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NASA investigates sending CubeSats to Phobos and back

NASA investigates sending Cube...
A CubeSat with solar sails could be sent on a mission to the Martian moon Phobos (Image: NASA)
A CubeSat with solar sails could be sent on a mission to the Martian moon Phobos (Image: NASA)
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Triple CubeSat with deploying solar sail (Photo: NASA)
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Triple CubeSat with deploying solar sail (Photo: NASA)
CubeSats could be sent to gather samples from the Martian moon of Phobos (Photo: NASA)
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CubeSats could be sent to gather samples from the Martian moon of Phobos (Photo: NASA)
A CubeSat with solar sails could be sent on a mission to the Martian moon Phobos (Image: NASA)
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A CubeSat with solar sails could be sent on a mission to the Martian moon Phobos (Image: NASA)
The Interplanetary Superhighway (Image: NASA)
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The Interplanetary Superhighway (Image: NASA)
an artists rendition of Montana State University's Explorer-1 [Prime] CubeSat (Image: Montana State University, Space Science and Engineering Laboratory)
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an artists rendition of Montana State University's Explorer-1 [Prime] CubeSat (Image: Montana State University, Space Science and Engineering Laboratory)
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A CubeSat design (Image: NASA)
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A CubeSat design (Image: NASA)

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.

an artists rendition of Montana State University's Explorer-1 [Prime] CubeSat (Image: Montana State University, Space Science and Engineering Laboratory)
an artists rendition of Montana State University's Explorer-1 [Prime] CubeSat (Image: Montana State University, Space Science and Engineering Laboratory)

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 Interplanetary Superhighway (Image: NASA)
The Interplanetary Superhighway (Image: NASA)

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.

Source: NASA-IACP

27 comments
L1ma
The problem with using the bungy cord method to tether the cube sats is that Newtons laws of motion still apply. In this case on the other cube sat, like a bat and ball. The casing may be built like a brick, but the Solar Cells, Aerials and Ion Engines/Thrusters with their delicate palladium gauze are not. If it was an Orion sized Spacecraft with the cube sat at the end of the tether, it could be reeled back in and the energy stored in flywheels and shock absorbers in the payload bay. Which may be the better method, with cube sats docked together they house a 300 metre monofilament carbon fibe cable with a tiny light penetrator with your sticky surface which is fired onto the moons surface, springs out and then reeled back in. Your choice NASA.
Slowburn
re; L1ma The two cubes do not need to reel themselves back together once they have escaped Phobos the sail craft can maneuver to dock with the lander cube. All that is needed to lift the lander is a simple friction brake to prevent shock when the tether runs out of slack.
Mr Stiffy
Ode to the idea of an unlimited budget, an unlimited R&D, an unlimited fuel supply and an unlimited amount of space craft. And a HUGE poo-poo to the DNA frying galactic radiation and the apparent light speed limitation.
L1ma
Re; Slowburn Friction breaking does work, but I would hope to apply it to the reel, not the line. A larger surface area is available in the reel which would also house both motor and flywheel, with less chance of the break pads breaking off and severing the cable. Because the escape velocity of Phobos is 11.3 ms it may be even better to drop the paired microsats onto the surface in the form of a lander by aiming ahead of the orbit of Phobos and simply drifting onto it, using one of them to fire the other half back to earth.
L1ma
re; Slowburn Still there is the problem of trying to even out forces in 3 dimensions on 2 objects now acting upon each other via a cord at several hundred meters above a large rotating body 30 minutes transmission away from your control room.
Slowburn
re; L1ma Sense they are in orbit around Mars with only the slightest bit of encouragement the two masses at the ends of the cord will assume an orbit at the center of the combined mass. One will be high the other will be low by severing the cord at the lander cube it will enter a predictable orbit and then sever the cord from the sail cube reducing the mass. If you used a reel get rid of it as well. Then as I said before fly the sail cube into docking.
DR.ZARKOF
It would be nice if this article gave a timescale for this mission. Are we talking years or decades to travel this energy free superhighway?
L1ma
re: Dr.Zarkof The usual gravity assisted path usually takes years off a journey, borrowing energy from a planet and giving speed to the Micro sat (The Voyagers and Gallaleo missions). The cubesat pictured has solar sails, sending it close to the sun would give it a vast initial speed boost especially if Mars is in opposition to the Earth both from gravity assistence and the solar wind, but you are right the point of the solar sails is free thrust through the path of least gravital resistance through planetary Lagrange points which as you correctly surmise will be years longer than necessary. In other words a mission which will take years to arrive, using space tethers which have the highest rate of failure of any space technology to the planet which also has the highest mission fail rate.
Slowburn
re; L1ma Given the energy budget minimum power required path is the minimum time path. This is why you don't want to use a reel to bring the two modules back together.
L1ma
re; Slowburn Not quite true, Most efficent and quickest are not the same thing, if you send your probe through the most energy efficient route I believe it would take 2 - 4 years, the quickest journey time to Mars is 6 months or less at near closest approach. This is with nuclear powered rockets, the quickest route is always dependant upon borrowing speed from a planets mass as well as having the most powerful propulsion, when Mars is at opposition the plan is to use a Venus slingshot orbit to gain velocity. If you use the slow route, and the Chinese get to know about it, the China mars orbiter return sample probe will use the fast option. Chang'e 2 has already completed its moon mission and has been moved to the Earths L2 point, the next version only needs larger fuel tanks(Chang'e 3 in 2013 is meant to have a sample return rover). However you get there it takes the same amount of energy to get to the Moon as to Mars. But its pointless to be the first to get there and the last to return, the only credo to using Micro sats is that you send a cloud of them at the same time with the knowledge that the high failure rate of Mars missions means at least one will return. So far we have no sucessful use of space tethers or solar sails either.