Deimos Dust for Mars Orbit Insertion

A friend proposes a fast transfer to Mars; 10 to 20% more delta V at Earth results in a much faster trip there. However, the arrival velocity would be MUCH higher, and difficult to shed by aerobraking without high lift (and high centripedal gees) to remain within the appropriate density layer of the atmosphere of this small, low gravity planet. The v²/r - g acceleration arriving at Mars would be much higher for Mars entry than for Apollo entry.

The difficulty is worse for large heavy spacecraft; there is not enough atmosphere for a low speed parachute landing.

Mars Missions in the Age of Robots

Going to Mars with microbe-infested humans is also a very bad idea, possibly the worst scientific crime of the millenium. If a motivation for a Mars mission is a search for life, it is likely that the only life found will be the microbes shed by humans. A robot probe can be autoclaved; humans cannot. However, humans can go to prepared habitats on Phobos, and control robots from there. Getting to Phobos (with aero-braking or hareno-braking ) will be less difficult than a Mars surface landing.

Before humans go to Mars, we must rule out the possibility of ancient life anywhere that human contamination can reach. The Martian atmosphere is thin, but it is windy and connects the whole planet. So, we should explore the most of the surface with robots, ruling out surface fossil life to a very high confidence level in every possible kind of niche. Mars is frozen; it is reasonable to assume that (with proper precautions) material below the surface ice level can be kept absolutely free of contamination. However, the robotic exploration of the planet will require very many robots a very long time.

If we screw this up, we destroy our chances of learning the truth - forever.

Phobos Low Station

But what if there is a cheap way to get robots to Mars, and humans to Phobos, to operate those robots from a close enough distance to simulate "walking on Mars in a space suit" via telepresence, taking advantage of rapid lightspeed communication from Phobos, or Low Mars Orbit, or from a manned station on a tether hanging down from Phobos? The round trip speed of light delay from a station at 1000 km, through a chain of relay satellites, to a surface station on the opposite side of Mars is less than 8 milliseconds. We are learning to operate robot submersibles in the ocean from much larger distances.

The "gravity" experienced on this hanging station would be "only" 2.2 m/s², only 60% of Mars surface gravity, but that also means the tether can be made without unobtanium. See AcousticClimber for a way to power climbers up and down that tether to the main base on Phobos, 5000 kilometers above, a 14 hour trip at 100 m/s .

From the station, another pendulum tether can reach all the way to the Mars surface, to deliver robots up and down. However, until the contamination issues are ruled out, it is probably best to keep humans far from the surface, or the lower station. Reducing speed-of light delay by a couple of milliseconds is probably not worth the complexity and risk. Humans on Phobos, robotic repair shop 5000 km an below (with fiber optic bundle bandwidth and 10 ms round trip speed), and 10 ms laser links to satellites and surface robots.


We continuously monitor the distance to the LAGEOS laser geodynamics satellites to micrometer precision, through a thick and turbulent atmosphere. These satellites are now the reference standard for all surface measurements on Earth. They are accurate enough to observe the slow drift of the continents via plate tectonics, and calibrate the GPS navigation system. They are inert balls of aluminum and brass covered with retroreflectors. It is conceivable that we could use them to calibrate GPS directly, and achieve nanometer precision with signal averaging.

We do vastly better with LIGO precision laser technology, with 1e-22 precision. LIGO has been compared to measuring the distance to Proxima Centauri to an accuracy of the thickness of a human hair. With distributed retroreflectors and an accurate computer simulation of the solar system, we can make hyperprecise calculations of positions and orbits. If we make this a priority, we can compute the position of spacecraft and electromagnetically launched objects to millimeters over gigameter distances. That means we can learn to hit a very small bulls-eye over solar system distances.

Harenodynamic Braking with "Deimos Dust"

In the early 1980s, Krafft Ehricke wrote about a "harenodynamic slide lander" for braking lunar landers to a stop on the Moon, without using propellant on board the vehicle. The vehicle would skid to a stop on a carefully prepared runway. That was in a low-accuracy era, but the spacecraft would need to be lined up to millimeter precision and milliradian trajectory angles to remain in contact with the runway all the way from orbital velocity to a stop.

