NEEP602 Course Notes (Fall 1996)
Resources from Space

Excerpt from:

NAS9-17779 - Phase III Final Report

6.0 Considerations for EVA on Phobos

NASA's present thinking about a human expedition to Mars includes a visit to the Martian moon Phobos, either as a precursor mission or as a sortie from the interplanetary exploration spacecraft. It is speculated that Phobos could serve as a staging ground and source of certain resources for the manned mission to Mars. However, a detailed scenario for human exploration of Phobos has not yet been formulated, largely because little is known about the environment there. Hopes for new information from reconnaissance and sampling by the Soviet probes to Phobos were disappointed when the spacecraft failed en route (1989).

Despite the saps in present knowledge, the prospects of a manned mission to Phobos are intriguing. The study team was requested to consider EVA on Phobos as a sidelight to our analysis of advanced EVA systems design requirements. Our guidance was not to present definitive requirements but to engage in creative brainstorming, raise in a preliminary fashion some of the basic issues for Phobos EVA, and offer some possible approaches or solutions as "food for thought." The results of this thoughtful exercise are presented here as practical matters to be considered by mission planners.


The environment of Phobos presents severe technical challenges for EVA and a set of concerns that differs quite markedly from those for Mars. Gravity is 0.001 and there appear to be no landscape features or surface amenities to enable a vehicle or crew to land there. A free crew member on the surface of Phobos will be in constant risk of "launching" himself; some sort of reliable restraint will be essential, but the surface may not be amenable to standard anchors. Some cracks and surface depressions could be filled with up to 5 meters of debris and dust, making translation and restraint difficult, especially if these actions raise dust clouds around the crew. The carbonaceous chondrite surface may pose some problems for the design of effective spacecraft tether and holding systems.

Phobos Environment (Verrerka, 1988)

  • Surface gravitational acceleration (0.001-g)
  • Carbonaceous chondrite surface (probably)
  • Several meters of lunar-like regolith
  • Probable debris-filled cracks and depressions
  • Escape velocity: 3.5 m/sec2 at the point facing Mars and 15 m/sec2 at the point facing away from Mars.

  • In 0.001-g, without effective surface attraction, EVA on Phobos probably will have more in common with orbital EVA than with activities on the surface of the moon or Mars. For the crew, the experience might be likened to working on a large, dusty, and unequipped spacecraft.

    The requirement for EVA at Phobos needs to be understood better and justified in view of the cost of developing supporting technology peculiar to a Phobos mission. A human expedition on Phobos raises a variety of unique questions.

  • What is the rationale for an EVA mission on Phobos?
  • Fuel and water resource
    Communications station
  • Can surface exploration be done on the surface?
  • How deep, how penetrable, and how retentive is the surface regolith?
    Must the lander stay off the surface (hovercraft)?
    Can restraints and mobility aids be installed on the surface (mountain climbing pitons or tether lifelines)?
    Will thrusters cause a debris problem?
    Will the surface be excavated (cratered) if spiked with a landing probe?
  • Given the decay of the Phobos orbit, will landing, EVA, or spiking significantly perturb it?
  • Should the habitat be a co-orbiting module? Should we transit to Phobos from a synchronous orbit?
  • Does Phobos have co-orbiting debris?
  • Other considerations:
    Hierarchial arrangement of mission alternatives depending on surface environment (amenable to landing, retentive to probes, dusty)
    (1) Crew vehicle lands and grips on surface
    (2) Crew vehicle tethered to surface from 30 - 50 m height (3-point tether system for energy conservation)
    (3) No landing; crew makes excursion in MMU-type backpack propulsion unit or hovercraft
  • Multi-frequency radar interrogation of regolith and landing sites before Phobos mission for site selection and verification
  • Sample collection and return techniques (scoops, bags, core tubes, etc., may not work well in milli-g)
  • Transport of support equipment to site (bagsltags, lights, mobility restraints, workstations, etc.).


    Three alternatives for Phobos surface exploration were discussed by the study team. They are presented here for consideration with the caution that EVA on Phobos raises more issues than can be solved with our current understanding of the characteristics of this unusual EVA setting.

    An exploration team has been sent to the Martian moon Phobos to set up science stations and communications antennas and to return surface samples to the Mars orbiting station for analysis and characterization.

    Composed of four crew members, the exploration team is similar in all respects to the Mars surface exploration crews, and their operations are similar with two major exceptions: the explorers operate from their spacecraft rather than a surface habitat, and all activity is performed in milli-g rather than one-third-g. For the purpose of this narrative, it is assumed that the Phobos excursion spacecraft is a Mars landing module that is employed prior to its use on the Martian surface.

    As the excursion craft approaches the orbit of Phobos, it makes attitude and velocity corrections to co-orbit with the moon. Detailed inspection of the surface of Phobos is made from this standoff position. Using visual and other instruments, the crew determine the composition and density of the surface soil. This task is necessary to verify earlier assumptions about the makeup of Phobos and the type of restraint systems that will be required during any manned surface exploration, surface craft landing attempt, or installation of science and communications packages. Photographic and video records are made of all phases of the inspection.

