A 4G network on the Moon is bad news for radio astronomy
Emma Alexander, University of Manchester
As you drive down the road leading to Jodrell Bank Observatory, a sign asks visitors to turn off their mobile phones, stating that the Lovell telescope is so powerful it could detect a phone signal on Mars.
Radio telescopes are designed to be incredibly sensitive. To quote the legendary astronomer Carl Sagan, “The total amount of energy from outside the solar system ever received by all the radio telescopes on the planet Earth is less than the energy of a single snowflake striking the ground.”
The total energy now is probably a few snowflakes’ worth, but nevertheless it is still true that astronomical radio signals are typically magnitudes smaller than artificial ones. If Jodrell Bank could pick up interference from a phone signal on Mars, how would it fare with an entire 4G network on the Moon?
That is the issue that is worrying astronomers like me, now that Nokia of America has been awarded US$14.1m (£10.8m) for the development of the first ever cellular network on the Moon. The LTE/4G network will aim to facilitate long term lunar habitability, providing communications for key aspects such as lunar rovers and navigation.
Network interference
Radio frequency interference (RFI) is the long-term nemesis of radio astronomers. Jodrell Bank – the earliest radio astronomy observatory in the world still in existence – was created because of RFI. Sir Bernard Lovell, one of the pioneers of radio astronomy, found his work at Manchester hampered by RFI from passing trams in the city, and he persuaded the university’s botany department to let him move to their fields in Cheshire for two weeks (he never left).
Since then, radio telescopes have been built more and more remotely in an attempt to avoid RFI, with the upcoming Square Kilometre Array (SKA) telescope being built across remote areas of South Africa and Australia. This helps to cut out many common sources for RFI, including mobile phones and microwave ovens. However, ground-based radio telescopes cannot completely avoid space-based sources of RFI such as satellites – or a future lunar telecommunications network.
RFI can be mitigated at the source with appropriate shielding and precision in the emission of signals. Astronomers are constantly developing strategies to cut RFI from their data. But this increasingly relies on the goodwill of private companies to ensure that at least some radio frequencies are protected for astronomy.
A long-term dream of many radio astronomers would be to have a radio telescope on the far side of the Moon. In addition to being shielded from Earth-based signals, it would also be able to observe at the lowest radio frequencies, which on Earth are particularly affected by a part of the atmosphere called the ionosphere. Observing at low radio frequencies can help answer fundamental questions about the universe, such as what it was like in the first few moments after the big bang.
The science case has already been recognised with the Netherlands-China Low Frequency Explorer, a telescope repurposed from the Queqiao relay satellite sent to the Moon in the Chang’e 4 mission . Nasa has also funded a project on the feasibility of turning a lunar crater into a radio telescope with a lining of wire mesh.