When one examines the plans that NASA devises for human missions beyond low Earth orbit, ISRU (in situ resource utilization) experiments or demonstrations are sometimes included but never incorporated into the imperative of the mission sequence, what engineers call “the critical path.” ISRU simply means that you make stuff you need in space from resources available in space. At current levels of development, such stuff would largely consist of high-mass, low-information materials, such as propellant and shielding. By reducing the amount of mass launched from the Earth through the use of ISRU techniques, we would save many billions of dollars of our limited space budget. So why aren’t we hearing more about this?
Certainly nothing in the chemistry or physics of ISRU indicates that it is impossible or unduly futuristic – most processes date back to antiquity (melting ice into water) or most recently, to 18th and 19th Century industrial chemistry (e.g., carbothermal reduction). But ISRU has never been actually attempted with extraterrestrial materials and real hardware in space. In other words, aerospace engineers haven’t done it; it’s too much a science-fiction concept for them (“Set the replicators on beef stew tonight, Mr. Sulu!”). They reactively imagine potential disasters issuing from its attempt, rather than appreciate its possible benefits. Engineers tend to be cautious when undertaking new designs that incorporate untried techniques, and in the case of human spaceflight, rightly so. However, when caution keeps you locked in a comfort zone, initiative and programs will atrophy and cripple our space progress.
Several years ago, Bob Zubrin, my friend and sometime public debate opponent, had a key insight. When you bring everything you need with you from Earth in order to get to Mars, the consequences of the rocket equation multiplies the gross departure weight by a large amount (it costs thousands of dollars per pound for delivery to LEO). If you don’t bring the fuel for the return trip home with you, your departure mass is much lower. For the trip home, Zubrin’s Mars Direct architecture utilizes rocket propellant made on the martian surface. In one fell swoop, he reduced the amount of mass needed in LEO to go to Mars by 50 percent; a significant finding.
The current NASA Mars Design Reference Mission utilizes 8 launches of a super heavy-lift vehicle (150 tons to LEO) to construct the 500-ton Mars craft in Earth orbit (another analysis projects 10-12 launches of same). More than 80% of this mass is propellant. If the HLV ends up costing a couple of billion dollars each, the cost of a human Mars mission is not only unaffordable – it will never be undertaken. And don’t think that the hallowed New Space “cheap access” to LEO will save you either; those vehicles are too small (much less payload than 150 tons) and will need to use storable propellant because the LOX-hydrogen (LH2) cryogens will boil away into space between their individual launches. Storable propellants have much less energy than LOX-H2 and thus, besides the complexity of logistical assembly from a greater number of flights, the total required mass in LEO multiplies frighteningly.
Engineers are reluctant to mandate ISRU as part of the Mars architecture because it’s never been done (risky) and it’s never been done because it’s too risky – the classic “Catch-22.” How can we move beyond this impasse? We can reduce the size of the “risk” roadblock by demonstrating ISRU on the Moon. An ISRU strategy on the Moon can be designed that not only retires risk for its future use on Mars, but can also provision the Mars trip from lunar materials, reducing even more the amount of mass needed to be launched from Earth. But why use the Moon to document the value of ISRU, rather than a near-Earth asteroid? Simply put, the Moon has the right materials and properties for performing this risk-reduction activity.
The results from the LCROSS impact experiment, in which a Centaur upper stage was crashed into a dark area near the Moon’s south pole, demonstrated that not only is water present in quantity, but there are other volatiles there as well, including methane, ammonia, carbon dioxide and some simple organic compounds. On the basis of their observed abundance, all of these substances are probably derived from comets that over geological time have hit the Moon and are preserved in the permanently shadowed, cold areas near the poles. Asteroids typically do not contain these exotic volatile substances; if water is present in asteroids, it is chemically bound in mineral structures and requires considerable energy and processing to extract and use.
Lunar ISRU can harvest not only water (and thus, oxygen) but also methane from the Moon’s polar deposits. Methane is the propellant Zubrin’s Mars Direct architecture uses for Earth return (although as the martian crust is water-rich, LOX-LH2 could also be used on Mars, making the lunar polar deposits doubly relevant). From the Earth-Moon L-1 point, either landing on or taking off from the poles of the Moon requires about a 2.5 km/sec change in velocity (Dv) while transfer from the surface of Mars to Mars orbit for rendezvous and return to Earth requires a Dv of 3.5 km/sec. Thus, the transfer energies for the two missions are comparable. This means that we can test the methane ISRU systems not only in principle – we can test the actual Mars equipment in practice three days away on the Moon and in cislunar space.
In effect, these lunar properties mean that a complete, end-to-end systems test of all the pieces of a Mars Direct-style architecture could be performed in cislunar space, overcoming the most critical obstacle – the “risk” of requiring ISRU in the critical path. In my opinion, ISRU is the most important and game-changing technology for future spaceflight. I will go so far as to say that a human Mars mission is inconceivable without incorporating ISRU in some form, most likely as a source of propellant but also for other potential uses (e.g., shielding, oxygen and water).
A Mars mission conducted in the Apollo mode (everything launched from Earth) is simply not possible, fiscally or politically. A national security imperative during Apollo allowed us to bludgeon technical problems to death with money. We no longer live in that world. Space programs must be affordable, which means that we cannot opt for the “easiest” or most familiar way to do something – we must be clever, frugal and use what is available.
During a recent hearing on the proposed new NASA Authorization bill, the two witnesses Steve Squyres and Tom Young both opined that lunar return was not a prerequisite for human Mars missions. They are both wrong. The critical path to Mars goes through the Moon, although not the way most engineers have been looking at it. They’ve viewed a lunar mission as a fancy “dress rehearsal” for the Mars mission, with people landing on the Moon to conduct a complex and carefully choreographed EVA, to practice how they plan to explore Mars, and then leave the Moon as soon as possible. In their view, the main object of the lunar mission is to get it over with.
To realize human Mars missions, a sustained lunar return is much more valuable than a “touch and go,” a “fly-by,” or a “hover-over.” Only on the Moon can we learn for the first time how to extract and use off-planet resources. We permanently retire ISRU risk and open up space by using lunar resources. We practice the entire Mars surface mission sequence with Mars hardware flying in space – from landing, to refueling and ascent. To go to Mars without ISRU requires too much mass in LEO, needing multiple launches of expensive, disposable vehicles. In consequence, it is unaffordable and thus, unlikely to ever happen. Such expense cannot be justified under virtually any conceivable political circumstance, save those associated with some national emergency, certainly an eventuality not to be hoped for.
And yes, for those following the breadcrumbs, by using this process we’ll develop a system that can routinely access all of cislunar space, including the lunar surface.
Note added, 26 June 2013: A version of this piece has been posted at Space.com