NASA's Hubble Space Telescope was responsible for many great discoveries during the past decade. Nevertheless, there is much revolutionary research left for the observatory to do. Predicting what Hubble will discover in the next millennium is truly a challenge; new results are generally unexpected and cannot be listed in advance. Still, here are some science expectations for the future:

EXPECTED NEW DISCOVERIES FROM SPECIFIC OBSERVATIONS

1) A front-seat view of the effects of a supernova explosion In 1987 astronomers discovered the closest supernova seen in four centuries. Follow-up observations by Hubble revealed several mysterious rings of illuminated matter around the supernova, still aglow after being flashed by the supernova's brilliant light. During the next decade matter flying outward from the supernova will ram into the central ring, a process that has already started. Astronomers anticipate that a fantastic celestial fireworks display will ensue, and Hubble, with its unparalleled eye for detail, will give astronomers the best view of this drama. For the first time astronomers will see in detail how the blast wave from a supernova interacts with the environment around it, and they expect to learn a great deal about the structure of the exploding star, its evolutionary history, and the nature of the enigmatic rings around it. Since supernovae of this type are also the main source of oxygen for the interstellar medium, from which later generations of stars and planets form, it is important to understand this interaction.

2) Are there planets around stars in the oldest clusters? Globular clusters are collections of hundreds of thousands of relatively old stars that are deficient in heavier elements. No planets have ever been detected around any star in a globular cluster. Hubble will potentially detect as many as 50 planets (if they exist) around globular cluster stars. Due to its uniquely sharp view, Hubble can resolve as many as 40,000 stars in one field in a globular cluster and follow the tiny variations (of about a percent) in their brightness as giant planets pass in front of stars. The detection of such planets will be of enormous significance, since it will not only demonstrate that planets can form even when there are fewer heavier elements than in the solar system, but also that planets can survive in crowded stellar environments. If planets are not detected, this will also place meaningful constraints on planet formation.

3) Filling in the missing links of life's origin Exploding stars create the elements necessary for life, but before these elements can be incorporated into newly formed planets and stars, they must cycle through a galaxy's "ecosystem." Astronomers suspect that many of the elements created in stellar explosions within the disk of a galaxy first get blown into the galaxy's halo. To date this halo gas has remained largely unobservable. The Cosmic Origins Spectrograph, the most sensitive ultraviolet-light spectrograph ever to be flown into space, will enable us for the first time to systematically study this crucial stage in the ecosystems of other galaxies. By observing quasar light shining through the halos of galaxies, astronomers will obtain "core samples" of the halo material and its composition, filling in this important missing link in our understanding of life's origin.

4) A giant step towards understanding how comets form Comets are small, icy objects that spend most of their lives well beyond the orbit of Pluto. They are generally recognized only on the rare occasions when one dives into the inner solar system, thereby developing a spectacular tail. One of the main regions where billions of comets reside is the Kuiper belt — a region extending from about the orbit of Neptune out to about 50 times the Earth's distance from the Sun. One of the main challenges of any theory for the formation of the solar system is to explain the formation and properties of this large number of comets.

Hubble observations of Kuiper belt objects in the infrared (using the refurbished Near Infrared Camera and Multi-Object Spectrometer and the Wide Field Camera 3) will determine the composition of these objects. This will be a huge step forward in the direction of determining their origin and formation process. Given the fact that the evolution of life on Earth has been dramatically influenced by comet impacts, an understanding of the origin of comets is vital.

MORE SPECULATIVE POTENTIAL NEW DISCOVERIES

1) Cosmic enigmas An examination of the Hubble Deep Field North — one of the two deepest images of the universe ever taken in optical/ultraviolet/infrared light, revealed the presence of a mysterious object with very unusual properties. While the object is readily visible in near infrared light (1.6 and 2.2 microns), it is totally undetected in visible light (wavelengths shorter than 1.1 microns). One intriguing possibility is that this galaxy (with a redshift of 12.5) is far across the cosmos, existing when the universe was only a few hundred million years old. Intervening galaxies may have absorbed most of its light.

The recharged Near Infrared Camera and Multi-Object Spectrometer will reveal if the object is indeed point-like, as in the case of a star, or has a "fuzzy" appearance, as one would expect for a galaxy. But more importantly, the Wide Field Camera 3 with its wide (and deep) field of view will be superb in searching for other objects of this type, determining how common they are, and whether they constitute a new class of objects.

