Is it really necessary to have a lander to perform radio-astronomic observations in moon's shadow? Isn't it easier to have an orbiting spacecraft instead and perform observation while it's orbiting behind the moon?
It's not necessary, but is significantly more radio-quiet than a lunar orbit. And secondly, though unfortunately not something we could really exploit this time, the stable temperatures of the lunar night greatly help with calibration for sensitive measurements like the 21cm Dark Ages signal
Isn’t the benefit here that you don’t have to deal with things such as significant Doppler shift, or having to maintain a supply of propellant for orbit-keeping?
My immediate thought was why not put it in the Earth-Moon L2 Lagrange point, like the James Webb Space Telescope, where it would be permanently shaded from RF from both the Earth and the Sun. But...
1. James Webb is in the Earth-Sun L2 point, where it is largely (though not completely) shaded from the Sun. A radio telescope at Earth-Sun L2 wouldn't be shaded from Earth RF. [edit: JWST is in a halo orbit which keeps it out of the shadow]
2. The Earth-Moon L2 point is shaded from the Earth, but not the Sun. So no benefit compared to the far-side lunar surface.
3. According to TFA, being on the lunar surface gets the telescope out of the solar wind, which is noisy at the low radio frequencies being observed.
There is perhaps some extra opportunity in a 10-14 day solid observation window, but I don't see why a satellite version couldn't still work in smaller windows.
Another reason could be testing for building a much large radio antenna on the moon's surface in the future which is mentioned to farther down in the article. The moon itself and it's dust has electromagnetic effects that might effect measurements and learning about them now could help future planning.
You'd build an array (see e.g. VLA mentioned in the article or SKA), and it is much easier to combine the data from an array if everything isn't flying around and so there are varying distances between the antennae.
Not for radio telescopes, but how is the current state of optical interference? Would it help if we didn't have to use adaptive optics to compensate for atmospheric turbulence (and have subtly different images at the different telescopes)?
Very readable explanation of why and how useful a dark-side lunar radio telescope would be.
TLDR: As a result of expansion of the universe, over 13B years the wavelength of neutral hydrogen signals has been stretched from 21cm to 'tens of meters'. On Earth, this part of the spectrum is cluttered with noise from Earth and Sun. For 14 days at a time...not a problem on the dark-side.
Well the point is it needs to be both. The telescope needs to be on the far side to shield it from Earth, and the dark side to shield it from the Sun. But yes, it's only on the dark side 50% of the time.
Personally, I don't find the phrase 'fully illuminated “dark side”' to be a convincing alternative to the physically more accurate term 'far side'. Of course NASA has only just emerged from the Earth's dark side as I write this (UK here, mid-morning), so I'm not expecting an immediate response from them.
And yes, I do know that 'side' is itself not entirely accurate because of libration [0] but that's a different hill to die on.
The far side is the darker side, though, at lunar night. Poetic proof: "The Earth shine might illuminate the light side of the Moon a little during the long night" (from Jules Verne, All Around the Moon https://www.gutenberg.org/cache/epub/16457/pg16457-images.ht...)
I do like poetry, but if we are looking at a crescent moon, in our night, it means that the bulk of farside is facing toward the sun, and will therefore be brighter than nearside
This describes lunar day on the far side of the moon, right? Excuse my ambiguity; I was comparing lunar nights only (inspired by the Jules Verne quote):
The far side is darker during lunar night (lit by starshine only; Full Moon on Earth) than the near side during lunar night (New Moon on Earth), because it receives both star- and max. Earthshine.
I'm not sure about Crescent Moon though: that only narrows the brightness gap slightly, right? Or I’ll have to ask if there’s an astronomer on board our flight.
Is it really necessary to have a lander to perform radio-astronomic observations in moon's shadow? Isn't it easier to have an orbiting spacecraft instead and perform observation while it's orbiting behind the moon?
It's not necessary, but is significantly more radio-quiet than a lunar orbit. And secondly, though unfortunately not something we could really exploit this time, the stable temperatures of the lunar night greatly help with calibration for sensitive measurements like the 21cm Dark Ages signal
on the other hand and orbiter could have a much larger effective telescope diameter by SAR.
