For decades, astronomers have advocated building radio telescopes on the far side of the Moon. This “radio-quiet” zone always faces away from Earth and would provide the perfect location to study a variety of astronomical phenomena that can’t be observed in low radio frequencies from our planet, or even by Earth-orbiting space telescopes. But the costs and logistics of such a project have pushed most of these concepts to the realm of futuristic dreams.
But now a group of astronomers and engineers have worked out a concept for a radio telescope placed on the lunar far side that could be as large as 100 square kilometers across, and it could be deployed from a robotic lunar lander and four two-wheeled rovers.
The Far-side Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) would use the rovers to deploy and operate an array of 128 dual-polarization dipole antennas on the surface of the Moon’s far side. The array would consist of a flat , thin (only a millimeter or two), antenna-embedded tape tether that also has optical communication and power transmission capabilities.
“The resulting interferometric radio telescope would provide unprecedented radio images of distant star systems, allowing for the investigation of faint radio signatures of coronal mass ejections and energetic particle events and could also lead to the detection of magnetospheres around exoplanets within their parent star’s habitable zone,” the team wrote in their pre-print paper, published on arXiv.
Additionally, FARSIDE would have the ability to characterize similar activity in our own solar system, from the Sun to the outer planets, including the hypothetical Planet Nine, said Dr. Gregg Hallinan, Professor of Astronomy at Caltech and one of the authors of the concept study.
“I would personally be most excited about the search for exoplanet magnetic fields of candidate habitable exoplanets,” Hallinan told Universe Today via email. “This may be a key ingredient for planetary habitability in our own solar system and we have practically no data yet on other exoplanets. It is why I pushed the design to very low radio frequencies that are 100x lower than those accessible from the ground or even Earth-orbit.”
Hallinan’s work on FARSIDE is based on his work in directing a newly upgraded array that attempts to perform a similar type of radio astronomy from Earth, called the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), located in California. The new upgrades will allow the observatory to image the entire sky every ten seconds, trying to detect the magnetic fields of Jupiter-like exoplanets.
“The design of FARSIDE is based on OVRO-LWA,” Hallinan said, “but will push 100 times lower in frequency from the Moon to chase those habitable planets. I am also very enthusiastic about the ‘dark ages’ science – the time before the first stars and galaxies formed — as that is the next big frontier in cosmology and is also completely inaccessible from the ground.”
While the idea for FARSIDE has been around for several years, new ideas for using robots and an uncrewed lander make the concept more feasible. The design for the robotic rovers for FARSIDE that would deploy the antenna tape is based on the Axel family of rovers that have been under development by the Jet Propulsion Laboratory for over 20 years. Some of the latest iterations for Axel allows the rovers to particularly excel at deploying instruments on rough surfaces, like the lunar far side. The two-wheeled tethered rovers can even work over steep and rugged terrains and even rappel down highly sloped terrain. JPL says the tethers provides mechanical support on steeper slopes and delivers power and communication from a host platform. Hallinan and team have designed the tether such that the instrument/antenna is also integrated within.
“By lying on the ground, the dipole actually has very good sensitivity looking straight up” to study the cosmos, Hallinan said.
The rovers would spool out four separate 12-km-long tethers, each with 64 remotely powered electronics nodes. The team analyzed four different designs for the array (see the illustration above), and the winner for the best performance was the four-arm spiral which “required the shortest rover trajectories, which implies easier accommodation of a smaller tether spool and less mass burden per rover.”
In addition to working with JPL, the team has also been working with Blue Origin, examining how the design of the FARSIDE telescope could be integrated into an existing lunar lander, and they used the Blue Moon Lander in their case study. Specifically, they worked with Steve Squyres, formerly the principal investigator for the Mars Exploration Rovers, Spirit and Opportunity, who is now the chief scientist for Blue Origin.
“We had a wonderful experience with NASA JPL, and with Steve and the Blue Origin team,” Hallinan said. “Obviously Steve brings a lot of practical nous to the table when it comes to the use of rovers too, which was an added benefit. We all got together every two weeks to discuss progress and to explore new directions to proceed. I was extremely happy with the results with the new design.”
Hallinan said the four small rovers provide much more redundancy than some of their previous concept designs. Additionally, the cold temperatures on the Moon (as low as 100 K) provided challenges, but engineers at JPL brought in a new design for the antenna receiver that has electronics that can operate at very low temperatures without adding in additional heating.
“We also were going to us multi-mission radioisotope thermal electric generator (MMRTG) to provide power to the base station, but the Blue Moon lander has solutions that allowed us to move in a new direction for that also,” Hallinan said. “As you can see from the paper, the new design is a major improvement.”
Hallinan said that one thing that may not be obvious in this relatively simple design is that this array is incredibly sensitive at very low radio frequencies.
“Dipoles typically get more sensitive as you get to lower radio frequencies,” he explained. “However, the main source of noise is the radio emission from own galaxy which is tremendously bright and gets brighter at lower frequencies at a rate that is faster than the rate at which the dipole gets more sensitive. Bottom line, you are typically getting less sensitive overall at lower frequencies from the ground! However, something magical happens at about 3 MHz. The galaxy becomes “optically thick” and does not get brighter below this frequency. But the dipoles keep getting better! The array is 100x more sensitive at 300 kHz as compared to 3 MHz thanks to this effect.”
Hallinan added that the night time lunar surface is the only place within the inner solar system where this can be done.
“In Earth orbit, ‘plasma noise’ from the solar wind is another source of noise that completely dominates,” he said. “A plasma cavity exists on the lunar surface, particularly on the night-time side that makes this possible.”
With all the work the team has done on the design, how quickly could the team turn around this design into a real mission?
“If we initiated the next formal step today, we could launch by 2028,” Hallinan said.