    When it has been determined that the surface can support manned inspection and installation of equipment, the crew of two EVA scientists prepare to launch their surface module just as they would for the Mars surface mission. The surface module, or a near-surface vehicle If more appropriate for the composition of the regolith, is launched; it will be returned to the spacecraft after the Phobos inspection for use in the Mars landing. For this first scenario, we assume that an actual surface landing is possible.

    Landing on the surface of Phobos, the crew deploy the required holding and vehicle restraint system, which may be automatically deployed devices such as screws or piton claws. Egressing from the surface module, the crew use tethers to leave the vehicle and begin implanting pitons and guys along the surface route. Both equipment and crew must be tethered to this lifeline during the mission. It is advisable to use semi-automatic equipment to install the pitons and guy wire along the selected translation route to reduce crew workload in the milli-g environment.

    The crew install the first science or communications package on the surface. restrain it to the surface, and deploy any required components. Activation of science packages can be done immediately after checkout; activation of communications packages may require an automatic delay if RF energy would pose a threat to the well-being of the EVA crew during any mission phases. The EVA crew report back to the orbiting station their progress and discoveries, and the two members in the orbiting vehicle closely monitor the surface crew's progress and status.

    The EVA crew then go to the next designated site and set up the next equipment package, installing the tether lifeline along the route. At the conclusion of the surface EVA, the crew use the lifeline to translate back to the surface module and prepare for ascent to the orbiting station.

    The requirement to be tethered to the Phobos surface while performing the exploratory EVA might be mitigated by using MMU-type propulsion systems. This approach would depend upon the regolith composition and any thruster plume disturbance on the surface, which might raise unacceptable amounts of dust and powder. The use of MMUs also might mean that the crew could work directly from the orbiting station, shuttling equipment between the station and the Phobos surface and not having to use a surface module. The feasibility of this second scenario would depend upon a much better understanding of the characteristics of Phobos than is currently available.

    A third scenario would not involve either a lander or an MMU-type of approach to Phobos, but would rely on deployment of a Module Web from an exploration and crew support module orbiting Phobos in a stationary alignment. The Module Web, shown in Figure 6-1, would consist of at least three cable loops fired from the orbiting module into the surface of Phobos, with one end of each loop remaining at the orbiter. With three loops firmly attached to the surface, the crew and equipment could be lowered to the surface on a taut tether from the orbiter. As the end of any one of the surface cables is retracted inside the orbiter, the other ends of all the cables, now harnessed together, are drawn to the Phobos surface. This tension and retraction method of going to the surface and being able to remain there would require upward thrust by the orbiter to compensate for the downward tug of the tension cables.

    Once on the surface, the crew can translate anywhere within the boundary of the triangle defined by the three surface points which hold the cable loops. Equipment can be shuttled to the surface on tethers and set up anywhere within the triangle. When the exploration and installation tasks are accomplished, the crew ride up the center tether and unharness the cables from the orbiter. The orbiter returns to the interplanetary vehicle or maneuvers to another location over Phobos, where the crew repeat the web building for another EVA.


    Given the hypothetical nature of the mission scenario, this discussion of possible requirements is not intended to be definitive. It is, however, pragmatically suggestive.

    6.3.1 General

  • Standard reference coordinate system definition (there is none for Phobos), e.g., parallels and meridians or equivalent
  • Phobos ephemeris (for mission planning)
  • Selection of designated landing sites based on science, processing, or operational considerations (may await a more detailed reconnaissance of Phobos)
  • Simulator for milli-g (Phobos surface gravity)

  • 6.3.2 Spacecraft Features

  • Spacecraft landing gear with integral equipment or mechanisms to:

  • - Effect positive "mooring" (attachment) to the Phobos surface. Note: Phobos surface properties are incompletely known. Before landing gear can be designed, the physical parameters of the surface regolith must be determined, or the gear must be designed to accommodate the range (variety) of surface soil mechanics properties anticipated.

    - Provide discharge of any electrostatic charges

    - Provide touchdown and liftoff indication. Rebound or bounce-off probably is a problem due to the low level of gravity.
  • Radar altimeter (or equivalent, e.g., laser)
  • VHF ranging (or equivalent) to exploit Phobos surface beacons and to support/assist contingency rendezvous with MMU. Primary spacecraft also should act as a VHF beacon for the MMU.
  • Airlock: must provide a means of removing dust/soil particles before reentry into the spacecraft pressurized volume. This may require a two-compartment airlock if adequate removal containment techniques cannot be included in a single compartment airlock.
  • IVA and EVA techniques and equipment for removing dust and soil from equipment, personnel. and spacecraft surfaces
  • External debris removal system
  • External spot and flood lights for Phobos 'night" operations
  • Specialized landing thrusters to effect low thrust levels without disturbing surface materials, i.e., similar to STS proxops techniques -- "canted" outward and downward thrusters fire the plume generally outward but with a downward component (net upward thrust). This technique is inefficient in terms of fuel use but provides braking without "hosing down" the area being approached. "Variable cant" thrusters would provide a method of fine vernier control (such as in the first stage of the Atlas booster). Upward firing thrusters of the same design would be required to control descent and to provide a positive hold-down force immediately after touchdown .
  • Specialized EVA support equipment for the low Phobos gravity; extendible/deployable device to provide EVA crew member restraint and mobility/translation capability.

