How radio astronomers will tune into the cosmic dawn from the far side of the Moon
China's history-making Chang'e-4 spacecraft touched down on the far side of the Moon last week and it opened up some exciting new avenues of scientific enquiry. While the rover goes to work examining the makeup of the lunar crust and mantle, a small radio instrument aboard a satellite parked in lunar orbit will allow scientists to listen into low-frequency signals that are blocked by the Earth's atmosphere. So, after falling on deaf ears for so long, what could these secretive radio waves tell us?
We first detected radio waves from space in the 1930s, but it wasn't until after World War II and advances in radar technology that things really got interesting. One well-known example of this was the detection of Cosmic Microwave Background Radiation in the 1960s, the lingering glow of the Big Bang, which earned the discoverers the 1978 Nobel Prize in Physics.
Since then, radio waves have been instrumental in furthering our understanding of the universe. By processing the information gathered by these telescopes, astronomers have been able to detect new kinds of objects, such as pulsars and quasars, and use radio waves emanating from distant hydrogen clouds to map out the structures of faraway galaxies.
While they have taught us a lot, the huge dish antennas we use to collect radio waves on Earth are only painting part of the picture. Our atmosphere blocks radio waves at lower frequencies before they can reach us, but scientists theorize that we could learn a lot from them, particularly about the early universe, if only we could tune in.
While suitably equipped instruments have been sent to space for this purpose before, their visits were short-lived. And though modern space probes are fitted with radio instruments, they are not designed with radio astronomy in mind. Conversely, a small instrument developed by astronomer Marc Klein Wolt's team at Radboud University and sent along as part of the Chang'e-4 mission to the Moon, very much was.
Developed in partnership with Dutch radio astronomy organization ASTRON and private company Innovative Solution in Space, the NCLE – Netherlands-China Low-Frequency Explorer – instrument hitched a ride to the Moon on the Queqiao satellite back in May. This satellite was sent ahead of the Chang'e-4 lander itself, because the Moon blocks all radio contact from Earth and so to land on its far side requires signals to be relayed somehow. In this case, scientists have done so via a satellite parked out beyond the Moon with clear communication lines both down to the surface and back to Earth.
We put a few questions to Klein Wolt about the traditional troubles in getting radio astronomy into space, how these challenges were overcome and his hopes for this relatively small but potentially game-changing instrument.
What attempts have been made to get radio telescopes into space before?
The last dedicated instruments date back to the 1970s, RAE-1 and 2. They went behind the Moon and back once, but did not have the same resolution that we have and were very short missions of a couple of days, and we need long integration times! In the meantime there have been radio instruments with relatively low frequencies, Cassini for instance has a low-frequency instrument, but these are not designed for our science and do not go to such low frequencies as we do.
Why has it been so difficult to get radio telescopes beyond the atmosphere and into space?
Costs mostly. Digital processing has also made some big steps over the last couple of years to allow for high performance with significantly less power, which has created more options for space applications. And in the process of finding funding for a space mission, we were in competition with the SKA project, which has almost the same key science cases, although it would never be able to go to such low frequencies as we can from space. So, finding a mission to space, even a piggy-back, has been difficult. We had been involved with plans for the European Lunar Lander but that got canceled.
Why was the Queqiao satellite able to overcome these challenges?
Basically, it was available! The Chinese have given us ample mass, power and communications budget, and are interested in the low-frequency radio science as well. So it was a win-win situation, in which we get our instrument behind the Moon at a relatively quiet location, not ideal but the best so far, and they participate in the science.
The atmosphere blocks certain radio frequencies from reaching Earth-based telescopes. What do we know about these unsighted radio waves and what secrets might they hold?
At frequencies below ~30 MHz the atmosphere blocks almost all the radiation from space, hence ground-based radio telescopes cannot do much there. But from our theoretical models we know that both the Sun and Jupiter will show very bright radio flashes at lower radio frequencies, furthermore we expect that a wealth of other radio sources that we already observe at higher frequencies, such as pulsars and the galaxy itself, will have radio emission below 30 MHz.
Finally, the signal we are after from the 21-cm line of hydrogen in the very early universe, from the period before there were any stars known as the "cosmological dark ages," is predicted to peak at 30 MHz. The signal from the ignition of the first stars, known as the "cosmic dawn," peaks around 70 MHz. In addition to all these "known-unknowns," opening up a new frequency regime will lead to the discovery of the unknown-unknowns.
How exactly will the satellite and lander be used to study low-frequency radio waves?
We have a radio instrument (NCLE - Netherlands-China Low-frequency Explorer) on the satellite, which consist of three, five-meter-long (16-ft) carbon fiber antennas to capture the radio signals, and a bunch of electronics for the data processing of these signals. In principle, this instrument can capture radio waves with frequencies from 80 kHz to 80 MHz.
We are collaborating with a Chinese scientist on the interpretation of the data from our instrument. But the lander also has a similar radio antenna on board, which will be sensitive from 0.1 to 40 MHz, and will aim to detect the bright emissions from the Sun and Jupiter. We are participating in the science team with the Chinese on this instrument.
What does this mean for the future of radio astronomy?
In the end we need much more then one antenna to do our science. The signal from the early universe we want to pick up is very weak and making detailed maps requires a large collecting area in the order of 1-10 square km (0.38-3.8 sq mi), but with one antenna behind the Moon we hope we can detect the global signal (peaking at ~30 MHz for the dark ages signal and ~70 MHz for the cosmic dawn). So with our instrument on the Queqiao satellite we are paving the way for future larger science missions, and in the process are opening up the last virtually unexplored regime for astronomy.