The Promise of Astronomy on the Moon

Thirty years ago it was a good idea, and it still is today.

Apollo 16 UV view.jpg
Image of the southern sky in the far ultraviolet, taken by the first astronomical telescope on the Moon, Apollo 16 mission, April, 1972.

Imagine that you are an astronomer. You want to gaze at the universe in crystal clarity. Yet you look at the heavens through a murky, partly opaque sky; you must deal with light pollution and the dynamic, wildly unstable platform of the Earth’s surface. It’s frustrating – you dream of the great views you know you could get from space. That’s the ticket! Plus, locating a stable, rock-solid base in space (where you could build extremely sensitive instruments) would be a huge bonus.

For years, the Moon was seen as the ideal place to build and operate sensitive telescopes. Its low gravity permits the building of giant telescopes with enormous seeing power. The stable, seismically quiet base of the lunar surface would allow for the operation of multiple telescopes in unison – arrays, effectively creating one giant telescope with an enormous aperture (a technique called interferometry). The cold, dark sky as seen from space – unimpeded by clouds, air or other meteorological phenomena – affords superb viewing conditions (as twenty years of fantastic Hubble Space Telescope images have documented). So with such considerations, one might conclude that conducting astronomy from the lunar surface would be one of the prime activities desired by the astronomical community. Right?

Well, not quite. Back in 1984, efforts to build a community of supporters for a base on the Moon included many astronomers who supported such efforts on the basis of the considerations listed above. Throughout the early days of the return to the Moon movement, astronomers such as Harlan Smith of the University of Texas and many others campaigned tirelessly for recognition of the value of lunar-based astronomy. These studies culminated in the seemingly outrageous idea for a telescope using a spinning disk of liquid with a reflective surface, lining the interior of one of the millions of bowl-shaped craters on the Moon. Such an instrument would extend for kilometers, making a gigantic “eye” to look at the universe. One might think such an idea is crazy, but liquid mirror telescopes already have been constructed on Earth.

Interestingly enough, the launch and success of the Hubble Space Telescope resulted in the loss of support for lunar astronomy. The biggest advantage of space-based astronomy is views of a dark, clear sky. Such views are available in free space just as easily as they are on the Moon. Moreover, the big advantage of a stable platform on the lunar surface for observations is partly negated by technical developments that permit the assembly of free-flying platforms in space. Such developments might mean that a short wavelength interferometer could be built and operated without the need to go into the (small) gravity well of the Moon. These and other technical innovations led to a general loss of support within the community for astronomical science on the lunar surface.

One might be forgiven for suspecting that a long-standing antipathy against human spaceflight might have had something to do with the attitude of many astronomers. They might possibly have feared that the advent of a new human spaceflight endeavor would divert funds from their lengthy wish list of robotic missions and automated observatories. However, the idea that the Moon is somehow useful to astronomers still holds an attraction.

Recent work has focused on the value of using the Moon’s unique environment to observe some parts of the electromagnetic spectrum that we cannot access from Earth or even near-Earth space. Very long wavelength emissions (meter- and multiple-meter-scales) cannot be seen from the Earth’s surface because the layer of charged particles surrounding the Earth in space (the ionosphere) blocks such radiation. Even in orbit, interference from the ionosphere prohibits observations because of this “noise.” However, the far side of the Moon is permanently shielded from Earth’s radio noise by over 3,600 km of solid rock. From such a truly unique vantage point, we will be able to listen to the whisper of radio noise generated in the aftermath of the origin of the universe.

The Chang’E 3 lander carries a small telescope designed to look at the other end of the spectrum, the far ultraviolet (as the name implies, wavelengths shorter than visible light). The Chang’E telescope is producing data and although of small aperture, it can observe the sky at these wavelengths. Apollo 16 emplaced a UV telescope on the Moon back in 1972 and took ultraviolet photographs of the sky from the lunar surface, including the Earth and images of the southern sky (which includes two satellite galaxies to our own Milky Way galaxy – the Magellanic Clouds). These instruments documented the possible value of such observations from the surface of the Moon.

Other astronomers have looked in detail at how one might begin to utilize the unique environment of the far side to map the earliest stages of the history of the universe. One concept sends a teleoperated rover to the far side with a dual purpose. We could collect samples from the floor of the biggest, oldest basin on the Moon (South Pole-Aitken basin, an impact feature over 2,500 km in diameter) to test ideas about the early cratering history of the Earth-Moon system.  While we’re there, we could also lay out an antenna array designed to map the sky’s low frequency radio emissions.

The far side L-2 mission concept involves humans stationed 60,000 km above the Moon to operate the rovers and deploy the antennas. These antennas are quite simple. They consist of dipoles (i.e., linear wires) several tens of meters in length, all connected to a receiver capable of listening to those low frequency bands minus the static and noise of the terrestrial RF environment. Over the course of a year, as the Moon orbits the Earth (and both orbit the Sun), nearly the entire sky could be mapped from this robotically emplaced astronomical instrument.

Despite some starts and stops, the promise of conducting astronomy from the Moon continues to draw the attention of imaginative scientists. Using one of the forthcoming commercial lunar landers, a group of private enthusiasts plan to deploy a small telescope on the surface. When we some day stand on the Moon, we will not only look down to study the complex history preserved there, but we will also look outward, into an endless universe, just as many science fiction authors envisioned.

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