2) "Far-out" giant planets? The dusty circumstellar disks observed with the Near Infrared Camera and Multi-Object Spectrometer and the Space Telescope Imaging Spectrograph reveal various gaps and ring structures, which are potentially attributed to the gravitational influence of large planets or protoplanets. A "problem" with this interpretation is that the structures are seen at large distances from the parent star, much farther away than the known giant planets in our own solar system. Interestingly, the analysis of some cometary orbits in the solar system also suggests the potential existence of a perturbing planet at a great distance from the Sun.

Are there unexpected families of giant planets in the far outskirts of planetary systems? Hubble surveys of faint moving objects could reveal such new planets around the Sun, and Hubble's highly sensitive infrared vision may even image such an object directly.

EXPECTED DISCOVERIES OF A MORE INCREMENTAL NATURE, BUT IN THE CONTEXT OF A "BIG PICTURE"

1) What makes the largest explosions in the universe? For decades astronomers have detected bursts of gamma-rays coming from the heavens, but until recently they had no idea where they were coming from. During the past two years Hubble has played a crucial role in pinpointing their origin in very distant galaxies.

Gamma-ray bursts are now recognized to be the largest explosions in the universe since the Big Bang, but astronomers still do not know what causes them. Hubble's superb vision will allow astronomers to determine the precise location of the "bursts" inside their host galaxies, and will thereby help to identify the nature of the exploding objects. Furthermore, observations in ultraviolet light shortly after the burst, and in the optical many months after the burst (both are possible only with the unique capabilities of Hubble), will complement observations with the High Energy Transient Explorer II and the Chandra X-ray Observatory. These observations will allow direct tests of the physical processes occurring during these cosmic fireballs, and will potentially help determine the rate at which stars form during the cosmic history.

2) What is the universe's ultimate fate? Edwin Hubble's discovery of the universe's expansion in the 1920s redefined astronomers' view of the cosmos. Until just a few years ago common wisdom held that the gravitational pull of each galaxy on every other galaxy must have been slowing down the expansion. However, some recent observations of distant supernovae have led many astronomers to wonder whether the universe's expansion is in fact accelerating under the influence of a somewhat mysterious repulsive force.

In order to settle this question, astronomers are busy working to discover and monitor ever more distant supernovae. Hubble is absolutely critical to this effort because its superior vision is crucial for distinguishing the supernova's light from that of its surrounding galaxy. Only by measuring with high precision the power of supernovae at distances spanning half the universe's age will astronomers be able to tell if the acceleration is real and thereby determine if the universe's ultimate fate is infinite expansion towards a cold death.

3) Mapping normal matter in the universe Large, ground-based surveys are currently mapping the distribution of galaxies in the local universe. But these galaxies represent only a fraction of the "normal" matter — the kind that makes up the Sun, the Earth, and human beings. A considerably larger proportion lies in the vast spaces between galaxies. Much of this matter is likely to be in the form of clouds that never formed galaxies. With the Cosmic Origins Spectrograph aboard Hubble, astronomers will be able to begin mapping out these large gaseous clouds, which will hopefully lead to an understanding of why some clouds form galaxies while others do not.

4) Where does the dark matter in the universe reside? By studying how galaxies move in response to gravity, astronomers have found that most of the matter in the universe is dark — does not shine like stars. Yet, it is this dark matter that holds galaxies and clusters of galaxies together. Astronomers have been refining techniques to study dark matter by measuring how severely its gravitational pull distorts light from distant galaxies, acting like a lens.

With the installation of the Advanced Camera for Surveys in 2001, Hubble will gain an invaluable new tool for unmasking the dark matter in the universe. This camera will surpass previous Hubble instruments in the size of its field of view and sensitivity and will take full advantage of Hubble's razor-sharp vision to detect the minute, but telltale, distortions that signal the presence of dark matter. In particular, the Advanced Camera for Surveys will excel in mapping out the dark matter that binds galaxies and clusters of galaxies. The luminous parts of galaxies are just like the mini-lights on a huge holiday tree, and the Advanced Camera for Surveys will enable astronomers to see the tree itself.

5) A broader view of the distant universe The most distant galaxies in the universe are detectable only in infrared light. Near Infrared Camera and Multi-Object Spectrometer observations detected some of the most distant galaxies ever seen. In the next few years astronomers hope to extend the frontiers of the known universe both farther and wider, once the near-infrared camera's cryocooler has been replaced and the infrared-sensitive Wide Field Camera 3 has been installed. With its much wider field of view, the Wide Field Camera 3 will allow for the simultaneous study of unprecedented numbers of distant galaxies, allowing astronomers to contrast and compare galaxies dating from within a billion years of the Big Bang. All of this will naturally pave the way for the Next Generation Space Telescope.