Telling aliens where we are

An alien spacecraft coming into the Solar System would have no trouble finding us.

All they would need is a radio.

For more than a century, we have been transmitting radio waves in all directions and over most of the radio spectrum. Hardly any of the signal from a radio transmitter actually gets received and used. Most of it goes off into space.

Today, the Earth is surrounded by an expanding sphere of radio waves with a radius of more than 100 light years.

In addition to advertising our presence in the universe, the sheer power and quantity of our radio emissions is making it hard for us to observe radio emissions from the cosmos.

Just as streetlights and sky glow make it hard to see the stars, a problem that today we call light pollution, our radio emissions are making it hard to see the radio sky.

The problem has been mitigated to some extent by allocating some parts of the radio spectrum for radio astronomical use. Radio transmitters cannot be operated at those wavelengths. However, that is not the entire problem.

Almost all electronic devices, including radio transmitters and receivers, computers, controllers and so on, emit what those in the radio business refer to as unwanted transmissions. Producing these is more or less inevitable, which means that the more devices are operating, the more of this unwanted stuff gets into the radio spectrum.

These transmissions do not care whether they fall in the wavelength ranges allocated to radio astronomy or any other use of radio waves.

What makes this issue worse is that more electronic devices are in use now than at any time in human history, and moreover, the number continues to grow rapidly.

The result is that observing radio waves coming from beyond the Earth is increasingly challenging. We pick sites such as secluded valleys where few live, and the surrounding hills block out interference from beyond. However, now these unwanted transmissions are also coming from aircraft and space.

Modern technical developments are reducing the problem, but we cannot get back to where we were.

That is why for many years astronomers have been discussing the prospects of doing radio astronomical observations from the other side of the Moon, where interference from Earth and satellite transmitters won't be a problem.

The Moon's rotation is locked. It takes as long to spin on its axis as it does to orbit the Earth. If we were on the side of the Moon we can see from Earth, the Earth would remain at the same position in the sky all the time.

On the far side, the Earth never rises above the horizon. This would be a great place to put a radio telescope, assuming we had the transportation problem solved and had a nice, comfortable permanent base in which to live and work.

This brings us to a very interesting experiment going on right now.

The Chinese Yutu 2 (Jade Rabbit 2) rover is prowling the other side of the Moon. Because the Earth will never be above its horizon, its radio signals have to come back to us via a relay satellite, Queqiao (Magpie Bridge).

This satellite is carrying a Dutch-Chinese radio astronomy experiment, making observations at wavelengths that are largely obliterated here on Earth by our own interference. 

This is exciting but also cautionary. The first radio telescope was made as a backyard experiment, and there are radio astronomers around now whose interest started with experiments in the back yard. No doubt their successors are out in their backyard today. What if their curiosity is killed by manmade interference?

We are gradually reclaiming our dark skies from light pollution. The radio pollution problem is another thing altogether, and a serious challenge.

• Mercury and Mars lie low in the southeast before dawn.
• Saturn and Venus are close together very low in the southwest in the sunset glow.
• The Moon will be Full on the 12th.