  • 6.3.3 Surface Installations

  • Landing site markers
  • Visual passive (flags, panels, long-range reflectors, etc.)
    Visual active (solar powered/charged light beacons: color-coded, strobes, etc.).
  • Navigation/ranging aids ("permanent" installations)
  • Passive (reflectors)
    Active (solar powered/rechargeable RF beacons).

    6.3.4 EVA Crew Aids

  • EVA restraint and mobility/translation aids

    Spacecraft-mounted extensible/deployable devices

    Extendible rails, booms (rigid/semi-rigid), translation and restraint aids for close proximity to spacecraft EVA, e.g., an RMS-type device with telescoping arms; inflatable booms in triplet attached at the outer end to create an open pyramid configuration; telescoping rigid segments attached by the crew member while moving out from the spacecraft.

    Tethers and a tether management system. The low gravity may create a "spaghetti bowlX' ff loose, flailing tethers; thus, some method of managing the tethers to avoid entanglements will be mandatory. One such concept, the Module Web and "tethervator," is mentioned and illustrated above
  • Anchoring devices

    Anchored supports (equipment, rails, tethers, facilities). Stakes, posts, and pylons (in increasing size/length), possibly with permeating adhesive, will be required to affix various items to the Phobos surface.

    Attachment (anchorinq) mechanisms will be required for a variety of surface materials (loose sand or dust, consolidated or compacted, etc.). The following items are suggested for consideration:

    Masonry nail gun (recoilless) for attaching restraint device mounts to hard rock surface

    Vacuum qualified "cement" to bond attachment fittings or the restraint device itself to hard rock surfaces

    Zero torque augers to penetrate deep regolith with helical or expandable anchors (deadmen)

    Recoilless post driver for shallow regolith with solid rock beneath.

    Safety cages, such as a geodesic frame, to serve as a work area with safety restraints, purchase points, and foot restraint mounting surfaces. These open framework cages could be emplaced around high-intensity
    work sites, facilities, and science stations to create a more effective and safer work environment. Candidate sites for these "cages" might include science stations, mining/extraction facilities, tank farms, and temporary sites of high activity.

  • Smart MMU (or future equivalent): An MMU-type device on Phobos must have rendezvous capability because it would have considerable range potential. It would need to have computer and IMU capability as well as radar or laser and VHF ranging.

    Ballast "turtle" to add mass to an EVA suited crew member. Five metric tons of ballast (mass) would give a crew member a "weight" of about 15 pounds.


    Assuming that daytime EVA on Phobos is more appealing for crew safety and efficiency than EVA in the dark, light will be a primary consideration in mission planning. Assessment of the orbital mechanics of Phobos and Mars reveals that part of the moon may be a more suitable site for initial EVA.

    The orbital period of Phobos is approximately 7 1/2 hours, of which about 3/4 hour is in Mars shadow at worst case (equinoctial alignment). At solstitial alignment, Phobos does not pass through Mars' shadow (see Figures 6-2 and 6-3).

    Phobos rotates once per orbit, with the same end always toward the center of Mars. Thus, Phobos has day-night cycles (7 1/2 hours) with any given site in direct sunlight for about 3 3/4 hours during each cycle (disregarding Mars shadow passage) (see Figure 6-4). However, the nadir end of Phobos receives reflected light from the Mars dayside surface during the time the nadir end is away from the sun. Thus, as the nadir end passes into its solar dark phase, it begins to receive light from the Mars surface. At local high noon on its dayside pass, the nadir end of Phobos is illuminated by reflected light from the sunlit Mars "disk," the diameter of which subtends an angle of about 40o.

    During the solstitial phase, Phobos' nadir end is in continuous light, and during the equinoctial phase, the nadir end is in darkness for approximately 3/4 hour of each orbit. Thus, the nadir end of Phobos appears to be a prime candidate site for an initial EVA mission if light is a major consideration.

    Note also that the nadir end of Phobos will be the most easily observable site from a low orbiting spacecraft (e.g., command spacecraft), and rendezvous from lower orbit is usually easier to execute from below (toward the nadir end).

    The thermal environment of Phobos should be roughly analogous to lunar conditions, but colder. The radiation environment also should be roughly analogous to the moon with a peculiar exception. It may be possible to execute a retrograde synchronous orbit around Phobos at a low altitude so as to stay always on the down sun side of Phobos. The purpose of this maneuver would be to use the mass of Phobos for radiation shielding. The orbital mechanics of such a maneuver need to be verified.

    While none of these considerations is an overriding issue in selecting a Phobos landing site, they are potential factors for mission planning.

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