QB50 has the scientific objective to study in situ the temporal and spatial variations of a number of key constituents and parameters in the lower thermosphere (90-320 km) with a network of 50 double CubeSats, separated by a few hundred kilometres and carrying identical sensors. QB50 will also study the re-entry process by measuring a number of key parameters during re-entry and by comparing predicted and actual CubeSat trajectories and orbital lifetimes.

Space agencies are not pursuing a multi-spacecraft network for in-situ measurements in the lower thermosphere because the cost of a network of 50 satellites built to industrial standards would be extremely high and not justifiable in view of the limited orbital lifetime. No atmospheric network mission for in-situ measurements has been carried out in the past or is planned for the future. A network of satellites for in-situ measurements in the lower thermosphere can only be realised by using very low-cost satellites, and CubeSats are the only realistic option.

A CubeSat is a miniaturised satellite (10x10x10 cm, weighing 1 kg) which offers all the standard functions of a normal satellite (attitude determination and control, uplink and downlink telecommunications, power subsystem including a battery and body-mounted solar panels, on-board data handling and storage by a CPU, plus either a technology package or a small sensor or camera). They can even have deployable solar panels, antennas or booms. Limited orbit control using micropropulsion, S-band instead of VHF/UHF and wireless data transfer inside the CubeSat are now beginning to be used. It takes about two years to develop a CubeSat from the provision of funding until launch. The hardware cost of a CubeSat is in the range 50-100 k€. Up to now, about 40 CubeSats have been successfully launched, worldwide an estimated 70-100 CubeSats are being readied for launch in the next few years.

A single CubeSat is simply too small to also carry sensors for significant scientific research. Hence, for the universities the main objective of developing, launching and operating a CubeSat is educational. However, when combining a large number of CubeSats with identical sensors into a network, in addition to the educational value, fundamental scientific questions can be addressed which are inaccessible otherwise. Networks of CubeSats have been under discussion in the CubeSat community for several years, but so far no university, institution or space agency has taken the initiative to set up and coordinate such a powerful network. CubeSat reliability is not a major concern because the network can still fully achieve its mission objectives even if a few CubeSats fail.

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