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NASA’s Planned Lunar Radio Telescope Will See the Universe’s Dark Ages

Image courtesy of NASA niac2020_bandyopadhyay.jpg
The layout of the Lunar Crater Radio Telescope.
NASA has a plan to install a giant radio telescope into a crater on the far side of the moon that won’t be obstructed by Earth’s ionosphere.

Astronomy fans and scientists were dismayed when Puerto Rico’s Aricibo radio telescope collapsed, but NASA has plans for a replacement that will blow away the capabilities of that facility by putting a telescope that is nearly three times as large into a crater on the far side of the moon.

The Lunar Crater Radio Telescope (LCRT) will be a kilometer in diameter, in comparison to 300 meters for the collapsed Aricibo radio telescope and 500 meters for the FAST facility in Guizhou, China.

NASA’s plan is for a lander to drop the necessary components into a suitable three-to-five-kilometer crater on the far side of the moon, where dual-axle robots called DuAxels would assemble the antenna in situ.

On the moon, the telescope can see the universe at wavelengths greater than 10 meters, which equates to frequencies of 30MHz and lower. This radiation is reflected by the Earth's ionosphere, which has prevented astronomers from studying the universe at these frequencies. Putting an observatory on the moon not only lifts the antenna above the ionosphere, but by locating it on the far side, the Moon acts as a physical shield that isolates the telescope from radio interference from Earth-based sources, Earth-orbiting satellites, and Sun’s radio noise during the lunar night.

“While there were no stars, there was ample hydrogen during the universe’s Dark Ages – hydrogen that would eventually serve as the raw material for the first stars,” said Joseph Lazio, radio astronomer at NASA’s Jet Propulsion Laboratory and a member of the LCRT team. “With a sufficiently large radio telescope off Earth, we could track the processes that would lead to the formation of the first stars, maybe even find clues to the nature of dark matter.”

Image courtesy of NASAniac2020_bandyopadhyay_2.jpg

These benefits led to the proposal of such a telescope as early as the 1950s, but this idea was dismissed as impractical, according to Saptarshi Bandyopadhyay, a robotics technologist at JPL who has been tasked with developing the mission plan for the LCRT.

“Specific technical challenges were identified that made an Aricibo-type telescope on the moon infeasible,” he recalled in a lecture outlining the project. “They said that selection of an existing lunar crater on the far side, design of thermal strain compensation to survive the temperature fluctuations from 100 degrees centigrade to minus 173 degrees centigrade over a lunar day, and rim-to-flow transportation were too difficult,” said Bandyopadhyay. “Moreover, Aricibo-type foundational elements, support structures, and restraint anchors are too heavy,” he added.

The LCRT’s design takes these challenges into consideration to avoid some of the obstacles. “In the LCRT database, there are over 82,000 craters in the three-to-five-kilometer range that are excellent candidates for LCRT. As a proof of concept, we built CAD models of the LCRT mesh inside these craters. We can safely conclude that finding existing craters on the far side of the moon is not a challenge anymore.”

With a crater identified, the LCRT team will use a telescope designed to be easier to construct than the Aricibo dish was. “We use a spherical trap-shaped reflector mesh so that the focus of this reflector is a half-radius below the center of the sphere,” said Bandyopadhyay. “The receiver antenna feed system is located at this focus point. Since both the wire mesh and the receiver antenna feed system are suspended inside the crater, we do not need Aricibo-type heavy foundational elements or support structure.”

Not only that, but the moon’s one-sixth gravity will make lifting the necessary mass easier than it would be on Earth. The reflector is a lightweight mesh with 2.5-meter spacing because that looks solid at the relevant wavelengths. “This is an important science requirement that enables the mesh to work like a perfect reflector since ultra-long 10-meter wavelengths cannot see the giant 2.5-meter by 2.5-meter holes in the mesh,” Bandyopadhyay remarked. “This makes the LCRT reflector considerably lighter than the continuous sheets used in Aricibo.”

The mesh is made of radial wires running out from the center and concentric rings tying the radial lines together. “The thickness of the radial wires is designed so that the shape of the free-hanging wire mesh conforms to the desired circular shape,” said Bandyopadhyay. “Since linear density changes uniformly due to temperature changes, the wire mesh passively maintains its shape across large thermal fluctuations.”

For the launch mission, the plan is a single launch with a vehicle carrying two landers. “One half carrying the reflector mesh and the receiver antenna lands in the crater flow,” he said. “The other half carrying some dual-axle rovers, and power and communications equipment, lands on the crater rim.” The dual-axle rovers were conceived with Mars missions in mind, but they are also perfect for climbing up and down steep terrain on the moon.

“DuAxel solves many of the problems associated with suspending such a large antenna inside a lunar crater,” said Patrick Mcgarey, also a robotics technologist at JPL and a team member of the LCRT and DuAxel projects. “Individual-axle rovers can drive into the crater while tethered, connect to the wires, apply tension, and lift the wires to suspend the antenna.”

Bandyopadhyay describes the deployment mission as falling into three acts.

“First we deploy guide and lift wires,” he said. “Our favorite concept is a tethered robot concept where the dual-axle robot goes into the crater and retrieves guide wires from the spacecraft lander.”

“Second, we lift the receiver antenna structure. We can use either stationary or mobile lift options.”

“Third, we will unfold the reflector mesh. One example is an origami approach. As we start to pull the guide wires, the [stowed reflector] disk starts to spin and release constraints. The mesh starts to unfold and expands to its full diameter of one kilometer.”

 With this infrastructure in place, the telescope will be able to gather information on the universe’s Dark Ages for the first time. “If successful, the LCRT would provide groundbreaking scientific insights into the evolution of the universe,” Bandyopadhyay said. Additionally, LCRT won’t just be the largest filled-aperture telescope in the world, it will be the largest in the solar system!

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