Observing the 21cm neutral hydrogen line from extra-terrestrial sources gives many clues on the composition of the universe. Due to the motion of the observed objects and the expansion of the universe this line is red-shifted because of the Doppler effect for objects that move away from Earth. The speed at which the objects move away from Earth becomes increasingly larger for objects further away. Because of the distance, these signals are also older. So observing hydrogen lines with a large red-shift gives an image of very remote objects in the distant past. In the Netherlands the LOFAR radio telescope has been implemented to make observations as low as 30MHz. The opaqueness of the atmosphere at lower frequencies and the natural and man-made interference make Earth-based observations at lower frequencies impossible. Even in Earth orbit, above the ionosphere, the interference is estimated to be too large. An alternative is envisaged in an instrument located at the far side of the Moon. The Moon then functions as a shield to block radio interference from Earth. To relay the measurement results from the far side of the Moon onto a terrestrial ground station, one or more Moon orbiting satellites are necessary. Landing on the Moon, especially autonomous landing on the far side of the Moon without direct contact with Earth is difficult and requires a lot of technological developments. The costs are likely to be quite high, and it may take a very long time to implement a radio telescope like this. So a few years ago an alternative was proposed that could still produce very interesting measurement results, yet sooner and at lower cost. This system is called the OLFAR swarm.
OLFAR is based on nano-satellites in Moon-orbit which form a synthesized aperture radio telescope that samples radio signals in the frequency range of 300 kHz to 30 MHz. The measurements are taken by nano-satellites which are in the part of their orbit where the Moon shields them from signals originating from Earth. After the measurements, the captured signals are distributed across the swarm in order to perform interferometry and other signal processing. After this, the results are transmitted to Earth by multiple synchronized nano-satellites, which may combine their transmitted power into a single beam. The baseline of this synthesized aperture radio telescope must be around 100km in order to have sufficient resolution to create a useful sky map. (Larger dimensions do not add to the resolution anymore due to the non-linearity caused by e.g. the interstellar medium and interplanetary medium). A study done on a system based on standard space system components (larger satellites), suggests that with 5 to 6 satellites sufficient measurements can be made to produce an interesting sky map of a yet unseen part of the spectrum, containing very old signals from very remote objects. In the proof of concept designs for OLFAR, an equal number of satellites is taken. For better performance around 50 satellites are desirable and the ideal configuration might even consist of 1000 satellites.
OLFAR within the Delfi Space Program
Within the Delfi Space program, building blocks for OLFAR are developed and demonstrated in space. Delfi-C3 was a starting point for nanosatellites in the Netherlands. Delfi-n3Xt demonstrates advanced bus capabilities and DelFFi demonstrates onboard propulsion and distributed control of two satellites. Beyond DelFFi, more dedicated systems for OLFAR are expected to be demonstrated onboard Delfi satellites, eventually leading to the operational phase of OLFAR in a few decades from now.