Ticket to Ride?

Astrophysicists mull a return to the moon

Ron Cowen

Scientists who study the moon and design the spacecraft to get there are typically worlds apart from astronomers who explore the realms of space beyond the solar system. The two groups attend different meetings, talk a different lingo, and usually get their funding from different divisions within NASA. But with a financially strapped space agency setting its sights—and the majority of its resources—on a highly publicized plan to return to the moon and establish a base there (SN: 12/9/06, p. 373: http://www.sciencenews.org/articles/20061209/fob5.asp), astronomers are looking for ways to jump on the lunar bandwagon.

This lunar scout mission is one of several that NASA will send in preparation for astronauts' returning to the moon. The mission will search for water and other resources at the south pole. Astronomers are now investigating how they might jump on the lunar bandwagon, using the moon or its environs to study distant stars and galaxies. NASA

"There's a serious concern that [astrophysics] will be left behind" if astronomers don't become part of the lunar initiative, says Webster Cash of the University of Colorado at Boulder.

"The NASA administrator has actually challenged the astronomical community to come up with scientific ideas that can benefit from a return to the moon," notes astrophysicist Mario Livio of the Space Telescope Science Institute in Baltimore. At an institute conference late last year and at an early March meeting of the NASA Advisory Council in Tempe, Ariz., Livio and other scientists debated the merits of a host of astronomical experiments that could be performed on the moon or in lunar orbit. Their options have become more limited by NASA's recent cancellation of several robotic missions to the moon.

The proposals included a telescope that would record light from the deepest reaches of the cosmos using a liquid mirror bigger than a football field. The device would be housed inside a crater at the moon's south pole. Another idea featured an array of radio telescopes deployed on the moon's far side, shielded from the chatter of Earth's radio signals. That array would search for radio emissions associated with the first stars in the universe.

Eschewing the moon's dust, craters, and surface gravity, other astronomers are setting their sights on lunar-orbiting craft situated at a gravitational sweet spot between the Earth and moon. In addition to viewing the cosmos from space, such craft could act as repair and refueling stations for observatories stationed farther away from Earth.

Liquid view

A bucket would be big enough to carry the material to make the 100-meter-wide telescope that designer Roger Angel of the University of Arizona in Tucson is proposing for the moon's south pole. Instead of being made of glass, Angel's mirror would consist of a low-temperature liquid. When set spinning in a wide container, the liquid would flow away from the center so that its surface would form one of the most prized shapes in astronomy—a parabola. The parabolic mirror would focus onto a single point of the light from objects at any distance.

Smaller-scale liquid-mirror telescopes have already been built on Earth and are far cheaper than comparable telescopes made of a single piece of glass or of several joined glass segments.

A liquid-mirror telescope on the moon has a huge advantage over such a device on Earth, notes Angel. Because the moon is airless, astronomers wouldn't have to worry about air currents or atmospheric disturbances that on Earth can generate waves in the spinning liquid, distorting its shape and reducing its capacity to focus light. Although the first lunar liquid mirror on the moon might be just 2 m across, telescopes 50 times as large could eventually be built, Angel says.

Whether on Earth, on the moon, or on some other orb, a liquid-mirror telescope can view only the patch of sky directly above it because the spinning mirror must always remain exactly horizontal in the local gravitational field. The moon's axis of rotation stays fixed with respect to the distant heavens, so a liquid-mirror telescope placed at one of the lunar poles would always see the same stars and galaxies overhead. Such a telescope could make an extraordinarily deep portrait of its overlying patch. Over months to years, it might see back to the time more than 13.5 billion years ago, when the earliest stars came to life, Angel says.

Radio-free moon

Looking for a respite from the cacophony of FM-radio and television broadcasts, radio astronomers have begun building arrays of antennas in western Australia and other radio-quiet locations. The moon's near side may not offer a distinct advantage over such spots, but the moon's far side is another story.