It is exciting to step back and imagine what the next 1,000 years might bring in astronomy and space science. Unfortunately, I am firmly of the opinion that we can really only predict advances over the rather short term, maybe 30 years or so under the conditions of the past century. Beyond that, totally unforeseen radical developments take over and dominate everything. However, the game is too much fun not to play, so I am willing to guess!
In astronomy, I am only willing to look forward 100 years, as it seems hopeless to guess beyond. Within that horizon, I expect we will have a detailed knowledge of all the significant bodies in the Solar System, the kind of knowledge we now have for Mars. I think we will have direct imaging of all the planets of every star within maybe 10 light years -- and I am almost willing to bet that practically all stars have planets of some kind.
I anticipate that we will have detailed three-dimensional positions and velocities for all the stars in our Galaxy, excluding only distant white dwarfs and brown dwarfs, and a rather complete dynamical understanding of the whole Milky Way. I hope we will understand galaxies and their evolution at least as well as we now seem to understand normal stars.
It is my opinion that space exploration is in every way analogous to the emergence of life from the oceans onto the land. Barring completely unforeseen and unguessable breakthroughs, I suspect this may be a somewhat slow process by our impatient human standards. Within the century I hope we will at least have independently self-supporting settlements at several locations in the inner Solar System.
When this landmark is attained, we will have a reasonable assurance that no natural disaster nor any human folly will ever render Humanity extinct. I fancy the complete settlement of the Solar System may take roughly as long as that of the Western Hemisphere of the Earth; so in 500 years I expect the New Worlds to be on a par with the Old: in population, culture, economic strength and by all other important measures.
Finally, I will stick my neck far out and predict that we will achieve interstellar travel, defined as sending humans to the nearer stars, in about 200 years, with an uncertainty of a factor of three in either direction (that is, between 60 and 600 years from now). In this I assume no fundamental breakthroughs allowing warp drives, faster-than-light travel, etc., but only some kind of fission or fusion-powered vehicle. If this should turn out to be accurate, the beginning of the next millennium should find the descendants of modern humans heavily settled throughout a sphere perhaps 20 light years in diameter, and expanding outwards at roughly 0.01c.
I feel I must at least mention at two obvious breakthroughs that seem fairly likely and that would radically alter the future in ways no one can predict. One would be the discovery of extraterrestrial intelligence. Another would be any large changes in the human species, due to some combination of unusually rapid evolution, application of fundamental biological knowledge, and interaction or fusion with artificial intelligence.
Bill Wheaton