Ticket to ride? Astrophysicists mull a return to the moon.
"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 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.
Always facing away from Earth, the far side would provide radio astronomers with a prime piece of secluded real estate, says Jackie Hewitt of the Massachusetts Institute of Technology. Instead of costly and unwieldy radio dishes, arrays of low-cost, easily transportable dipole antennas--simple metal rods--could provide sophisticated data when combined with advanced computer analysis.
Scientists might use moon-based antenna arrays to study the I radio waves that accompany coronal mass ejections--the solar storms that occasionally strike Earth and damage satellites and power grids. The radio studies could more closely pinpoint the arrival times and severity of these storms.
Most tantalizing is what these radio observations might reveal about the early universe--in particular, the epoch before there were any stars and the period during which the stars ignited. Neutral hydrogen gas, which filled the universe from about 400,000 years after the Big Bang until the birth of stars, emits and absorbs radio waves at a particular frequency. The birth of stars quenched that early radio-wave activity.
Because stars across the cosmos didn't all turn on at once, different parts of the universe became radio quiet at different times, theorists say. Radio telescopes can receive signals from distant regions, and therefore early times, of the universe. Tuned to low frequencies, such telescopes on the dark side of the moon might discern the predicted variation in stellar start-up times.
Engineers have already begun to imprint metal dipole antennas on rolls of plastic to produce the vast arrays required to detect primordial radio signals. To put an array on the moon, "we'd stuff the plastic onto a rocket and then unroll the wires and dipoles" like a carpet when astronauts got there, Hewitt says.
She also suggests a more immediate possibility: An array of antennas strapped to the outside of a lunar-orbiting craft could search for signals from the early universe every time the satellite passed behind the moon.
DARK VIEW Radio telescopes might not be assembled on the far side of the moon for more than a decade. But some of the first astronaut crews returning to the moon could install devices on the near side that might shed light on the accelerating expansion of the universe (SN: 5/22/2004, p. 330).
Some astronomers attribute the accelerated expansion to a mysterious entity called dark energy. Others reject that notion, instead proposing that Einstein's theory of gravitation may have to be modified. Gia Dvali of New York University and his colleagues propose that gravity doesn't precisely follow Einstein's theory because some of the field leaks away into extra, hidden dimensions (SN: 5/22/04, p. 330). Leaky gravity would produce the same cosmic-expansion effects as dark energy and would have other consequences. For example, it would slightly alter the way that the moon wobbles in its orbit about Earth.
New measurements of the Earth-moon distance, which varies as the moon's elliptical orbit takes that satellite slightly closer to and slightly farther away from our planet, could reveal the altered wobble, says Tom Murphy of the University of California, San Diego.
Astronauts have already measured the Earth-moon distance using lasers that bounce off mirrors installed on the moon 38 years ago by the Apollo 11 astronauts. But the mirrors have degraded over time. A new system of mirrors, which could be installed on one of the first return missions, would measure the distance to a few millimeters, 10 times the accuracy of current measurements. That may be accurate enough to reveal whether the behavior of gravity differs from that predicted by Einstein's theory.
ORBITAL ADVANTAGE Some astronomers argue that many critical observations can be made better and more cheaply from lunar orbit rather than from the moon's surface. A moon-based telescope can perform better than a similar detector on Earth, notes Dan Lester of the University of Arizona in Tucson. With the Hubble Space Telescope now demonstrating that an orbiting observatory can be pointed accurately, temperature controlled, and reliably serviced, are astronomers better off "putting something down on the moon's surface or placing it in free space?" Lester asks.
For some kinds of telescopes, such as the liquid mirror, the moon's gravity and solid surface offer an advantage. But some visible-light and infrared telescopes might be more easily assembled and operated in the weightless environment of space. They then wouldn't have to contend with lunar dust, which tends to seep into and degrade optical instruments. To scan wider swaths of the sky, space-based detectors would also be easier to move or steer than instruments on the crater-packed moon, Lester says.
Some lunar orbits offer a particular advantage, he and other astronomers note. At a region about 84 percent of the way toward the moon, Earth s gravity balances that of the moon. A spacecraft in such an orbit, known as the Earth-moon L1 point, requires relatively little fuel to maintain its position (SN: 4/16/05,p. 250). Moreover, little energy is required to send a craft at L1 to a similar balance point between the Earth and the sun, known as Earth-sun L2.
The L2 orbit offers an unobstructed view of the heavens and easy communication with Earth. The Wilkinson Microwave Anisotropy Probe, which measures the radiation left over from the Big Bang, resides there, as will the James Webb Space Telescope, the proposed successor to Hubble. Craft can travel economically back and forth between L2 and L1, along part of what astronomers call the interplanetary highway. Astronauts could travel from Earth or the moon to park in a spacecraft at L1.
For example, Cash outlines how a pair of spacecraft residing at L2 could hunt for an image of a planet circling a star outside the solar system. The faint light from these orbs is swamped by the glare of their parent stars. Astronomers often place a coronagraph, or artificial mask, inside a telescope to blot out the light of the parent star. But such devices are costly.
In Cash's proposal, one craft would carry a large space telescope while its companion, 20,000 to 50,000 km away, would carry a giant shade. Astronomers would position the shade to keep light from a distant star out of the telescope. Any planets around the star would then spring into view.
A shield 30 to 50 meters across would enable the telescope to see a body as small as Earth's moon orbiting a nearby star, Cash reported in the July 6, 2006 Nature.
Thrusters on the starshade craft would consume much fuel to maintain its desired position within a meter. "When it gets low on fuel, it flies from L2 to L1, about a million-km trip, where the astronauts will rendezvous to fill up the tank," says Cash. "Then it flies back out to L2 again. That takes the starshade 6 months but allows another 3 years of operation."
Astronomer Neff deGrasse Tyson of the American Museum of Natural History in New York City, says that he's unsure how the new emphasis on the moon will play out for astronomy. On the one hand, he says, "I have a great concern that the moon looms so large on our horizon that we may be distracted by it and end up having no destination [for space exploration] beyond it."
But Tyson worries that if NASA is devoting much of its resources to a return to the moon, "and astronomers are not anywhere to be seen, felt, or heard in that shift," then their research might just drop out of sight.
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|Date:||Mar 24, 2007|
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