The Case for Two Asteroids and a Planet
There are three realistic options for our next exploratory destination in space: a return to the Moon to establish a permanent presence; the current plan to visit a near-Earth asteroid; and what early space engineer S. Fred Singer calls “PhD missions.” These are trips to the martian moons Phobos and Deimos without immediately trying to land on Mars — an idea that has not been seriously considered in recent years.
Our next major project in space should be a return to our nearby and newly intriguing Moon — this time to stay. We know how to get there and visit. Now we need learn to live and work for extended periods on an airless surface dominated by regolith, while still relatively close to home and the potential for support and rescue. We can study our nearest neighbor in depth, not least to understand our star system’s history of major impacts. We should begin to use lunar resources, like oxygen and probable heavy metals from the cores of large asteroids that have struck the Moon, to support activities in low Earth orbit. Doing so would establish regular transportation routes to a single location with regular launch windows, and the earliest beginnings of inner solar system trade and commerce.
If we decline to take this logical incremental step, the martian moons make a better destination than a near-Earth asteroid.
In theory, and considering only propulsion, a number of near-Earth asteroids are among the easiest destinations in the solar system. In practice, with the propulsion, life-support technology and other skills that are available today or likely in the near future, NASA has found it difficult to find a suitable near-Earth asteroid that is much larger than the Orion-derived spacecraft that would visit it.
A one-time visit to an asteroid in deep space is more risky than returning to the Moon, but probably well within our capabilities. However, recent discoveries have shown that asteroids are extremely diverse. A single expensive mission to send a geologist to a single asteroid would scarcely be representative, and therefore could be scientifically misleading.
Because of their small sizes and changing launch windows, it is difficult to imagine regular transportation to, or much permanent infrastructure on, an asteroid. The international space station has shown that it is permanent infrastructure that drives the development of low-cost commercial transportation. Ideas similar to the Commercial Orbital Transportation Services (COTS) contracts, which encourage commercial development of new vehicles to deliver cargo to low Earth orbit, got nowhere until the space station was partially built, providing a ready commercial and political market.
Asteroids are a long-term investment. With today’s technology, surveying asteroids is better left to automated missions, which could visit and conduct reconnaissance of a large number at relatively low cost.
Phobos and Deimos are either captured asteroids or permanent moons that are very like asteroids. A PhD mission could accomplish most of the goals of any near-term asteroid mission. Since the two moons are very different, one mission visiting both of them might well be the near-equivalent of two separate asteroid missions.
Mars, and thus its moons, has regular and predictable launch windows, once every two years. This allows the establishment of permanent infrastructure and regular transportation that could support a commercial industry — a “PhD COTS.”
Without repeat visits to a defined location — e.g., regular supply runs to a base — there is no predictable market to support a transportation industry. Without regular transport, it is hard to experiment with the use of local resources and their return to low Earth orbit. Without permanence, it is difficult to establish commercial and political momentum to keep a project going. Witness the political staying power and resistance to change of the semi-permanent international space station and of the Soyuz spacecraft needed to get there, versus the ephemeral Apollo operational missions. Permanent bases develop permanent constituencies and markets; visits do not.
Strictly in terms of change in velocity, the martian moons are easier to get to and return from than most asteroids and the lunar surface — if we ignore travel time, supplies, medical support and radiation. These are not minor issues — to put it mildly — but these problems are remarkably similar to those involved in an asteroid trip, albeit greater in magnitude.
Using a minimum energy trajectory, the one-way transfer time to the martian system is approximately nine months. Lockheed Martin’s Plymouth Rock proposal, using two Orion spacecraft and higher than minimum energy trajectories to reach an asteroid, envisioned flight times of approximately three months each way. A feasibility study by Rob Landis, et al., of NASA calculated slightly longer but comparable minimum travel times.
Plymouth Rock envisions a crew of just two, limited by the ability to carry supplies. All other things being equal, a first flight to the martian moons would require three times as many supplies in each direction. To increase safety on a long trip, it should also have a larger crew. Early PhD missions would require a habitation module, and a large upper stage with storable propellant or other technology to brake at Mars and, later, initiate the trans-Earth injection to home.
An initial PhD mission would undoubtedly be more expensive than an asteroid mission — but you would get two asteroids plus Mars. After the first flight or two, it could be dramatically cheaper.
A semi-permanent PhD base would extract oxygen — the heaviest consumable needed in quantity — and possibly water from local rocks. Phobos and Deimos appear to have dry surfaces, but that may be a consequence of impact processing. Very little is known about these bodies, but the rim of Stickney Crater on Phobos has a carbonaceous chondrite-like spectrum, implying there may be water in bedrock under the regolith. Russia’s Phobos-Grunt mission to return samples from Phobos is currently scheduled to launch on a Zenit rocket in late December. Upon its return in August 2014, samples of Phobos’ regolith may elucidate the question of water.
As regular scientific and supply flights depart for the Mars system, the learning curve may be very rapid: The final three J-class Apollo flights, in spite of their extraordinary ambition, had far lower incremental costs than the earlier Apollo missions.
Likewise, new technologies can be introduced to reduce PhD mission costs, e.g., aerobraking, first at Mars, then on the higher velocity return to Earth. Local oxygen would supplement, then replace, fuel oxidizer and life support volatiles brought from Earth.
A PhD base would demonstrate all the skills and technology required for regular interplanetary flight without initially attempting the difficult, expensive and dangerous steps of landing and surviving on the alien martian surface. It would also test everything needed for regular asteroid flights.
From Phobos, Mars fills almost half the sky. Deimos, with a near-synchronous orbit and hemispheric views of the planet, is an excellent place from which to control Mars rovers without the long communication delay from Earth.
Automated Mars landings staged out of Phobos could use local fuel to return samples of many different martian geologic provinces at far lower cost per mission than flights staged from Earth. The samples would be processed on Phobos for return to Earth, once again utilizing PhD-derived fuel. If martian biology were found in a sample, a PhD base could be more fully isolated from Earth’s biosphere than the space station. Likewise, a PhD strategy could help protect Mars from terrestrial contamination.
Experimental automated factories landed on Mars and controlled from Deimos could separate fuel from the martian atmosphere or regolith and store it for the return legs of future landings. That alone would dramatically reduce the cost of even the first human landing on Mars.
In truth, both asteroids and PhD are a step too far, and we should consolidate our skills on the moon before heading out into interplanetary unknowns. However, if we do choose to stretch ourselves and take these great risks, a PhD base buys both “small body” and planetary science on or near three worlds at one low-energy destination. Dramatic views of astronauts working just above Mars would be combined with the best opportunities for unexpected discoveries. Going to the martian moons would give us early experience in interplanetary spaceflight and lay foundations for solar system commerce in oxygen and other resources.
All of this for only a little more cost and risk than a single asteroid barely larger than your spacecraft.
Donald F. Robertson is a freelance space industry journalist based in San Francisco. Further examples of his work are at www.DonaldFRobertson.com.