In the undulating, dust-covered Descartes Highlands, 380 kilometers southwest of Tranquility Base, where Apollo 11 landed half a century ago, a lonely gold-plated telescope has sat inert since 24 April 1972, when Apollo 16 astronauts John Young and Charles Duke blasted off the surface and left it behind. It was a small part of their 3-day mission, but a milestone for astronomers: the first observatory on another world.
The designer of the telescope was George Carruthers, a young researcher at the Naval Research Laboratory in Washington, D.C., who had made his name building ultraviolet (UV) telescopes for sounding rockets, which make short flights above the UV-blocking atmosphere. A big question at the time was whether the hydrogen in interstellar gas clouds was made of individual atoms or molecules of hydrogen (H2). The answer lay in its UV spectrum, which is difficult to capture from a sounding rocket.
Following the Apollo 11 landing, NASA called for science experiments to fly on future moonshots. Carruthers saw an opportunity to answer the hydrogen question—and gain a unique look at the gases surrounding Earth. Once his proposal was accepted, Carruthers had only 2 years to build the UV sensor—which converted photons to electrons and recorded that signal on film—and train Young and Duke to use the telescope. To block the sun's glare, the astronauts had to set it up in the shadow of the lunar landing module. Every so often, they would return to point the telescope at another target on Carruthers's list. At the end of the mission, the astronauts extracted a film cartridge and left the camera to its fate. That roll of film contained 178 frames, a meager haul, but enough for Carruthers to confirm the existence of interstellar molecular hydrogen—and reveal first-ever UV images of Earth's ionosphere and aurorae, the solar wind, and distant galaxy clusters.
His pioneering lunar observatory will not be the last. The Moon beckons astronomers because it is dry, airless, and seismically quiet. It has room for sprawling arrays of multiple instruments, and it turns slowly, allowing long exposures. For radio astronomers, its far side is a piece of heaven, entirely shielded from interference from terrestrial transmitters.
For many years, however, a lunar observatory has been a dream deferred. Just months after Carruthers's telescope was installed, Apollo 17's Eugene Cernan and Harrison Schmitt left the Moon in a cloud of dust, and the age of lunar exploration suddenly came to an end. That didn't stop astronomers from plotting and planning, but astrophysicist Peter Chen of NASA's Goddard Space Flight Center in Greenbelt, Maryland, says, "For a long time, [a lunar observatory] was not something you brought up in polite conversation."
The Moon is becoming a hot target again, and this time the players include not just NASA, but also other space agencies and commercial space companies. Astronomers are once again along for the ride. On 14 December 2013, China's Chang'e-3 touched down in the Mare Imbrium carrying a near-UV telescope. Although mostly a technology demonstration, the 15-centimeter scope has been collecting data on quasars, blazars, novae, and other hot bright objects and beaming them back to Earth in quantities Carruthers couldn't have imagined in the days of film.
That's just the start of what Steve Durst, president of the Waimea, Hawaii–based International Lunar Observatory Association, calls "this Moon-rush we're experiencing now." His organization hopes to launch a tiny, 1-kilogram precursor telescope with Moon Express of Cape Canaveral, Florida, or one of the other startup companies vying for NASA contracts to take payloads to the Moon. Once missions start to go to the Moon's south pole, the association hopes to send a 2-meter radio dish and a small optical scope to the peak of the 5000-meter Malapert Mountain. The peak receives around-the-clock sunlight, useful for solar power, and its unobstructed view of Earth could aid data transmissions. And the Indian Institute of Astrophysics in Bengaluru is developing a small near-UV imager that the launch company OrbitBeyond would carry on its first mission in 2020.
Astronomers are arguing for far larger radio observatories. From the Moon's quiet far side, they hope to peer back to the universe's dark ages, before the first stars began to shine. Some even envisage building giant optical telescopes, larger than any on Earth, that could not just find Earth-like exoplanets, but see features on their surfaces.
"I've been thinking about this for 35 years and we're finally seeing some traction," says astronomer Jack Burns of the University of Colorado in Boulder. "The feeling is that the Moon isn't that hard to do anymore."
The idea of an observatory on the Moon goes back at least to German astronomers Wilhelm Beer and Johann Heinrich von Mädler, who drew the first accurate lunar maps in the 1830s. They realized that, lacking an atmosphere, the Moon would offer a pristine view of the sky. When the space age suddenly made the idea seem possible, U.S. astronomers held multiple lunar astronomy workshops beginning in 1965. At a large gathering in Annapolis in 1990, astronomer Harlan Smith of the University of Texas in Austin, a champion of the Hubble Space Telescope that was then due for launch, charted the progress of astronomy toward ever-larger telescopes in ever-more-remote and high-altitude locations. "The steady unfolding of this tradition should soon lead us to the Moon—the best place in the solar system from which to do many if not even most kinds of astronomy."
