Discovering the Sky at the Longest wavelengths with a lunar orbit array
  Due to ionosphere absorption and the interference by natural and artificial
radio emissions, astronomical observation from the ground becomes very
difficult at the wavelengths of decametre or longer, which we shall refer as
the ultralong wavelengths. This unexplored part of electromagnetic spectrum has
the potential of great discoveries, notably in the study of cosmic dark ages
and dawn, but also in heliophysics and space weather, planets and exoplanets,
cosmic ray and neutrinos, pulsar and interstellar medium (ISM), extragalactic
radio sources, and so on. The difficulty of the ionosphere can be overcome by
space observation, and the Moon can shield the radio frequency interferences
(RFIs) from the Earth. A lunar orbit array can be a practical first step of
opening up the ultralong wave band. Compared with a lunar surface observatory
on the far side, the lunar orbit array is simpler and more economical, as it
does not need to make the risky and expensive landing, can be easily powered
with solar energy, and the data can be transmitted back to the Earth when it is
on the near-side part of the orbit. Here I describe the Discovering Sky at the
Longest wavelength (DSL) project, which will consist of a mother satellite and
6~9 daughter satellites, flying on the same circular orbit around the Moon, and
forming a linear interferometer array. The data are collected by the mother
satellite which computes the interferometric cross-correlations (visibilities)
and transmits the data back to the Earth. The whole array can be deployed on
the lunar orbit with a single rocket launch. The project is under intensive
study in China.
