# What is the reason the Chinese want to measure the <30Mhz radiation behind the moon?

On the Earth it is difficult to detect radiation below 30MHz from space due to our atmosphere. But why do they want to measure this radiation behind the moon? It has something to do with the big bang, but isn't the frequency of CMB not around 160GHz?

The Chinese are planning to launch a satellite with a 30 MHz antenna on it in 2018.

Aren't there already enough decent measurements of the CMB? So what would they like to find concerning the Big Bang?

• Can you give some references or URLs for context? Jun 28 '16 at 23:31

## 2 Answers

Frequencies below 30 MHz have a hard time penetrating the so-called ionosphere - the series of layers of ionized atmosphere that exist at high altitudes.

This is good news for folks like me, amateur radio operators, because it allows us to speak into a transmitter at home, and let the ionosphere bounce the radio waves back to Earth, then again up into the ionosphere, and so on, eventually reaching another radio operator on the opposite side of Earth.

But it's bad news if you're a radio astronomer trying to receive such frequencies from distant objects - the ionosphere simply bounces those waves back into space.

So, apparently, you could raise a receiver above the ionosphere, in a satellite, and have it receive signals from outer space. But there's a catch. At 30 MHz and below, the wavelengths are huge. 27 MHz - the wavelength is 10 meters. 14 MHz - wavelength is 20 meters. 3.5 MHz - 80 meters. And so on, the lower the frequency, the bigger the wavelength.

The size of the antenna needs to be comparable to the wavelength if you want to receive signals from one direction only, so as to be able to tell where the signal is coming from. This also helps with efficiency. A very good antenna needs to be much bigger than the wavelength for it to be directional and efficient.

The problem is, even with the ionosphere, there's a lot of signal "leaking" out of the Earth, through ionosphere, into space; some of it is bounced back down, but some of it penetrates the ionosphere and escapes into space. A satellite in low orbit would be swamped by transmitters from Earth, completely erasing any signals from deep space. You'd need a humongous antenna to ignore the signals from the nearby Earth. This is not feasible.

It would be a lot better if you could just put the Moon in between Earth and your antenna. Even these low frequency transmissions cannot penetrate the Moon, and can't go around the Moon either. So the Moon acts like a screen, casting a shadow on the antenna and eliminating parasite transmissions from Earth.

Now your satellite can receive low frequency transmissions from deep space.

TLDR: Too much noise near Earth at these frequencies, need to hide the antenna behind the Moon.

So the instrument in question is actually an antenna being built and managed by Dutch scientists. The Chinese satellite will carry it behind the moon. It will make measurements at 30 MHz. As you have said in your question, this portion of the EM spectrum is largely unexplored, thanks to us not being able to view at this wavelength from Earth. There are a number of possible things (pdf of a PowerPoint presentation) to do with such an antenna. However, with regards to the origin of the universe, at such a frequency, they can make

pathfinder measurements of the red-shifted HI line that originates from before the formation of the first stars (dark ages, reionization)

This is one of the ways to examine the state of the early universe, before stars had begun to form. The afterglow of heat from the Big Bang had faded, and most of the universe was composed of Hydrogen. However, this hydrogen had yet to form stars, so everything was dark - hence the name "the dark ages". However, examining this HI line will allow scientists to map the early universe just after recombination.

This is being done from behind the moon, because the astronomers want to use the Moon to block radio transmissions from Earth which might interfere with the signal they receive.