Deimos vs Mars - What's The Cost?

The following is unorganized and incomplete but improving soon

Mars has been the goal of many space advocates for almost a century. It is seemingly the most similar other planet to Earth, and some dream of making it into a second Earth. To date, our "terraforming experience" consists of making the Earth more like Venus; in spite of the concerted efforts of 100 billion people past and present, "progress" has been slow. Terraforming is hard. Creating a global industrial civilization is also; that also took the multi-millenium efforts of 100 billion people. We can export much of our scientific, technological, and cultural heritage, but Martian conditions will require g

Mars is most valuable as a potential "second genesis" of life. Ruling Mars life "in or out" in the presence of substantial human contamination (worst case from human fecal bacteria spread planetwide after a crash) may be impossible, a potential scientific and economic loss worth trillions of dollars to a civilization entering the biotechnology age.

In light of the "progress" we have actually made, and how we made it, perhaps we should look for more forgiving opportunities before we screw up Mars as well.

Small Body Opportunities

Rocket progress is slow; the Saturn 5 could put 140 tonnes in LEO in 1967, the much-delayed Falcon Heavy can put 64 tonnes into orbit in 2018. Silicon progress is faster; 100 MHz processors in 1996 (when Robert Zubrin published "The Case for Mars"), and 30-GHz-equivalent multi-core processors in 2018. The optimal "silicon-to-spaceship" ratio has increased by a factor of 600.

Our experience with human biology, estimated by tilt-bed studies at NASA Ames and calibrated by ISS, teaches us that the biological optimum for human beings is walking and running in a 1 gee gravity field ( more "stress" than standing still at one gee), not somewhere between zero and one gees. Mars, at 3.7 m/s², is more like 0.0 m/s² ISS than 9.8 m/s² Earth. Its radiation and atmosphere and temperature extremes are also closer to ISS.

For multi-year survival, we will need to create a one-gee environment, a spinning wheel. For multidecade survival, and safe reproduction, we must shield that spinning wheel with considerable mass, and support that mass structurally, Supporting mass on Mars is expensive; on milligravity bodies like asteroids or Deimos or Phobos, very easy, assuming that dust and sand can be loosened up.

Mars versus Deimos Landings

Mars has an atmosphere marginally useful for aerobraking. However, the Martian atmosphere varies greatly, so the resulting orbit or landing point will not be precise. Perhaps the best way to get good results for a Mars surface landing is to aerobrake to orbital capture, then make a smaller second aerobraking manuver for a precision landing at a designated point (for Zubrin's 1996 recommended mission, that would be at the precise location of a previously-delivered return vehicle). That will require some rocket thrust for landing at a precise spot; Zubrin estimates about 0.5 km/s of thrust (CHECK THIS). Total delta V, about 7 km/s.

Zubrin's clever "Mars Direct" idea for a Mars surface mission is to bring hydrogen and a nuclear power source, and manufacture the return fuel and oxidizer with those and CO₂ from the thin Martian atmosphere.

Leaving the Mars surface, and boosting into a faster-than-Hohmann trajectory for return to Earth requires the kinetic energy ( ½ M V²) for 5 km/s escape, plus the kinetic energy of about 4 km/s of "hyperbolic excess"; the result is a launch velocity that is the "root sum square" of these two velocities if the acceleration is completed near the Martian surface. sqrt(5²+4²) is approximately 6.4 km/s. That is also the velocity that must be shed in a one pass direct entry using an aerobraking shell.

Robots to Mars, Astronauts to Deimos, Samples to Deimos

The first goal should be to land hoards of small robots on Mars, and look for samples and data. A complete, state-of-the-art biotech lab on Earth uses massive machines, and are designed for human operation in Earth-surface conditions; the small robots cannot do that without spending billions of dollars attempting to duplicate all this equipment in miniature, and qualify it for space launch. That seems very unlikely.

Instead, we should take voluminous data and gather samples with robots, and determine what tools and talents we must send to the vicinity of Mars to evaluate them. The best nearby spot is Deimos; with an array of laser-link relay satellites in 500 kilometer Mars orbit, the round trip communication time to any surface robot is 200 milliseconds, not the 40 minute round trip delay to Earth at conjunction. We tolerate longer delays through geosynchronous satellites to communicate with (and partly control) undersea robots, a rapidly developing technology that Mars explorers can duplicate.

We may not need boots on the ground, but evaluating samples for the unexpected requires skilled scientists and a well equipped, hands-on wet lab. We can transport samples from the surface with very small rockets, accelerating from the surface up to 4.7 km/s and arriving at Deimos with a relative velocity of 0.44 km/s about 7 hours (not 7 months) later. The thin Mars atmosphere and the weak Martian gravity allows us to move small sample packages with small rockets.

Landing on Deimos

Landing a base and humans on Deimos, and a crew return after a similarly lengthy mission, needs less delta V. This mission cannot use the Mars atmosphere to manufacture fuel, but it does not need to. It does open up other possibilities.

Assume a similar interplanetary arrival at Mars, but rather than shed all 6.4 km/s with one or two aerobraking passes, we shed only 1.72 km/s in one aerobraking pass, going into a highly elliptical orbit around Mars with a periapse just above the Mars atmosphere, and an apoapse at Deimos orbital radius, an orbit similar to those taken by . The apoapse velocity at Deimos radius is 0.91 km/s, Deimos orbital velocity averages 1.35 km/s, so the average additional circularization velocity is 0.44 km/s . Deimos is small and its gravity is weak, so a "landing" requires a mere 7 meters per second of acceleration, the same speed as dropping 7 feet on Earth. All told, arrival requires less than half a kilometer per second of delta V.

This low acceleration will greatly simplify filling sandbags and supporting them over a roof over a centrifugal habitat - which should also be prepared by robots long in advance, before humans arrive, tired from their long journey. The humans are there to do science and repair their equipment to stay alive, not build stuff. The robot preparation mission will also deliver the return vehicle, infrastructure, and the supplies and low-tech scientific instruments that are unlikely to be obsoleted by the delay between the robot launch and the astronaut launch, perhaps years later.

After the astronaut mission is completed, they return to Earth on a similar trajectory from Deimos down to Mars, another 0.44 km/s. This time, the periapse will be well above the lower Martian atmosphere, aerobraking is not wanted. Their vehicle will near Mars at 4.6 km/s, and they will use rockets to boost that to 6.4 km/s, a delta V of 1.8 km/s. They will slow to 4 km/s as they leave the Martian gravity well, on an interplanetary trajectory back to Earth. Total delta V thrust, including arrival and departure, only 2.7 km/s.

Rather than the 7 km/s needed to land on and leave Mars, a Deimos mission requires only 2.7 km/s total. It won't be able to manufacture fuel, but it does not need to. Keeping cryofuels cold? Outside of Martian atmosphere and gravity, a mirror and radiators can point away from the Sun, towards deep space, and cool the return stage to similar temperatures to the James Webb space telescope, about 50 Kelvin. That is below the freezing temperature of methane and oxygen, and the sublimation rate under pressure will be low.


DeimosVsMars (last edited 2018-02-19 19:19:57 by KeithLofstrom)