spherical aberration that was carelessly ground into its mirror.
Hubble astronomer Stephen P. Maran of the Goddard Space Flight Center in Greenbelt, Md., says, "We want to fix it and make it do the whole job." However, he adds that, while Hubble scientists look forward to the on-orbit repair of the telescope in late 1993 or early 1994, "there isn't any time or money that is going to waste [now]."
Discovery of the super-hot star is one of the more spectacular results of Hubble observations. Dr. Maran's Goddard colleague Sally R. Heap presented the picture shown here at a press conference held at the National Aeronautics and Space Administration's Washington headquarters last month to commemorate the Hubble telescope's launch on April 24, 1990.
The picture shows the nebula NGC 2440. This dust cloud fluoresces with energy that astronomers believed had to come from an embedded hot star.
But ground-based telescopes saw a blurred image in which astronomers could neither pick out the star nor estimate its temperature. That star leaps out as the central bright dot in the sharp Hubble image - an image made even sharper by computer restoration.
Dr. Heap said that, to the astonishment of astronomers, the star turned out to be a collapsed dense object called a white dwarf, running a temperature of 200,000 degrees C.
Heap also showed Hubble's view of tightly packed star clusters that had appeared in blurry ground-based views to be single large stars. Hubble photographs show groups of about 50 stars each - young heavy stars with masses up to 100 times that of our sun.
Astronomer Daniel W. Weedman of Pennsylvania State University at University Park, said that, to judge from the Hubble images, "it appears that it's as easy, if not easier, to make heavier stars as to make lighter ones." He called this "a truly new discovery" that upsets assumptions about star formation.
Hubble's sharper view has also given cosmologists a stronger clue to formation of matter in the early universe. Cosmologists are interested in deuterium - a doubly heavy form of hydrogen - because they say it could have been made only in the "Big Bang" explosion of primordial mass that theoretically formed our universe. All deuterium now present is left over from that epoch.
Moreover, the abundance of that deuterium relative to the abundance of ordinary hydrogen allows cosmologists, using "Big Bang" theory, to estimate the maximum amount of all ordinary matter that can be present in the universe today. Measurements with the Hubble telescope have given cosmologists their most accurate assessment yet of the deuterium/hydrogen ratio.
Jeffrey Linsky, a National Institute of Standards and Technology astronomer located at the University of Colorado in Boulder, reported the finding in Atlanta last January at the American Astronomical Society's semiannual meeting.
The Hubble research group that he heads found a cosmic abundance of 15 deuterium atoms for every million ordinary hydrogen atoms, with an uncertainty of less than 10 percent. This implies that the universe today has 10 times too little ordinary matter for its self gravity to halt its expansion. If there is more matter than this, as some theorists say they believe, then it must be an unknown exotic kind of substance.
Another kind of possible exotic object is the so-called black hole. There is no direct evidence that such a thing exists. But Hubble observations have provided the best indirect evidence yet obtained that massive black holes lurk at the core of energetic galaxies.
Many astronomers have suggested that energy released by stars and other matter falling into such black holes could account for the highly energetic behavior associated with these galaxies.
Tod Lauer and C. Roger Lynds of the National Optical Astronomy Observatories in Tucson, Ariz., and Sandra Faber of the University of California at Santa Cruz reported at the Astronomical Society meeting in Atlanta that the distribution of stars and light revealed in Hubble images strongly imply the presence of a black hole with with 2.6 billion times the mass of the sun at the core of the giant galaxy M87.
Last month, the Space Telescope Science Institute in Baltimore announced that Drs. Faber and Lauer had located a second possible black hole with 3 million solar masses at the center of galaxy M32. This is a small companion galaxy to the giant Andromeda galaxy that dominates the galaxy cluster to which our own Milky Way galaxy belongs.
Summing up the status of Hubble-based research today, Bahcall says that many other discoveries may lie in data already gathered and being processed. He likened the situation to a birthday party at which every guest receives a present to take home but doesn't know what it is until it is unwrapped.
"We're just beginning to unwrap our presents," he says.
Meanwhile, all Hubble scientists are looking forward to what a repaired telescope will give them.
Shuttle astronauts, now training for the repair mission, could reach the telescope as early as November 1993. They plan to replace failed gyros, install new solar panels, replace an instrument, and install corrective optical equipment.
Picking up on Bahcall's party analogy, Daniel Weedman says of the data that the repaired telescope should provide: "We're looking forward to the greatest birthday party there ever was because now we have a taste of what the cake will be like."