In its simplest form, a CubeSat is a cubical picosatellite that usually has an edge of 10 cm, a volume of a liter, and a mass less than 1.33 kg (2.9 lb). Several CubeSats can be connected together when needed to form a larger vehicle. They are built to strict specifications, so that CubeSats can hitchhike to space together with larger payloads without interfering with the primary mission, which helps keep the typical cost of a CubeSat mission to around US$100K. While CubeSats are usually released in relatively low Earth orbit, this is not a fundamental limitation.
So how do CubeSats get to Phobos on a budget? The study mission is based on the use of two coupled CubeSats, one of which is specialized as the drive vehicle and the other as the sample collector. A European study of a small, solar-powered ion motor for small satellites such as CubeSats has recently appeared, that suggests its use for lunar missions. The JPL study, however, focuses on solar sails.
Once placed in Earth orbit, the drive vehicle deploys a solar sail, which produces a thrust that can be controlled in magnitude and direction by embedded nanoactuators. This thrust slowly increases the altitude of the coupled CubeSats and directs them toward a suitable Lagrange point – perhaps the Earth-Moon L1 point located between the Earth and the Moon.
The Lagrange points in the solar system are passageways into the gravitationally defined Interplanetary Transport Network. This network is a collection of very low-energy orbits which connect the Lagrange points of the solar system. When the CubeSats are inserted into such a transfer orbit, virtually no energy input is required to travel to a similar Lagrange point near Mars. The energy difference between the CubeSats in Earth orbit and the CubeSats in Mars orbit when transferred via the Transport Network is supplied by other planetary bodies through gravitational slingshot maneuvers, so requires no energy input. True, the transfer orbit will be lengthy and indirect, and typically requires far longer than would a traditional Hohmann transfer orbit. However, the Hohmann transfer orbit to Mars orbit would require a change in velocity of about 6 km/s (3.7 miles/s) – a very expensive requirement.
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