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The potential science from future lunar missions stretches far beyond the moon.
A new radio telescope on the far side of the moon could capitalize on NASA's new Artemis era of lunar exploration, say scientists who hope to one day use such a telescope to potentially probe deeper into the universe than even the newly-operational James Webb Space Telescope can.
"The argument for putting a radio telescope on the far side of the moon is to look at lower radio frequencies that are otherwise heavily polluted by human radio transmissions on Earth," Steven Kahn, a physicist at Stanford University in California, told Space.com.
Kahn led one of the panels of the recent astronomy and astrophysics decadal survey (Pathways to Discovery in Astronomy and Astrophysics for the 2020s), focusing on electromagnetic observations from space. A proposal for a lunar telescope named FARSIDE, led by Jack Burns of the University of Colorado, Boulder, was submitted as a potential 'probe-class' mission costing $1 billion to $2 billion. Burns has been working on a plan for a radio telescope on the moon since the 1980s, but ultimately FARSIDE wasn't selected for recommendation by the decadal survey.
However, a brand new concept is now being evaluated called the Lunar Crater Radio Telescope, which is led by Saptarshi Bandyopadhyay of NASA's Jet Propulsion Laboratory in California.
"We're wanting to build a 350-meter-diameter radio telescope in a 1.3-kilometer-wide crater on the far-side of the moon," Bandyopadhyay, who is a robotics technologist, told Space.com. (That would be a 1,150 foot wide dish in a 0.8 mile crater.)
The original plan was for an even larger telescope, 0.6 miles (1 km) in size, but the sheer scale of that instrument proved unfeasible because of the total mass that would need to be launched from Earth. Fortunately, "As we looked at the science, we realized that a diameter of 350 meters would be sufficient to give us the science that we want to get," Bandyopadhyay said.
That science involves seeing back in time to a distant era known as the Epoch of Reionization. Shortly after the Big Bang there were no stars and galaxies, only a vast fog of neutral hydrogen. This period has been dubbed the "Cosmic Dark Ages." Eventually the hydrogen began coalescing to form stars and galaxies, lighting up the universe and ionizing the neutral hydrogen. It is this early era that the Lunar Crater Radio Telescope would hope to see.
Ordinarily, neutral hydrogen emits radio waves at a wavelength of 8.3 inches (21 centimeters), but cosmic expansion will have lengthened the wavelength of the radio waves emitted by hydrogen in the Dark Ages to extremely long wavelengths of tens of meters.
Detecting this long-wavelength light on Earth is difficult, partly because the ionosphere (the upper realm of our atmosphere) can reflect light of these wavelengths back into space, and partly because terrestrial radio interference can obscure it. The thing to do would be to build a giant radio telescope on the far-side of the moon, where there is no ionosphere, and where the moon itself can shield the telescope from interference from Earth.
"The recent astrophysics decadal survey talked about the need to understand what the dark ages looked like, and that to do this kind of work, a global single-receiver measurement is required," Bandyopadhyay said. "That's exactly what we're proposing."
Bandyopadhyay's plan is to send a spacecraft to a suitable crater on the lunar far side. The spacecraft would land inside the crater and then fire multiple cables with anchors into the rim of the crater, where the anchors would penetrate securely into the lunar regolith. The cables would then pull tight, creating a framework to hold the wire mesh radio dish, which would be folded like origami into the lander and open as the cables pull taut. A feed antenna would be deployed above the dish and a spacecraft overhead would first provide a beacon signal to help calibrate the telescope and then act as a relay for data and commands, since Earth cannot be seen from the lunar far side, hidden as it is behind the body of the moon.
At least, that's the plan. The Lunar Crater Radio Telescope is currently in phase II of NASA's Innovative Advanced Concepts (NIAC) development program, through which it has received $500,000 of funding for maturing the necessary technology. But that's a far cry from the multi-billion dollar budget that would be required to make the telescope mission a success.
Because of the hefty price tag, the telescope requires strong support from the scientific community to win a recommendation in the next decadal survey.
"What we have to do right now is present a very strong case why this should be done," Bandyopadhyay said. "So hopefully when the next decadal survey comes around, we have put enough time and effort into this that they recommend the mission."
However, the same issues that worked against FARSIDE could also work against the Lunar Crater Radio Telescope, specifically that while it will do important science, it is also too niche, Kahn said.
"Given its limited focus, it is going to be hard to prioritize that and get it ranked number one [in the next decadal survey in 2030]," he said.
However, there may be another option, suggested Kahn (who is not directly involved with the project), in that it dovetails with NASA's desire to do more exploration on the moon. "There are programmatic reasons to want to do more frequent flights to the moon, and so the hope is to try and capitalize on that," he said.
This approach is embodied in NASA's Commercial Lunar Payloads Services (CLPS) program, which will see more than 50 small science payloads be delivered to the moon by private contractors over the next three years. Three of these payloads will head to the 194-mile-wide (312 km) Schrödinger Crater, which is an impact basin on the far-side of the moon near the lunar south pole, in 2025 on a lunar lander built by the Massachusetts-based aerospace company Draper.
"From a science perspective, the far-side of the moon is one of the primary locations that we want get information from," Debra Needham, a planetary scientist at NASA's Marshall Space Flight Center in Huntsville, Alabama, told Space.com.
Schrödinger is an interesting target, and is thought to be the second youngest impact basin on the moon. The three payloads, named the Farside Seismic Suite (FSS), the Lunar Interior Temperature and Materials Suite (LITMS) and the Lunar Surface ElectroMagnetics Experiment (LuSEE), will investigate the relationship between the formation and size of the crater, as well as the interior structure of the moon.
"The impact that formed Schrödinger was so large that we think it penetrated all the way through the moon's upper crust and into the mantle," Needham said. "Exploring that will be really important in helping us understand the bulk chemistry of the moon, what the moon is made from inside and out, and how that helps us understand how the moon formed and how it cooled off from its hottest, earliest days when it was volcanically active and a much more dynamic place than it is today."
For example, the seismometers will listen for moonquakes caused by meteorite impacts or by the strain on the moon's interior caused by gravitational tides emanating from Earth. As the seismic tremors reverberate through the moon's interior, the signal that FSS will detect can tell scientists about the structure, composition and density of the moon's interior.
Meanwhile the LITMS experiment is armed with a heat-flow probe with which it will be able to take the internal temperature of the moon for further details about the interior of the moon.
"The heat-probe measurements during Apollo were made at an anonymously hot region of the moon," Needham said. "There's a chemical anomaly on the near side that gives off heat, so LITMS will be our first time getting measurements outside of that area."
The final experiment, LuSEE, will study dust in the moon's exosphere, which is the thin layer of gas and dust that clings close to the surface and is sputtered off the ground by micrometeorite impacts and electrostatic forces.
Together, this trio of small robotic experiments could pave the way for further exploration of the far side. Such exploration would follow in the footsteps (or should that be wheel-tracks?) of China's Yutu 2 rover, which became the first spacecraft to land on the far side when it touched down on Jan. 3, 2019, in a crater called Von Kármán.
NASA's Artemis 1 mission to the moon will launch on Monday (Aug. 29) during a two-hour window that opens at 8:33 a.m. EDT (1233 GMT). You can watch live launch coverage courtesy of NASA and follow Space.com's live updates on the mission.