August 31, 2011
Part I: Operational Considerations
The current controversy over the direction of our national space program has many dimensions but most of the discourse has focused on the means (government vs. commercial launch vehicles) not the ends (destinations and activities). Near-Earth objects (NEO, i.e., asteroids) became the next destination for human exploration as an alternative to the Moon when the Augustine committee advocated a “flexible path” in their 2009 report. The reason for going to an asteroid instead of the Moon was that it costs too much money to develop a lunar lander whereas asteroids, having extremely low surface gravity, don’t require one. The administration embraced and supported this change in direction and since then, the agency has been studying possible NEO missions and how to conduct them.
On the surface, it might seem that NEO missions answer the requirements for future human destinations. NEOs are beyond low Earth orbit, they require long transit times and so simulate the duration of future Mars missions, and (wait for it)… we’ve never visited one with people. However, detailed consideration indicates that NEOs are not the best choice as our next destination in space. In this post and two additional ones to come, I will consider some of the operational, scientific and resource utilization issues that arise in planning NEO missions and exploration activities and compare them to the lunar alternative.
Most asteroids reside not near the Earth but in a zone between the orbits of Mars and Jupiter, the asteroid belt. The very strong gravity field of Jupiter will sometimes perturb the orbits of these rocky bodies and hurl them into the inner Solar System, where they usually hit the Sun or one of the inner planets. Between those two events, they orbit the Sun, sometimes coming close to the Earth. Such asteroids are called near-Earth objects and can be any of a variety of different types of asteroids. Typically, they are small, on the order of tens of meters to a few kilometers in size. As such, they do not have significant gravity fields of their own, so missions to them do not “land” on an alien world, but rather rendezvous and station-keep with it in deep space. Think “formation flying” with the International Space Station (ISS) without the option to dock.
The moniker “near Earth” is a relative descriptor. These objects orbit the Sun just as the Earth does and vary in distance to the Earth from a few million km to hundreds of millions of km, depending upon the time of year. Getting to one has nothing to do with getting to another, so multiple NEO destinations in one trip are unlikely. Because the distance to a NEO varies widely, we cannot just go to one whenever we choose – launch windows open at certain times of the year and because the NEO is in its own orbit, these windows occur infrequently and are of very short duration, usually a few days. Moreover, due to the distances between Earth and the NEO, radio communications will not be instantaneous, with varying time-lags of tens of seconds to several minutes between transmission and reception. Thus, the crew must be autonomous during operations.
Although there are several thousand NEOs, few of them are possible destinations for human missions. This is a consequence of two factors. First, space is very big and even several thousand rocks spread out over several billion cubic kilometers of empty space results in a very low density of objects. Second, many of these objects are unreachable, requiring too much velocity change (“delta-v”) from an Earth departure stage; this can be a result of either too high of an orbital inclination (out of the plane of the Earth’s orbit) or an orbit that is too eccentric (all orbits are elliptical). These factors result in reducing the field of possible destinations from thousands to a dozen or so at best. Moreover, the few NEOs that can be reached are all very small, from a few meters to perhaps a km or two in size. Not much exploratory area there, especially after a months-long trip in deep space.
That’s another consideration – transit time. Not only are there few targets, it takes months to reach one of them. Long transit time is sold as a benefit by asteroid advocates: because a trip to Mars will take months, a NEO mission will allow us to test out the systems for Mars missions. But such systems do not yet exist. On a human mission to a NEO, the crew is beyond help from Earth, except for radioed instructions and sympathy. A human NEO mission will have to be self-sufficient to a degree that does not now exist. Parts on the ISS fail all the time, but because it is only 400 km above the Earth, it is relatively straightforward to send replacement parts up on the next supply mission (unless your supply fleet is grounded, as currently it has been). On a NEO mission, a broken system must be both fixable and fixed by the crew. Even seemingly annoying malfunctions can become critical. As ISS astronaut Don Pettit puts it, “If your toilet breaks, you’re dead.”
Crew exposure is another consequence of long flight times, in this case to the radiation environment of interplanetary space. This hazard comes in two flavors – solar flares and galactic cosmic rays. Solar flares are massive eruptions of high-energy particles from the Sun, occurring at irregular intervals. We must carry some type of high-mass shielding to protect the crew from this deadly radiation. Because we cannot predict when a flare might occur, this massive solar “storm shelter” must be carried wherever we go in the Solar System (because Apollo missions were only a few days long, the crew simply accepted the risk of possible death from a solar flare). Cosmic rays are much less intense, but constant. The normal ones are relatively harmless, but high-energy versions (heavy nuclei from ancient supernovae) can cause serious tissue damage. Although crew can be partly shielded from this hazard, they are never totally protected from it. Astronauts in low Earth orbit are largely protected from radiation because they orbit beneath the van Allen radiation belts, which protect life on the Earth. On the Moon, we can use regolith to shield crew but for now, such mass is not available to astronauts traveling in deep space.
When the crew finally arrives at their destination, more difficulties await. Most NEOs spin very rapidly, with rotation periods on the order of a few hours at most. This means that the object is approachable only near its polar area. But because these rocks are irregularly shaped, rotation is not the smooth, regular spin of a planet, but more like that of a wobbling toy top. If material is disturbed on the surface, the rapid spin of the asteroid will launch the debris into space, creating a possible collision hazard to the human vehicle and crew. The lack of gravity means that “walking” on the surface of the asteroid is not possible; crew will “float” above the surface of the object and just as occurs in Earth orbit, each touch of the object (action) will result in a propulsive maneuver away from the surface (reaction).
We need to learn how to work quickly at the asteroid because we don’t have much time there. Loiter times near the asteroid for most opportunities are on the order of a few days. Why so short? Because the crew wants to be able to come home. Both NEO and Earth continue to orbit the Sun and we need to make sure that the Earth is in the right place when we arrive back at its orbit. So in effect, we will spend months traveling there, in a vehicle with the habitable volume of a large walk-in closet (OK, two walk-in closets maybe), a short time at the destination and then months for the trip home. Is it worth it? That will be the subject of my next post.
Destination: Moon or Asteroid?
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