How glass could help the search for extraterrestrial life

We tend to think of a decade as a long time. Astrophysicists? Not so much.

We tend to think of a decade as a long time. Astrophysicists? Not so much.

The telescopes and space-based observatories they use can peer back billions of years. The planning required to build those telescopes often takes place over the course of three decades or more.

The idea for the Hubble Space Telescope (HST), for example, came in the 1940s. Engineering and planning began in the 1970s. It launched in 1990. Its successor, the James Webb Space Telescope (JWST), is expected to launch in 2020. Planning began in 1996.

It should be no surprise, then, to learn that NASA is in the planning stages for JWST’s “grandchild.” This one, slated to launch sometime in the mid-2030s, will search for rocky planets outside our solar system that might harbor life.

SCHOTT’s glass could be on board, as it often has been for the last 50 years.

Scientists are exploring whether to use SCHOTT ZERODUR as a mirror substrate for the Advanced Technology Large Aperture Space Telescope or ATLAST.

ZERODUR is a preferred material as a mirror substrate because of its high dimensional stability. Most materials tend to shrink or expand in response to fluctuations in temperature. When peering over billions of light years however, slight temperature dips caused by a telescope travelling through the moon’s shadow could cause other glasses to change shape.

This year, we’re celebrating the 50th anniversary of ZERODUR’s invention. In 1968, SCHOTT produced a mirror substrate with a diameter of nearly 4 meters on behalf of the Max Planck Institute for Astronomy.

Today, the main components of almost all major reflector telescopes worldwide are made of ZERODUR glass-ceramic. ZERODUR has been studied and tested extensively. In fact, it has flown successfully on over 30 space missions, including the Chandra X-Ray Observatory and the HST, and it’ll fly on JWST as well.

ZERODUR is an inorganic glass ceramic with 70 to 78 percent of high-quartz micro-crystallites 50 to 80 nm in size, embedded in a remaining glassy phase. The micro-crystals contract when they are subjected to heating, whereas the glass itself expands. The size and number of the micro-crystallites are carefully adjusted to achieve an extremely low thermal expansion.

This process can be used to make mirror blanks with more than 88 percent of their weight removed. It’s a perfect match for space applications, where weight is a limiting factor for launch vehicles. For example, a 1.2 m (approximately 3 feet 9 inches) ZERODUR lightweight mirror weighs about 99 pounds.

But before an ATLAST design is finalized and materials selected, the multi-disciplinary teams working on the design require several technical advances. The teams must decide whether to build a corona scope, which blocks incoming starlight, or rely on a “starshade” that will float around the observatory to shade it.

They must help guide the development of increasingly sensitive scientific instruments capable of detecting infrared, visible, and ultra-violet light. And they must decide on a telescope architecture itself – whether to build it from tiles, or rely on a single monolithic mirror substrate. Above all, these new developments, and hundreds of others, must work as part of an integrated system, in the harshness of space, after surviving launch into orbit.

Testing of these new concepts could take much of the next decade. But a decade? That doesn’t sound like so long. After all, ATLAST will contain scientific instruments that will be able to detect molecular oxygen, ozone, and methane – biosignatures that are strong indicators of a planet’s habitability. This great observatory could help solve questions we’ve been asking ever since there were scientists to ask them.

June 11, 2018


David Schimmel
Advanced Optics
SCHOTT North America, Inc.