Before long, however, orbiting space telescopes such as Hubble, Spitzer, and Chandra were scoring major successes across the electromagnetic spectrum. Few could see the gains of shipping a telescope all the way to the Moon for a risky landing when a cheaper trip to low-Earth orbit would work just as well.
One group has continued to hold out for the Moon: radio astronomers. On Earth, radio telescopes must tussle with signals from terrestrial TV and radio transmitters for clean looks at the sky. The problem is worst for observations at low frequencies, below 50 megahertz (MHz). At those frequencies, the ionosphere, a layer containing charged particles high in the atmosphere, can refract, disperse, or completely block radio waves. Yet such low-frequency waves hold clues to events in the deepest, darkest era of the universe's history.
Several hundred thousand years after the big bang, the roiling plasma of charged particles it created had cooled enough for electrons to latch onto protons and make hydrogen atoms, releasing a burst of light that we now see as the cosmic microwave background (CMB). Then, the dark ages began. For several hundred million years there was nothing in the universe but dark clouds of neutral hydrogen and a few other atoms, slowly being pulled together by clumps of dark matter.
The starless universe wasn't completely dark. On rare occasions the spins of the protons and electrons in neutral hydrogen atoms flip from being parallel to antiparallel, emitting a photon at a frequency of 1.4 gigahertz—the so-called 21-centimter radiation. This dark ages signal is just a whisper. But, theorists calculate, a sufficiently large array of antennas should be able to detect it. They could also trace its disappearance later in cosmic history, as the first stars ended the dark ages by bursting into light and ionizing the hydrogen around them with their UV radiation. The details of the process could reveal whether the first generation of massive stars did most of that ionization, or whether much more powerful quasars, extremely bright galactic cores centered on voracious black holes, also played a role.
The epoch contains far more data than the CMB, which only records a brief moment in cosmic time. "This is a 3D volume with primordial fluctuations in there, just waiting to be measured," says astronomer Jacqueline Hewitt of the Massachusetts Institute of Technology in Cambridge.
Charting the dark ages' 21-centimeter radiation "is essential. It has to be done at some point," says radio astronomer Heino Falcke of Radboud University in Nijmegen, the Netherlands, who works on the Low Frequency Array (LOFAR), a vast radio telescope spread across northern Europe. The problem is that after 13 billion years of cosmic expansion, photons from the dark ages arrive at Earth stretched to frequencies below 50 MHz, where they not only have to contend with the ionosphere, but also with confounding signals from ham radio, aviation, and long-distance communications. LOFAR, for example, has struggled to detect the 21-centimeter signal from that epoch. The Hydrogen Epoch of Reionization Array, which Hewitt and others are building in South Africa, may have a better shot. But Falcke and others are looking to the Moon.
Burns's 35-year campaign for a lunar radio telescope won him NASA funding in 2008 to form a team called LUNAR to work out how to build it. The researchers designed a lunar telescope array that would include hundreds of simple dipole antennas laid flat on the ground. They demonstrated how autonomous rovers could lay out strips of conducting film to act as antennas. In a 2013 experiment, astronauts on the International Space Station remotely guided a rover that laid out antenna strips on a simulated moonscape at the NASA Ames Research Center in Mountain View, California, to show how a future crew onboard NASA's proposed Moon-orbiting Lunar Gateway station could supervise construction.
A more recent NASA-funded project, the Network for Exploration and Space Science (NESS), is drawing up plans for lunar radio telescopes that could probe exoplanets as well as the early universe. Planets that have a magnetic field emit low-frequency radio waves when electrons spiral around the field lines. Because planetary magnetic fields require an internal dynamo, like a spinning liquid metal core, detecting a field could offer clues to an exoplanet's interior—and, Burns says, "could play a key role in assessing [its] habitability." Earth's magnetic field, for example, not only protects life from cancer-causing high energy particles, but is also thought to have shielded water in the atmosphere from being lost to space.
Earth-based radio arrays might be able to detect the radio emissions from a Jupiter-size exoplanet, with a mighty magnetic field. But a lunar observatory might pick up the fainter—and lower frequency—emission of a rocky exoplanet.
NESS is working on two proposals. The first is a $60 million suitcase-size satellite with antennas on four sides that will tune in to a range of wavelengths by unfurling like frog tongues to lengths of up to several meters. It will orbit the Moon and, when above the far side and shielded from Earth, will try to detect the dark age 21-centimeter signal. Known as DAPPER, the Dark Ages Polarimeter Pathfinder satellite should be able to map the radiation in enough detail to determine how dark matter tugged the primordial hydrogen clouds into clumps. That could help distinguish between rival models of cosmic structure formation.
The second is a billion-dollar class mission to put a basic radio telescope on the surface of the Moon. The Farside Array would use the techniques pioneered by the LUNAR consortium: small rovers laying out a total of 128 dipole antennas across an area 10 kilometers wide, supervised by astronauts in lunar orbit. This month, NESS submitted a proposal to Astro2020, the U.S. astronomy community's once-a-decade priority-setting exercise, in the hope of winning wider support.
