Abstract: After the Big Bang, the Universe entered the so called dark ages, during which structures grow until the first generation of stars, galaxies and black holes formed. The 21cm line of the neutral hydrogen provides a unique probe to the dark ages and cosmic dawn. However, observing this redshifted 21cm line is extremely difficult. On top of the huge foreground radiation, there is the ionosphere and the interferences at low frequency which hampers ground-based observations. The radio band below 30 MHz is still largely unexplored. I shall present the Discovering Sky at the Longest wavelength (DSL) mission, also known as the Hongmeng mission in Chinese, which aims to unveil the sky at this low frequency band by using an array of microsatellites flying in formation on an orbit around the Moon. The mission will also make highly precise 21cm global spectrum measurement in the 0.1-30 MHz band and in the 30-120 MHz band, without being hindered by ionospheric distortion, radio frequency interferences, or ground reflections. The satellite array will also make high resolution full sky maps in the 0.1-30 MHz band, which will provide new observational clues to the study of the Galactic interstellar medium, extragalactic radio sources, and solar and planetary activities. Observation in this new window may also reveal unknown new objects or phenomena. The DSL mission science team welcomes the participation of the international science community.