See you on the dark side of the moon: China's lunar radio observatory

...For nearly as long as there has been radio, there have been antennas trained on the sky, looking at the universe in a different light than traditional astronomy. Radio astronomers have used their sensitive equipment to study the Sun, the planets, distant galaxies, and strange objects from the very edge of the universe, like pulsars and quasars. Even the earliest moments of the universe have been explored, a portrait in microwave radiation of the remnants of the Big Bang.

And yet with all these observations, there’s a substantial slice of the radio spectrum that remains largely a mystery to radio astronomers. Thanks to our planet’s ionosphere, most of the signals below 30 MHz aren’t observable by ground-based radio telescopes. But now, thanks to an opportunity afforded by China’s ambitious lunar exploration program, humanity is now listening to more of what the universe is saying, and it’s doing so from a new vantage point: the far side of the moon.

BOUNCING BOTH WAYS

As any amateur radio operator can tell you, the key to direct global radio communication is the Earth’s ionosphere – those layers of charged particles that ebb and flow 50 to 600 miles (80 to 1000 km) above our heads. Produced by the constant stream of radiation flowing from the Sun and interacting with the Earth’s magnetic field, the ionosphere has long been known to refract radio waves. The degree to which radio waves are refracted depends on things like the structure of the ionosphere, which changes diurnally, as well as the angle at which the radio waves strike the charged particle layers. But refraction also depends heavily on the wavelength of the incident waves, with the 10-meter band, or 28 MHz, normally considered the upper limit for useful ionospheric bounce.

The refraction of radio waves below 30 MHz or so represents the core problem for ground-based low-frequency radio astronomy. (Nomenclature note: while commercial and amateur radio operators consider the space between 3 MHz and 30 MHz to be the “high frequency” (HF) band for historical reasons, the frequencies reflected by the ionosphere are very low for ground-based radio astronomy.) The ionosphere is nearly as efficient a reflector of radio waves coming from space as it is to those from terrestrial sources, and so acts as a blanket, isolating us from what the universe is telling us in those wavelengths. To be able to do any useful observations below 30 MHz, radio astronomers need to cast off that blanket, and the easiest way to do that is to build a space-based radio telescope.

https://hackaday.com/2019/12/23/see-you-on-the-dark-side-of-the-moon-chinas-lunar-radio-observatory/

• Our thanks to Stephen, G7VFY for spotting this item