NASA recently announced that it has embraced the idea of an asteroid retrieval mission as the central goal of its human spaceflight program for the next decade or two. According to the agency’s leadership, this mission will accomplish a number of important objectives, including delivering a science bonanza, demonstrating a technology useful for planetary defense, creating a large cache of materials in space that can provide in situ resources to support space exploration activities and achieving the president’s goal of flying a mission to a near-Earth asteroid as a way of breaking out of geocentric space and demonstrating human deep-space capabilities necessary for subsequent missions to Mars.
Since this initiative will cost many billions of dollars and, by diverting the entire multibillion-dollar human spaceflight program for decades, impose an opportunity cost amounting to many tens of billions of dollars, it is imperative that these claims be examined critically to see if any of them are true.
Let us therefore consider each of them in order.
There is no doubt that the “asteroid” mission, which involves using an electric propulsion spacecraft with a power supply half that of the international space station to push a 3.5-meter-radius object (i.e., a rock, not an asteroid) to a lunar-like orbit, and then visiting it with astronauts flying sorties in the Orion capsule sometime in the third decade of the 21st century, would eventually return some science. However, vastly more science could be achieved, much sooner, at much lower cost and risk, simply by sending a flotilla of small robotic spacecraft to collect kilogram-sized samples from multiple real asteroids and return them all the way to Earth.
While the electric propulsion system proposed for the so-called asteroid mission can be used over a period of several years of continuous thrusting to alter the trajectory of 3.5-meter rocks, objects representing planetary threats have masses thousands to millions of times greater, and it would not even be practical to despin them to allow continuous thrusting to begin, let alone deliver to them sufficient propellant or power to change their trajectories. There are tens of thousands of asteroids with radii over 100 meters, each with a mass of over 15 million tons. Assuming that such an object is in an Earth-like orbit, it would require a velocity change of about 1 meter per second to move the periapsis of its orbit by a distance equal to the diameter of Earth, and thus have a chance at turning a direct hit into a near-miss. Using electric propulsion, about 500 tons of propellant would be required, and the 40-kilowatt system employed by the asteroid mission would need to thrust continuously for 250 years to deliver the necessary push. A much more practical approach would be to send a missile armed with a conventional or nuclear warhead (depending upon the size of the object) to give the asteroid a sudden solid shove by blasting a small portion of its mass off its side. Unfortunately, in order to preserve a false rationale for the asteroid mission’s electric propulsion system, such more potent approaches to planetary protection are being neglected.
The 3.5-meter rock moved to near-lunar orbit in the asteroid mission would have a mass of about 500 tons, which is about 20 times the mass of the system that would need to be launched to low Earth orbit to move it. This might appear to be a good trade, but the rock would likely be only about 5 percent water by weight, so in terms of potentially useful mass delivered to space it would only be a match. However, hydrogen and oxygen launched to low Earth orbit are already in useful form as pure cryogenic propellants, whereas the water in the rock would have to be extracted by processing 3 meters depth of rock, then collected, electrolyzed and cryogenically liquefied, all of which would require a system of considerable power and complexity. Furthermore, in its proposed near-lunar retrograde orbit, the propellant produced from the rock would be in the wrong place to support useful space exploration activity. In fact, the delta-V needed to leave low Earth orbit to reach the rock propellant depot would be about the same as the delta-V needed to leave low Earth orbit and fly directly to Mars. Therefore, even if the rock propellant depot were there today, ready to provide propellant for free to any Mars-bound mission willing to stop by to refuel, it would not make any sense to go there.
In situ resource utilization is a key technology to space exploration, but the resources to be used need to be located at the destination of interest, not somewhere else. Martian missions need to use resources located on Mars. Lunar missions need to use resources located on the Moon. A rock in a retrograde lunar orbit is of no resource utilization interest to anyone.
As to the claim that the asteroid retrieval mission achieves the goal set by President Barack Obama in 2010 of breaking out of geocentric space, that is simply untrue. In point of fact, aside from potentially providing a fat contract to an excessively influential electric propulsion company (see my op-ed “The VASIMR Hoax,” SpaceNews, July 13, 2011), the entire purpose of the initiative is to find a way to shirk the challenge of human interplanetary flight.
The asteroid retrieval mission is not a competent way to advance science, planetary defense, in situ resource utilization or human interplanetary flight. It thus represents an enormous waste of time and money that could prevent NASA’s human spaceflight program from achieving anything worthwhile for decades. Congress must not accept this. Hearings need to be held, with the NASA administrator required to defend his plan in the presence of technically qualified critics. If the plan is found to be irrational, then lawmakers need to insist that it be replaced with a space agency strategy that actually makes sense.
The American people want and deserve a human spaceflight program that really explores new worlds. It is past time that NASA stepped up to the plate and accepted that challenge.
Robert Zubrin is president of Pioneer Astronautics and the Mars Society and author of “The Case for Mars.” His latest work, “Mars Direct: Space Exploration, the Red Planet, and the Human Future,” was recently published by Penguin.