European and Chinese researchers are thinking along similar lines. Last year, a team led by Falcke added a radio astronomy experiment to China's Queqiao probe, which was parked near the Moon to relay signals to and from the Chang'e-4 lander on the lunar far side. Now that Queqiao has completed its communications duties, Falcke says, the Netherlands-China Low-Frequency Explorer (NCLE) can start observations later this year. The NCLE will not get a clean 21-centimeter signal because it is not completely shielded from earthly interference, but it will be a testbed for observing in the lunar neighborhood in preparation for a bigger goal: a lunar LOFAR. "We're discussing with China about follow-up projects," Falcke says.
The European Space Agency (ESA) is also considering options. In May, the agency published a 10-year strategy for science on the Moon; its goals include testing low-frequency radio receivers on the far side. "To make the first observations [from the far side] would be fantastic," says James Carpenter, strategy officer in ESA's human and robotic exploration division in Noordwijk, the Netherlands. A larger array of antennas is "a long term aspiration," he adds.
Optical astronomers don't have the same all-or-nothing need for a lunar observatory. But that hasn't stopped them from imagining what they could see with a telescope as clear-eyed as Hubble, but with a mirror larger than could ever be launched into Earth orbit. The Moon's low gravity and rock-solid stability mean support structures for a giant telescope could be simple and cheap. But here's the catch: The mirror would have to be made in situ.
One solution, proposed by a NASA-funded team last decade, is to build a liquid mirror. The idea is simple: Construct a large shallow circular dish, fill it with a liquid, and set it gently spinning. Centrifugal force will pull the surface into a parabolic shape. Liquid mirror telescopes on Earth use mercury, which is naturally reflective. The biggest so far was the 6-meter Large Zenith Telescope in British Columbia in Canada. That testbed telescope, built in 2003 and now decommissioned, paved the way for the 4-meter International Liquid Mirror Telescope, which will take its first view of the sky from the Devasthal Observatory in India later this year. Although a liquid mirror is limited to looking straight up, the rotation of Earth—or the Moon—scans it across the sky.
Mercury won't work on the Moon—it would evaporate in the lunar vacuum and is too heavy to transport from Earth. In its place, the NASA team came up with a class of organic compounds called ionic liquids, essentially molten salts, that would remain liquid in the frigid lunar night. Ionic liquids are not reflective, but could be glazed with silver to make an ideal reflecting surface. Superconducting bearings could levitate the dish and keep it spinning frictionlessly. "In principle there is no limit on the size," says team member Ermanno Borra of Laval University in Quebec City, Canada. "This would be totally impossible in space, but not that expensive on the Moon."
The Canadian Space Agency (CSA) followed up the NASA study by looking at the practicalities of building such a scope. "There were no showstoppers and the mechanical tolerances were more relaxed than for a space telescope," says Paul Hickson of the University of British Columbia in Vancouver. A 20-meter telescope, CSA concluded, would require no more than 3.5 tons of material to be transported to the surface. An even larger instrument, as big as 100 meters across, "would be in a class of its own," Hickson says, able to study the very first stars that formed and coalesced into galaxies at the end of the dark ages.
Chen wants to make giant lunar mirrors from moondust, minimizing the mass needed to be brought from Earth. In a small laboratory test last decade, he mixed a simulated lunar soil, or regolith, with epoxy to produce a paste the consistency of melted chocolate. He spun the mixture in a dish, like a liquid mirror, then cured the epoxy with UV light, which is plentiful on the Moon. The result was a 30-centimeter parabolic dish that could be coated with aluminum to produce a mirror. Other parts of the telescope structure could also be made from regolith paste using a 3D printing machine, he says. More recently, Chen has made thin moondust mirrors that can be reshaped by actuators on the back surface to compensate for temperature changes and for gravity as the mirror moves.
Such mirrors could be made in batches, Chen says, and combined into optical interferometers, which could achieve extraordinary resolution when viewing distant objects. An interferometer harnessing mirrors 1 kilometer apart would have the resolution of a 1-kilometer mirror, even if not its light-gathering power, enabling astronomers to scrutinize the surface of a nearby Earth-like exoplanet. But combining the light of widely spaced telescopes requires monumental precision, which small earthquakes can disrupt. That's one reason why Earth-based optical interferometers remain experimental and span no more than tens of meters. Much larger interferometers might be feasible on the Moon, with its seismic quiet. "The Moon is the only place you can do it," Chen says.
Although many of these schemes may sound like science fiction, would-be lunar astronomers feel a sense of urgency. Other parties also hope to exploit the Moon, for purposes that could disrupt astronomy. Mining could loft dust and cause tremors; Moon bases would generate radio interference that could affect the far side.
For radio astronomers, their sensitive instruments increasingly deafened on Earth, that is a bleak prospect. Falcke says: "If the far side gets spoiled, I don't know where we would go."