We easily forget how young the age of big optical telescopes is and therefore how little we know about the dim reaches of space—even right here in our own solar system. For most of the modern age of astronomy, the 200-inch (about 5 meters) Hale Telescope on Palomar Mountain in California, completed in 1949, was as good as it got. In the 1960s and 1970s telescopes started popping up on Kitt Peak in Arizona like prickly pear cacti (there are more than two dozen telescopes on the mountain), but none had a mirror more than 4 meters across.
The Keck I, with a 10-meter mirror, was the first serious advance in nearly five decades; it opened for business on Mauna Kea in Hawaii 11 years ago. Its neighboring twin, Keck II, was not completed until 1996. Now this dormant Hawaiian volcano is home to four huge optical telescopes: the two Kecks along with Subaru and Gemini North. Seven more giant telescopes, with mirrors ranging from 6.5 meters to 8.2 meters, dot the mountaintops of Chile. None of them were in operation before 1999. The Large Binocular Telescope, being built on Mount Graham in Arizona, will pair two 8.4-meter mirrors for unparalleled views of distant and faint objects. And this flurry of construction is just the beginning of a golden age of optical astronomy.
Seeing far—and well—into space completely alters our view of reality. Just this year we were amazed by the announcement that Sedna, a mysterious object nearly as large as Pluto, is orbiting almost three times farther out than the ninth planet from the sun. It is tempting to call it a new planet, but it is more truthful to say that Sedna shows that we don’t really know exactly what a planet is. We have begun to discover that there is far more to our solar system than we might ever have guessed, including, perhaps, planetlike objects as large as Mars way out in the Oort cloud, hundreds of times more distant than Pluto.
We don’t necessarily need large telescopes to find objects like Sedna, which was located using just a 48-incher (about 1.2 meters), but we do need large telescopes to see them well and understand what they are. There’s very little sunlight to illuminate objects beyond Pluto, so capturing every photon counts. As a result, astronomers have learned to think big—and to think ahead. The National Optical Astronomy Observatory, which manages many of the U.S. telescopes for the National Science Foundation, is developing a design for a 30-meter telescope, and the European Southern Observatory has initial plans for a 100-meter telescope. Who knows what strange and wondrous worlds will come into view when these new instruments start penetrating the gloom?