NASA's Innovative Advanced Concepts Program provides funding to study a small number of highly advanced spaceflight concepts, with the goal of understanding the technological possibilities which will guide the development of future space missions. Under this program, a JPL (Jet Propulsion Laboratory) researcher has proposed the use of a pair of CubeSats for an autonomous mission to retrieve samples from Phobos, Mars' larger moon.
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.
Once the coupled CubeSats are in the vicinity of Mars, how do they manage to collect a sample of Phobos' surface material? Landing on Phobos is not a difficult feat, as the escape velocity is just over 10 meters per second (22 mph). However, the solar sails do not provide the force required to lift the lander CubeSat from Phobos, so it will have to be pulled free by the momentum of the drive CubeSat.
In this scenario, the two CubeSats will skim the surface of Phobos from a hyperbolic orbit. As the lander CubeSat approaches the surface, its motion relative to the surface is stopped by the action of a preset spring which pushes apart the two CubeSats. It is on the surface with very little relative motion, so it has "landed." A robotic scoop might be used to collect a surface sample, but even a sticky surface could do so.
If the two CubeSats remain connected by a tether, when the tether pulls tight both CubeSats will pull free of Phobos' gravitation. The CubeSats then navigate back to the Interplanetary Transit Network, and enter an appropriate zero-energy trajectory to return to Earth. Presumably the CubeSats would eventually dock with the ISS for recovery and transport to the surface.
The mission outlined here is not necessarily the easiest nor the most effective way to sample Phobos using a pair of CubeSats. However, it does illustrate the possibility of such scenarios. The real payoff of the study is to demonstrate the enormous potential of non-brute force approaches to space exploration - there are other ways to travel to other planets than by launching a nuclear-powered spacecraft.