Low-Frequency Space Interferometry
Studying the Universe at low radio frequencies can teach us a lot about astrophysics, but we cannot observe below about 30 MHz from Earth as the ionosphere effectively blocks this frequency band. This is why we are interested in low-frequency radio astronomy from space. The NCLE instrument, designed and built by RRL and its partners, is currently collecting low-frequency radio data from beyond the Moon - but what are the next steps?
Low-frequency radio emissions carry a lot of valuable information on astrophysical systems and processes, such as the properties of the interstellar medium, the magnetic field structure of the Milky Way, interactions of the Solar wind with planetary magnetospheres and the magnetosphere of the Sun itself - with the biggest one being the structure of the early Universe from redshifted neutral hydrogen line emission (the ‘dark ages’ and ‘cosmic dawn’ spectra). In order to effectively pursue these science targets, we need to move beyond Earth’s ionosphere and measure this part of the radio spectrum from space. With a single antenna, we can characterise the low-frequency radio spectrum and identify several different sources of radio emission. However, if we want to study a weak source we will need to subtract other components from the spectrum before we can see it - and it can be difficult to disentangle these contributions from each other. To move beyond purely spectral data and get directional information that helps us with this, multiple antennas are needed. RRL is looking into multiple mission concepts that tackle space interferometry using multiple elements on different platforms at low radio frequencies.
One of these concepts under study is for the Chang’e 7 mission, where a lander, an orbiter and a separate small orbiting platform work together at the Moon with radio antennas on each element. Such a system may be used to extensively map and characterise the low-frequency radio sky, allowing us to separate various low-frequency sources from each other and pave the way for the study of redshifted hydrogen from the Dark Ages and the Cosmic Dawn.