Isn’t the benefit here that you don’t have to deal with things such as significant Doppler shift, or having to maintain a supply of propellant for orbit-keeping?
How significant would doppler shift be for a lunar orbit?
My immediate thought was why not put it in the Earth-Moon L2 Lagrange point, like the James Webb Space Telescope, where it would be permanently shaded from RF from both the Earth and the Sun. But...
1. James Webb is in the Earth-Sun L2 point, where it is largely (though not completely) shaded from the Sun. A radio telescope at Earth-Sun L2 wouldn't be shaded from Earth RF. [edit: JWST is in a halo orbit which keeps it out of the shadow]
2. The Earth-Moon L2 point is shaded from the Earth, but not the Sun. So no benefit compared to the far-side lunar surface.
3. According to TFA, being on the lunar surface gets the telescope out of the solar wind, which is noisy at the low radio frequencies being observed.
There is perhaps some extra opportunity in a 10-14 day solid observation window, but I don't see why a satellite version couldn't still work in smaller windows.
Another reason could be testing for building a much large radio antenna on the moon's surface in the future which is mentioned to farther down in the article. The moon itself and it's dust has electromagnetic effects that might effect measurements and learning about them now could help future planning.
You'd build an array (see e.g. VLA mentioned in the article or SKA), and it is much easier to combine the data from an array if everything isn't flying around and so there are varying distances between the antennae.
Not for radio telescopes, but how is the current state of optical interference? Would it help if we didn't have to use adaptive optics to compensate for atmospheric turbulence (and have subtly different images at the different telescopes)?
Very readable explanation of why and how useful a dark-side lunar radio telescope would be.
TLDR: As a result of expansion of the universe, over 13B years the wavelength of neutral hydrogen signals has been stretched from 21cm to 'tens of meters'. On Earth, this part of the spectrum is cluttered with noise from Earth and Sun. For 14 days at a time...not a problem on the dark-side.
It’s far side, not dark side. The moon doesn’t have a dark side anymore than the earth does
Well the point is it needs to be both. The telescope needs to be on the far side to shield it from Earth, and the dark side to shield it from the Sun. But yes, it's only on the dark side 50% of the time.
NASA uses "dark side" (meaning far side, not night side) when facing the public [0]:
> A series of test images shows the fully illuminated “dark side” of the Moon that is not visible from Earth.
> The far side of the Moon was first observed in 1959
[0] https://science.nasa.gov/earth/earth-observatory/the-dark-si...
> the fully illuminated “dark side”
Personally, I don't find the phrase 'fully illuminated “dark side”' to be a convincing alternative to the physically more accurate term 'far side'. Of course NASA has only just emerged from the Earth's dark side as I write this (UK here, mid-morning), so I'm not expecting an immediate response from them.
And yes, I do know that 'side' is itself not entirely accurate because of libration [0] but that's a different hill to die on.
[0] https://en.wikipedia.org/wiki/Libration#Lunar_libration
Agreed.
The far side is the darker side, though, at lunar night. Poetic proof: "The Earth shine might illuminate the light side of the Moon a little during the long night" (from Jules Verne, All Around the Moon https://www.gutenberg.org/cache/epub/16457/pg16457-images.ht...)
I do like poetry, but if we are looking at a crescent moon, in our night, it means that the bulk of farside is facing toward the sun, and will therefore be brighter than nearside
This describes lunar day on the far side of the moon, right? Excuse my ambiguity; I was comparing lunar nights only (inspired by the Jules Verne quote):
The far side is darker during lunar night (lit by starshine only; Full Moon on Earth) than the near side during lunar night (New Moon on Earth), because it receives both star- and max. Earthshine.
I'm not sure about Crescent Moon though: that only narrows the brightness gap slightly, right? Or I’ll have to ask if there’s an astronomer on board our flight.
> NASA uses "dark side" (meaning far side, not night side) when facing the public [0]:
That's not helpful, at all.
For 14 days a month, it's the dark side. That's the whole idea. Astronomy 101.