Destination Deimos (part 1)
by James S. Logan and Daniel R. Adamo
|Not only are we not experienced interplanetary travelers—at least, not yet—but no human being has been farther from the Earth’s surface than San Francisco is from Los Angeles in over forty years.|
Fast forward to the 21st century. After being mired in low Earth orbit (LEO) since 1972, long-duration crewed voyages across interplanetary distances are being seriously contemplated. If successful, history will undoubtedly change course once more. But the warning analogy is clear: in extending human space travel from LEO to interplanetary destinations, we face difficulties no less formidable than did Magellan.
Magellan’s voyage was undertaken with primitive 16th century technology. Likewise, the first forays into interplanetary space will be accomplished with rather primitive legacy technologies, many of which date from the 1960s. Propulsion and re-entry techniques for human spaceflight haven’t fundamentally changed since the dawn of the space age.
Since the first cosmonaut rocketed into space in 1961, more than 500 people on more than 270 missions have accumulated in excess of 100 person-years of spaceflight experience. But the vast majority of that experience has been in LEO. The average time on the lunar surface for the twelve Apollo astronauts was only 2.08 days; the average lunar extravehicular activity (EVA) time only 13.5 hours. Humanity’s total experience on another celestial body is less than seven percent of a single person-year. Not only are we not experienced interplanetary travelers—at least, not yet—but despite hundreds of billions of dollars spent, no human being has been farther from the Earth’s surface than San Francisco is from Los Angeles in over forty years.
Project Constellation, NASA’s return to the Moon program, was canceled in 2010 for reasons much more complicated than “been there, done that.” Although scientific justifications for going back to the Moon persist, programmatic limitations conspired to reduce the effort to little more than an expensive redo of the same sortie-style missions that were accomplished 45 years ago. The lack of enthusiasm in the general public, Congress, and even the halls of NASA itself (where the authors were at the time) was due, in part, to the uneasy feeling it just wasn’t bold enough. There was little to excite those passionately desiring human footprints on Mars or those advocating human missions to near Earth asteroids. If science was the ultimate justification for going back to the Moon, detractors argued that could be accomplished just as efficiently and vastly cheaper by robots directed from Earth.
Mars remains the beacon for human exploration. Generations of science fiction writers and multiple successful space probes and rovers have established Mars in the core of our collective consciousness. More than any other place, Mars beckons humanity. Although an eventual human landing on Mars is inevitable, most experts agree it is currently a bridge too far. However, there is an alternate path to Mars, one that not only has intrinsic value itself, but also one that would enable the first wave of human visitors to the Mars system to conduct extensive “humans-in-the-loop” real-time telerobotic exploration of the Martian surface from a natural staging area: the Red Planet’s outer moon Deimos.
Deimos has many virtues that make it the logical gateway to the Mars system. It is a full 15 by 12.2 by 10.4 kilometers in size, making it much bigger than near Earth asteroids NASA is considering visiting in the near term. Unlike the vast majority of such asteroids, a launch window to Deimos opens up consistently every 2.14 years. Many experts believe Deimos actually is an asteroid, perhaps a carbonaceous chondrite, captured eons ago by Mars’ gravitational field.
Only 20,000 kilometers above the Martian surface, Deimos is less than five percent the distance from the Earth to the Moon. The round trip light time, very important for line-of-sight telerobotics, is only a little over a tenth of a second, almost 18 times less than from Earth to the Moon and back.
Surprisingly, Deimos is easier to get to energetically from LEO than the lunar surface. Escape velocity from the surface of Deimos is a paltry 20 kilometers per hour. Unlike our Moon, no dedicated lander is required. You don’t “land” on a small celestial body with no gravity well; you “berth” with it. Phobos, the inner and larger Martian moon, resides much deeper in the Red Planet’s gravity well and therefore requires 6.5 percent more round-trip propellant, a seemingly small yet significant penalty when considering multiple trips and the distances involved.
|Those opposed to tele-exploration of Mars from its moons often lament this concept sends humans 99.99 percent of the distance between Earth and Mars and then stops. Our answer is the majority of funding and consumables expenditures, complexities, and risks accrue in traversing the last one hundredth of one percent.|
Like our Moon, the same side of Deimos faces Mars at all times. Because its orbit is just above Mars synchronous orbit (MSO), Deimos would appear to move very slowly east to west as seen from the surface of Mars. From the perspective of Deimos, Mars would appear to slowly rotate eastward at only 2.7 degrees per hour. A Mars surface feature would undergo two sunrises and two sunsets, remaining continuously visible, before rotating out of view. If several surface assets were positioned at regularly-spaced longitudes, Deimos-based human teleoperators could circulate westward from one to the next and explore 24/7. Over a period of nearly five and a half days, the entire planet is seen except for extreme polar regions. This “short-range” human telepresence would meet all Mars surface exploration objectives currently documented in NASA Mars Design Reference Architecture (DRA) 5.0, Section 3 (“Goals and Objectives”). Phobos is so close to Mars it unfortunately has a much narrower view of the planet. Because Phobos rises and sets four times a day, acquiring and operating surface assets from the inner moon would be much more challenging than from Deimos.
