NIAC Phase I Award: Phobos L1 Tether Experiment

Phobos L1 Operational Tether Experiment (Credit: Kevin Kempton)

Phobos L1 Operational Tether Experiment (PHLOTE)

Kevin Kempton
NASA Langley Research Center
Hampton, Va.

Value: Approximately $125,000
Length of Study: 9 months

Description

A sensor package that “floats” just above the surface of Phobos, suspended by a tether from a small spacecraft operating at the Mars/Phobos Lagrange 1 (L1) Point would offer exciting opportunities for science (SMD), for human exploration (HEOMD) and for advancements in space technology (STMD).

Detailed information on the Martian moon Phobos is limited even though it is considered an important destination for near term human exploration. A PHLOTE spacecraft would perform fixed point station keeping at the Mars/Phobos L1 point to allow a tethered sensor package to “float” just above the moon’s surface and also park instruments on the surface for in situ science measurements.

This can include ground penetrating radar for subsurface composition measurements to determine how thick the layer of fine grained regolith is for future landings. Other key instruments would be dosimeters for understanding the radiation environments for future human missions, cameras, and a spectrometer for surface mineral analysis.

If deployed after a human landing, a PHLOTE spacecraft could provide a constant “eye in the sky” for ground controllers to monitor mission deployments and operational activities.

The PHLOTE mission concept has only now become feasible due to recent technology advances, many of which have been supported by NASA’s STMD. Key technologies that make this mission concept feasible include: The Navigation Doppler Lidar (NDL) Sensor for the providing precise spacecraft position and rate knowledge relative to Phobos. This high precision is needed to maintain position at the L1 point; Carbon Nanotube (CNT) braided yarns for a structurally strong tether that doubles as a power and data conduit, Ultralightweight solar arrays, and highly efficient electrospray micro-propulsion thrusters for long term “hover mode” station keeping.

The Martian Moon Phobos offers a key waypoint toward enabling human surface landings on Mars. In particular Stickney Crater, which always faces Mars due to Phobos’ synchronous rotation, provides an excellent stepping stone destination as a precursor to a human Mars landing.

There is very limited information on the composition and the environments at Stickney Crater on Phobos. Since Phobos has a composition similar to carbonaceous chondrite meteorites, it is believed that it could provide minerals that can be used for In Situ Resource Utilization (ISRU) to recover key elements such as Oxygen for use as return trip propellant. The mission concept below would answer many of these questions as well as provide TRL advancement in key technology areas for human exploration.

This mission concept is a synthesis of new technologies that would provide a unique platform for multiple sensors directed at Phobos as well as Mars. Since the Mars/Phobos L1 point is only ~3.1 km from the surface of Phobos, the PHLOTE tether length only needs to be a few kilometers long. A tether configuration with its Center of Gravity at the Mars/Phobos L1 point can place a sensor package on the moon’s surface or float it just above. Due to Phobos’ very low gravity, the tether will be under very low tensile loads.

Using a longer tether, this concept can be similarly used for other missions such as Mars/Deimos or at the Pluto/Charon L1 point where both bodies are tidally locked which means a PHLOTE spacecraft with a much longer tether could descend into Pluto’s tenuous atmosphere and sample its chemistry at all elevations unlike a traditional probe.

If selected, a feasibility study for the Phobos L1 Operational Tether Experiment (PHLOTE) mission would be performed that will define the PHLOTE mission, determine the technology needs and assess the technology readiness. The study would also model the system, identify risks, as well as explore new science opportunities that could be done with this unique sensor platform.

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  • Brainbit

    I like this, it uses a number of as yet unproven techs perhaps to many? It would be incredible hard as there are so many unknowns and potential miscalculations. I don’t think carbon nano tubes are needed. It can only work over the one spot on Phobos unless the lander is mobile. To get into the L1 spot coming from earth would require impressive calculations if they are not carried by a human mission and only using EMPT (electrospray micro propulsion thrusters) and air braking? But this is potentially a good spot to find out what can be done with EMPT or even SEP. Could a lander be landed on Phobos from its L1 point and even return to the L1 point using EMPT or even a SEP? Is a tether really needed?

  • JamesG

    Probably not needed specifically, but a tether will provide a “free” * way of maintaining the position over the long term against all the perturbations that would eventually knock it out of the L1 point if it were free floating in its own orbit.

    * free in the sense that it wouldn’t need any propellant, but it would have to carry the tether’s mass and its deployment mech. all the way out there.

  • Brainbit

    If the tether is one way then the gravity on the tether, all be it small will pull the satellite out of orbit with the same force it would take to land what ever they put just above the Phobos surface, else they will need a mass in the opposite direction, as this is the L1 point that’s towards Mars. There could be a better balance point if they don’t use the L1 point by attaching the tether to Phobos and orbiting the satellite closer to Mars. unfortunately there may not be, as the orbital speed to stay in such an orbit may not be the same rate as the Phobos orbit. To long a tether and you will speed up the de-orbit of Phobos, to short a tether and the satellite will crash into Phobos. As you say even to keep the satellite in orbit will require fuel and once that fuel weight is spent then that will also effect its orbit. One idea I have is if the tether is attached to Phobos then it would be possible to double orbit the satellite on the end of the tether, for example if you tie a weight on the end of a string and let it swing on the end while holding the other end and you start moving your hand in a circle then the weighted end starts to circle also, as the weights orbit increases then the centrifugal force equals the angle of swing if you then run your finger holding the string down from the center of rotation towards the weight the orbit speed increases and the pull on the string increases due to the shortening length and increasing speed of rotation of the weight. Applying this principal to our satellite by having a connecting ring on Phobos that can move up and down from the surface and a rotating point on the surface circling like your hand will impart a circular spin to our satellite where we can then control its orbit by varying its circular speed and length of the point of rotation. The energy needed to create the spin could come from solar power and an electric motor on Phobos. No mass needed.

  • JamesG

    You’re over thinking it. I believe what the author is suggesting is that the Phobos-Mars L1 point and Phobos’ geosync alt. for its tidally locked rotation are coincidental. The tether then just becomes a “leash”, not so much a elevator cable as it envisioned for places like Earth.

  • Brainbit

    Yes absolutely. Sorry, I will try harder to control my imagination.

  • The mass at Phobos Mars L1 would feel zero net acceleration. All the tether and sensors between L1 and Phobos would feel an acceleration towards Phobos. To stay aloft this would need tether extending towards Mars from L1. A 7 km tether would do it. Carbon nanontubes are not needed. For the small stresses in this scenario, Zylon would be more than adequate. See Lower Phobos Tether