Space versus Ground Telescopes

In 1609, an Italian astronomer, Galileo Galilei, pointed a telescope toward the sky. It is said that he was the first to do so. With it, he saw mountains and craters on the moon, and the Milky Way.

People have been innovating the telescope ever since.

Ground-Based Telescopes

Today, thousands of ground-based telescopes operate across the globe, with astronomers capturing new views of the universe—and new knowledge—every day. The Large Binocular Telescope, or LBT, is one of the largest telescopes in the world, built in Galileo’s home country of Italy and shipped to the state of Arizona in 2002. Operated by the University of Arizona, the LBT has been used in a myriad of astronomical discoveries, including the first images of a planet in the making captured in 2015.

Ground-based telescopes have long been the workhorses of astronomical research.

Compared to space-based telescopes, ground-based telescopes have much to offer. They can be built bigger and for less money. They’re easier to maintain and upgrade. Practically speaking, they also have a much lower risk of being damaged by one of the 500,000 pieces of debris flying through the cosmos—or space junk.

But increasingly, scientists and engineers are turning to space as the new frontier for advanced telescopes. The trend toward space telescopes began in the 1960s, when astronomers started attaching giant balloons to telescopes as a means to carry them above Earth’s lower atmosphere.

Their goal: get a sharper view.

While ground-based telescopes are generally placed in isolated, elevated locations with little light pollution—the LBT, for example, stands at 11,000 feet in the Pinaleño Mountains—they still face the challenge of atmospheric distortion.

When a telescope on the ground looks to the cosmos and takes a picture, the light it captures has first travelled through air in the atmosphere. If that air is at all turbulent, it blurs the light. This is termed “atmospheric distortion.” It’s the reason stars seem to “twinkle” when we look at them from Earth, and also the reason many ground-based telescopes can’t take super-sharp images of objects in space. For some research projects, image quality is not all that important, but for researchers who need high-resolution photos to do their work, many ground-based telescopes present a problem.

In the decades that followed the giant-balloon strategy of the 1960s, it became clear that what was really needed was free-flying space telescopes—that is, telescopes that can orbit on their own. So, NASA’s Great Observatories program was born. The Great Observatories program led to the creation of four big, powerful space-based astronomical telescopes, including the Spitzer Space Telescope and the Hubble Space Telescope that launched over 25 years ago, in 1990.

Space-Based Telescopes

Space-based telescopes like Hubble get a much clearer view of the universe than most of their ground-based counterparts. They’re also able to detect frequencies and wavelengths across the entire electromagnetic spectrum. Ground-based telescopes can’t do the same, because the Earth’s atmosphere absorbs a lot of the infrared and ultraviolet light that passes through it.

Nevertheless, space-based telescopes are expensive to build and difficult to maintain. NASA’s Hubble, with infrared eyes built by the University of Arizona, is the first space telescope designed to be repaired by astronauts in space. Other space telescopes cannot be serviced this way.

But as technology advances, both telescopes on the ground and in space are continuing to improve.

Newer, more advanced ground-based telescopes implement adaptive optics, a technique to sharpen blurry images right as they’re being taken. An adaptive optics system uses lasers, supercomputers, and an array of mirrors to correct atmospheric distortion.

To sharpen an image with adaptive optics, astronomers will pick a point of light, called a “guide star.” They’ll measure and monitor how its light waves bends as they move through Earth’s atmosphere, and then send that information, via supercomputer, to the telescope’s mirrors. In response, the mirrors bend in a way that straightens out the light waves as they are reflected onto the telescope. Of course, there isn’t always a guide star near enough to the object that an astronomer is studying, and, in that case, astronomers use lasers as a guide star instead.

Adaptive optics technology can be seen on the ground-based Giant Magellan Telescope that’s slated for completion in 2021. Remarkably, with mirrors constructed by the University of Arizona’s Richard F. Caris Mirror Lab, the ground-based GMT will have ten times the resolving power of space-based Hubble.

The next telescope that will push the boundaries of technology for astronomical observation is NASA’s James Webb Space Telescope, scheduled to launch in 2018. It will be used by thousands of astronomers worldwide, studying every phase in the history of the Universe.

Whether in space or on the ground, telescopes remain the cornerstone of astronomical discovery. Want to learn more? The University of Arizona runs some 20 telescopes.