Those opposed to tele-exploration of Mars from its moons often lament this concept sends humans 99.99 percent of the distance between Earth and Mars and then stops. Our answer is the majority of funding and consumables expenditures, complexities, and risks accrue in traversing the last one hundredth of one percent. Likewise, the base camp at Mount Everest is 99.96 percent the distance between Houston, Texas, and the summit. Yet it is obvious the majority of challenges are in that final fraction of a percent. Successful ascents to the summit of Mt. Everest occur only after significant resources and supporting infrastructures are deployed at various base camps, and the majority of these resources never reach the summit.
Depending on the resources discovered, Deimos could be utilized as a staging area for the entire Mars system, including the surface. Deimos may hold valuable resources within its interior, and transportable main belt asteroid resources may lie just outside the orbit of Mars. In situ resource utilization (ISRU) at Deimos could be a game-changer for Mars exploration. If water ice deposits are found beneath the surface, a Deimos “gas station” could facilitate Earth return transits and even human missions to the Martian surface far sooner than anticipated.
A mission architecture encompasses not only hardware but also the operational aspects: the way a mission is flown to optimize efficiency, flexibility, safety, and success. The architecture also serves as a technology driver by identifying crucial aspects or elements in need of advancement or further definition through research and development. The chief feature of elegant mission architecture is synergy, the process of bringing together various elements so the whole is more than the sum total of the parts. A truly innovative mission architecture makes feasible what otherwise wouldn’t be possible. Human missions to the Mars system remain beyond the cusp of the standard approach, a combination of chemical propellant, minimal radiation shielding, frequent EVAs and no ISRU (other than sunlight.) Not only is the standard approach insufficient for human interplanetary spaceflight, it is dangerous.
We propose, therefore, a bold and daring yet essential departure from the standard approach: a program-level architecture for human tele-exploration of Mars from Deimos resulting in humanity’s first viable toehold in interplanetary space as a precursor to permanent human presence on Mars itself.
Any innovative mission architecture derives from challenges. The two biggest for human interplanetary spaceflight are flight dynamics, always constrained by the cold, hard physics of the rocket equation; and bioastronautics, the psycho-physiological realities of human adaptation, or lack thereof, to the deep space environment.
Once in orbit about a planetary body such as Earth or Mars, distance to an interplanetary destination generally depends on change in velocity (so-called delta-V) and transit time. The higher the delta-V and the more massive the spacecraft, the greater the amount of propellant required. Cargo can tolerate long transit times to minimize delta-V but humans cannot.
|Establishing a human foothold at Deimos will require a series of increasingly sophisticated robotic precursor missions.|
A draconian example of the rocket equation's tyranny is Apollo 17. The Apollo moonships were self-contained exploration systems. Everything needed to get to the lunar surface and back (crew, supplies, lander, structures, entry vehicle, and propellant) was included in a single launch package that stood as high as a sixteen-story building weighing 2,961,860 kilograms on the launch pad. Just twelve minutes after liftoff, 86 percent of the launch mass had been shed as either spent propellant or jettisoned stages. At mission end less than two-tenths of one percent of the original liftoff mass (the Command Module with the crew) returned to Earth.
For trips to the Mars system and back including required stay times, self-contained exploration systems are simply not possible.
To reduce the total mass of any human Deimos Transit Vehicle (DTV) to feasible levels, consumables, return propellant, and other resources required for extended stay times and Earth return must be pre-emplaced at Deimos. This aspect of our mission architecture should not be surprising. Since its inception in 1927, transatlantic air transport has essentially been a one-way proposition. Even today, round trips across transoceanic distances are possible only because pre-emplaced return consumables are available for transport resupply at the destination. Interplanetary transport will share the same dependency.
Establishing a human foothold at Deimos will require a series of increasingly sophisticated robotic precursor missions with probes capable of everything from detailed mapping and remote sensing of the moon to multiple sample analyses of surface and subsurface strata, excavation and preparation of a radiation-protected subsurface habitat (see below) and storage facilities for pre-emplaced crew consumables, and Earth return propellant for visiting crew. Support infrastructure (e.g. power, communications, thermal control, refrigeration, environmental control, and life support) must be transported, deployed, and proven operational prior to human visitation. With round-trip light times from Earth ranging from 7 to 41 minutes, preparation of Deimos Base will present worthy challenges to robotics, automation and artificial intelligence experts.
The sheer amount of consumables required to support a crew of three for a 933-day Deimos mission, plus the standard five percent safety margin, is formidable. Utilizing standard NASA open loop life support numbers extrapolated for interplanetary flight, a total of 96,658 kilograms of water, oxygen, food, crew supplies, gasses lost to space, and systems maintenance equipment is required. However, a DTV would only take 25 percent of the total amount on the outbound leg. The remaining 75 percent, (for Deimos loiter and Earth return phases) would be pre-emplaced prior to any human visitation.
