Using In Situ Resource Utilization and Robotic Mobile Manufacturing on the Moon
now that we’ve put all the cool kids to sleep, let’s talk about space stuff
(this post is an exploration of understanding why an idea I had may or may not make sense, I tried to make it fun, clarification requests are very welcome)
Last night during dinner a friend and I were talking about future space missions and the kinds of missions we would like to see. An idea that I suggested would be to send one or more rovers to the far side of the moon to turn the Lunar surface into a massive radio telescope. I imagined a small rover that would go around reshaping lunar regolith (aka moon dust) into small indentations that would help to concentrate radio waves. Once the parabola was formed the robot would position a small module to capture the reflected radio waves. While individually these small radio telescopes would have relatively low sensitivity, my thought was that if you had massive fields of regolith antennae you could start to get a pretty interesting image of the night sky.
As it turns out I am not the only one who wants to deploy a rover on the far side of the moon. The Lunar Farside project at UC Boulder wants to send a rover to the far side of the moon and deploy a collection of radio receivers to build a massive telescope array to try to look into our universe soon after the big bang. Now the Farside project has their rover “simply” driving around and dropping off its sensors in a pattern otherwise only leaving footprints. So we have to ask, why aren’t they doing something closer to my “brilliant idea”? (well it’s 90% likely a combination of the kind of radio astronomy they want to do and that my idea doesn’t make sense from a practical engineering standpoint, but I’m going to deep dive so I learn something)
I will preface my breakdown that I am by no means an expert on radio astronomy or even experienced, like many of my posts I am going off of buzzwords and a desire to think things through (with a healthy dose of Wikipedia articles)
A preface on radio telescopes. Radio telescopes work by focusing one or more radio frequencies on a sensing element that converts those radio waves into electrical signals that are then processed by computers to figure out what they were looking at.
What I envisioned when BSing with my friend was that we would have a rover that would roll across the Lunar surface, like Wall-E, picking up dirt and forming it into things. This lunar robot would reshape regolith into a shape that would be useful for whatever the radio telescope receivers would look like, instead of Wall-E’s trash cubes.
That sounds pretty dope until we start thinking about time and energy to make our radio mounds. There is a relationship between the area of your radio telescope and the telescope’s gain (aka how good your telescope is at detecting signals). Generally speaking the bigger your telescope the fainter the signal you can find.
The equation for calculating gain is
Gain= (4*Pi*Area of Antenna/Wavelength detected^2)*Aperture efficiency
Aperture efficiency basically says, on a scale of 0 to 1 my sensor is detecting some quantity of the energy actually aimed at it. For modern antenna the aperture efficiency is generally between 0.5 and 0.77 aka, around half to three quarters of the energy that the detector is supposed to “see” is actually being registered.
As an example researchers at SETI think that we will have good luck finding signals from alien life looking for radio waves with a 21 cm wavelength or a frequency of 1420 MHz.
If we try to make a circular indent 2 meters in diameter, we can use the equation Gain = Aperture Efficiency*(Pi*Diameter/Wavelength)^2
Therefore our gain would be between 447 and 689, which is certainly an improvement, but for comparison a 25 meter diameter telescope looking for the same signal, would have a gain of around 107,000 (assuming an Aperture Efficiency of 0.77. Assuming we could guarantee that if we just had a boatload of these smaller telescopes working together how many would we need?
Best Case (again I feel I’m missing something, but I want to show forward momentum) Gain Bigger Telescope/Gain Smaller Telescopes = Number of smaller telescopes needed
107,000/689 gain/telescopes= 157 telescopes
If we were to put our 2 meter diameter telescopes on a grid pattern, the grid would be pretty big (26 meters by 24 meters). To get the same sensitivity of a single 25 meter diameter radio telescope (assuming my logic is correct), we would need to cover around 624 square meters of the moon in radio telescopes, that’s about 25% larger than what the single 25 meter telescope. That’s not great, but not super terrible. Greater sensitivity usually means more cost.
Ok, so maybe our bigger array of telescopes will take less material and hopefully less material means faster completion. Using CAD software and some wild assumptions it looks like each telescope would need to move between 150 and 500 liters of regolith to make the indent for each telescope. To make the 157 unique mini telescopes we would need to move at least 122 cubic meters of regolith. If we were to try making a single 25 meter diameter indent in the lunar regolith we could potentially get by moving only 70 or so meters of regolith.
Great, then let’s just make that single giant telescope out of dirt, not so fast. Our ability to reliably place regolith isn’t that great (I mean we’ve only really done lab projects and Earth based competitions). Back in the 1990s NASA had egg all over its institutional face after it was determined that Hubble’s optics were just a little off, and they had to spend billions to fix things. Fixing something in Low Earth Orbit is likely to be easier to get around to than fixing a foul up on the moon. The advantage of having a swarm of small dishes that if one or two aren’t great, you still have over 99% functionality.
What does this mean for the UC Boulder team, that my presumption at the beginning of the post was correct, in that my idea wasn’t as viable as I had hoped. The UC Boulder team is planning on using a collection of Dipole antennae to investigate a range of radio phenomena. Apparently dipoles do not require the same focusing tools (I tried to follow the articles on dipole radio telescopes and came out pretty confused)
This doesn’t mean that we won’t eventually have potentially thousands of mechanically simple radio telescopes on the moon aiding in our ability to understand the universe. Where I think lunar radio telescopes will be able to really take off is when we have more consistent manufacturing capacity on the moon. It would be reasonable to develop a simple design that could link together to form fairly large parabolas of telescopes. Many Earth based radio telescopes are mostly large collections of shaped aluminum, with all of the expensive bits concentrated in a small package. If at some point in the future we have the ability to cast and machine aluminum semi affordably we will see a fleet of telescopes that will put the Earth’s largest arrays to shame.
I hope this was interesting, this was more me trying to understand what I didn’t previously understand, in a narrow way, if you have any questions or corrections they are welcome.
Additional links
https://www.skatelescope.org/aperture-arrays/ an article from the square kilometer array on other ways to build really large telescopes
https://www.atnf.csiro.au/outreach/education/pulseatparkes/radiotelescopeintro.html background on Radio Telescopes from the Australia National Telescope Facility (see not everything is Wikipedia)
https://www.astronomynotes.com/telescop/s6.htm an explanation on why telescopes are better when they are bigger (technically this is for optical telescopes, but many of the principals can be applied)
https://www.astronomynotes.com/telescop/s7.htm here we go for radio telescopes
https://www.almaobservatory.org/en/
they have a cool education guide on radio telescopes https://almaobservatory.org/wp-content/uploads/2016/11/edu_0072.pdf
this article explains how the gain calculation function was derived https://nvlpubs.nist.gov/nistpubs/jres/64D/jresv64Dn1p1_A1b.pdf
https://www.researchgate.net/post/What_is_the_relationship_between_antenna_bandwidth_and_bit_rate a thread on discussing how to estimate how much data you can transmit over a given frequency (this is more for another project I was working on, but I wanted to have a call back point)
https://www.giangrandi.org/electronics/anttool/gain.shtml this page has a calculator for estimating bandwidth for a given design (not sure how good it is, but it was cool enough to include)