An interesting idea, but we can do better. We can use low velocity mortars or short launchloop style accelerators to launch packets of carefully sorted lunar dust into the center of the path of a lander's heat shield, details here. A vehicle travelling from Earth will arrive at 2520 m/s and require careful aiming; however, a 2500 m/s accelerator (lunar escape velocity) can launch a small fraction of that material at the vehicle heat shield when it is far away, perhaps with tiny trajectory adjustments via laser ablation. That can put the vehicle "in the groove" for a precise landing. With more packets of material launched this way, perhaps over many hours, we can shed some of the vehicle's velocity, or change the vehicle's orbital plane to line up with our dust mortar "runway".

We can do even better for Mars orbit entry, with powder packets launched from Deimos.

Escape velocity from Deimos orbit to an interplanetary orbit is 1.35 km/s. Deimos is 1.47e15 kg of rock and sand, which can be mechanically converted into arbitrarily small grains of dust, sorted by size electrostatically in high vacuum. That material can be launched by a slower, very long distance dust mortar at vehicles inbound at 8 km/s and 30,000 km distant, slowing them at 0.1 gee to the apoapse velocity of a Phobos-intersecting orbit. As they near Phobos, more dust packets may be launched to slow the vehicle's periapse velocity to roughly match Phobos's orbit. From there, a few tens of meters per second of rocket delta V can complete the Phobos rendezvous and "landing" ... more like a docking for this very low gravity object.

Phobos might not have the "brag rights" of a direct landing on Mars, but the returning astronauts will not be slowly tortured to death by angry astrobiologists, either. An electromagnetically stabilized rotating habitat anchored to Phobos, with staged acclimatization from the zero gee trip from Earth to a 1 gee environment for a healthy multiyear mission means the astronauts might have a chance of returning home without being crippled for life.

Note that launch loops will enable complete 1-gee Aldrin-cycler wheel-habitats for the trip from Mars to Earth and back. Humans did not evolve for zero gee, and keeping exposures down and shielding thick will be a safer way to travel.

Appendix: Phobos Tether

Imagine a tapered tether hanging radially/vertically from Phobos to a terminus above the martian equator. We will assume Torayca T1000 tether material, derated and tapered (smaller at bottom terminus, larger at the Phobos attachment).

Mars gravitational parameter



Mars surface radius



Mars rotation velocity



Phobos radius



Phobos velocity



T1000 density



T1000 rated strength



T1000 derated strength (WAG)



T1000 specific strength



Libreoffice spreadsheet

(radius 9376 km) hanging towards an altitude 300 km above the equatorial surface of Mars (radius 3696 km) made of Torayca T1000 material ( 6.5 GPa tensile strength, derated to 4.5 GPa, 1800 kg/m3 density ) and supporting a 1000 kg mass at the end will have a total tether mass of 32700 kg, about 33 times the mass of the payload plus the climber. Horizontal velocity at the bottom of the tether is 840 m/s inertial and 600 m/s relative to Mars' rotating surface. Without parachutes or wings to slow down, a dropped payload arrives at the surface with a velocity of 690 m/s, approximately mach 3.

A pendulum tether will be perhaps twice as heavy (WAG), but could "swing" so it touches down on the surface at Phobos perigee with zero relative velocity. Thus, a climber could arrive on Phobos without wings or rockets.

Note added 2021 July 31: a payload travelling down a tether will leave Phobos with 2137 meters per second velocity. As it descends the tether, it will pull the tether forwards. Eventually, it will "pendulum" backwards. WAG: if the decent rate is tuned just right, the backswing could match the surface.

Further - if the tether is rotating like a "swing chair" amusement park ride, the vehicle could be released with north or south velocity. Perhaps(???) it is possible to release a winged glider with enough velocity to enter Mars atmosphere and glide a considerable distance to the north or south - probably not as far as the poles, but enough to survey a wide band around the equator. Or into orbit, for observation and telemetry relay.

DeimosDust (last edited 2021-07-31 11:23:03 by KeithLofstrom)