The “Open Loop” approach is essential, especially for the first several crewed missions. Complex recycling technology with limited person-years of continuous operation could break down, essentially dooming the crew. It therefore requires the adequate spare parts be pre-emplaced before crew use. Open Loop may be simple, but it works. At Deimos, especially if ISRU resources are available, the risks of implementing closed-loop technologies earlier may be acceptable.
Space radiation is the chief biomedical show-stopper for human interplanetary spaceflight. Space is a seething cauldron of ionizing radiation with energies sufficient to destroy molecular bonds and strip electrons off atoms, creating free radicals and generally wreaking havoc on biological systems. Damage results in genome instability, increased mutation rates leading to cancer, and accelerated changes usually associated with aging, including deterioration of the central nervous system.
Although the Earth’s magnetic field deflects lower energy particles toward the poles, for high-energy space radiation it’s the mass shielding effect of Earth’s atmosphere that protects the biosphere from harm. All creatures on Earth benefit from a natural, passive and constant 1,030 grams per square centimeter radiation shield. We have devised a simple radiation protection scale pegged to the shielding equivalent provided by Earth’s atmosphere. A shielding equivalent of 1,030 grams per square centimeter provides 100 percent of Earth radiation protection, called RP100. At 5,500 meters above sea level, half of the atmosphere is above and half is below, the equivalent of RP50. By contrast, the most protected areas of the International Space Station provide RP2 and the spacesuit provides less than RP0.1.
NASA career space radiation exposure limits are based on the concept of Risk of Exposure Induced Death, or REID. This is a statistical metric pegged to a single radiation effect: death from cancer directly attributable to the exposure. NASA’s astronaut career dose limits accept a lifetime increase of cancer mortality of three percent.
The average daily dose rate, measured inside the Mars Science Laboratory (MSL) spacecraft on its 253-day, 560-million kilometer journey to Mars, was a whopping 1.8 millisieverts (mSv) per day, 180 times the average daily radiation dose at sea level. This is equivalent to getting a whole-body CT scan once every five to six days.
A typical round trip to Deimos consists of an outbound leg of 203 days, a Deimos stay time of 497 days until the next Earth return window opens, followed by a 233-day Earth return. Assuming the average radiation dose rate at Deimos is half the interplanetary dose rate (a reasonable assumption considering the Mars surface dose is roughly one-third the interplanetary dose—Mars does have an ultra-thin atmosphere and it is much more massive than Deimos), a rough estimate of the total mission radiation dose without additional shielding can be calculated. That figure is approximately 1.232 sieverts (Sv), a number that exceeds the current LEO career radiation dose limits for all astronauts except males aged 55 years or older. It also equates to more radiation than one would receive in 342 lifetimes on Earth.
|RP5 is a particularly attractive compromise level of protection for the entire crew compartment in transit. It provides the most reduction in radiation dose for the least mass.|
Long-duration exposures to microgravity result in deleterious effects in multiple organ systems and subsystems including loss and alteration of skeletal and cardiac muscle, irreversible trabecular bone demineralization, decreased immune function, and permanent changes in vision. Evidence suggests the combination of radiation and microgravity is worse than either alone. Remembering Magellan’s experience, attempting interplanetary flight without adequately addressing these issues risks inadvertently ushering in a dangerous, prolonged and ultimately preventable “Scurvy Phase” of space exploration.
Although pharmaceuticals and even genetic modification may play a role in the long term, for the foreseeable future mass shielding, however disdainful, appears to be the only viable option for enhanced crew radiation protection. As a general rule some shielding is better than none, and more shielding is better than less.
By invoking a single operational requirement, providing RP100 at Deimos (the equivalent protection of Earth’s atmosphere at sea level), accomplished by pre-emplacing the Deimos habitat inside the moon rather than on the surface, total mission radiation dose is reduced by almost 40 percent compared to surface exposures without enhanced radiation protection. The total number of exposed days is also reduced by 53 percent. Provided the equivalent of about seven meters of Deimos regolith separates the crew from interplanetary space, the RP100 requirement will be met. This same level of protection can be provided on the Moon and Mars by burying habitats 4.12 and 2.65 meters below the surface, respectively.
During interplanetary transit radiation protection is more problematic. Severe weight and volume constraints make mass shielding seem impractical for long-duration missions. NASA adheres to the ALARA principle (as low as reasonably achievable) regarding radiation exposure. As our understanding of basic mechanisms of radiobiology expands and supporting technology improves, the level of radiation protection considered ‘reasonably achievable’ continues to evolve.
RP5 (51.5 grams per square centimeter shielding equivalent) is a particularly attractive compromise level of protection for the entire crew compartment in transit. Not only will it protect the crew from acute radiation sickness for all but the most intense solar storms, it is an inflection point beyond which additional shielding doesn’t decrease the dose equivalent significantly until much higher levels of shielding are attained. In other words, RP5 provides the most reduction in radiation dose for the least mass. Considering other attributes of our proposed mission architecture (see below), RP5 (2.5 times the protection provided by any spacecraft to date, over 10 times the protection of the Apollo lunar lander and 50–100 times the protection of the spacesuit) can now be considered within the realm of reasonably achievable.