NASA Asteroid Grand Challenge hits the soggy and uncertain road running
By Brian Dodson
July 3, 2013
On June 18, the NASA Asteroid Grand Challenge was announced to a flood of media inattention. This was probably to be expected, as NASA actually said very little about it. Maybe so as to not attract the ire of forces in the US Congress that are trying to shut down the largest portion of this Grand Challenge; namely the capture and relocation of a seven-meter (23 ft) asteroid to a stable lunar orbit for study and as a practice site for asteroid exploration and exploitation. We've dug up the formal Request for Information (RFI) associated with the Grand Challenge, which gives a better idea of where NASA wants to put its money.
The RFI, which refers to this program as an "initiative" rather than a "Grand Challenge," is looking for input concerning system concepts and innovative approaches that will address the two primary goals of the initiative: the Asteroid Redirect Mission, and defending Earth against catastrophic asteroid impact.
The overall goals include, in the words of the RFI, "Advancing technologies and capabilities applicable to future exploration, as well as science, commercial, and other U.S. government space activities; Integrating these efforts to lead to the first-ever human mission to an asteroid; and accelerating efforts to improve detection, characterization, and mitigation of potentially hazardous asteroids to help plan for the defense of our planet against the threat of catastrophic collisions (planetary defense)."
Asteroid observationTo help define a program that meets these goals, NASA is requesting information in six general areas, the first of which is asteroid observation. Here the emphasis is on techniques and technology, either space- or Earth-based, that will lead to the identification of "all" near-Earth asteroids, where all appears to mean "down to around 10 meters (33 ft) in size." This is a large leap past the charge given by Congress to NASA to find all near-Earth asteroids larger than 140 m (459 ft), this being the size estimated to be capable of wiping out a city should it impact the Earth's surface.
RedirectionNext on the list is Asteroid Redirection Systems, which appears to take up the bulk of the initiative with four subsections and two closely related bullet points. Again in the words of the RFI, "NASA is interested in concepts for robotic spacecraft systems to enable rendezvous and proximity operations with an asteroid, and redirection of an asteroid of up to 1,000 metric tons into translunar space." A weight of 1,000 metric tons converts into a stony asteroid about eight meters in diameter, so this is essentially the asteroid capture program with which Congress is having so much trouble.
The first redirection system NASA is requesting ideas for is a solar electric propulsion system that could be made ready to launch in 2017 or 2018. The system must be able to be put into orbit by a single Space Launch System launch, although a smaller launch vehicle would be preferable (cheaper). Power capacity of the solar electric system should be about 40 to 50 kW. For comparison, each solar panel on the ISS can supply 33 kW of electric power. Admittedly, these are not small at 34 x 12 m (111 x 40 ft), and weigh about 1,100 kg (2,400 lb), but clearly the required level of power is available.
NASA is after a drive with a specific impulse of about 3,000 seconds, which is on the low end for ion engines, but this area of endeavor has been pushed towards smaller specific impulse by the need for the larger thrust/weight ratios required to enable tomorrow's missions. The ion engines on Deep Space 1 operated at a power of 2.1 kW, and delivered just under ten milliNewtons of thrust, about equal to the force of an 8 g US Sacajawea dollar coin resting on a hand.
The solar electric propulsion system is required to operate within 0.3 Astronomical Units of the Earth's orbit, and be otherwise suitable for an asteroid relocation mission. I'm sure that if we had such a thing lying around, many other missions could easily benefit.
NASA is also looking for input on integrated sensing systems to determine an asteroid's size, shape, mass, inertia, spin, surface properties, and composition. These will be used not only in targeting asteroids for redirection, but also to provide closed-loop feedback during remote asteroidal operations.
Asteroid deflectionIn the hope that satellite servicing technology will be in place during the decade or so course of this initiative, ways to adapt those technologies to wrangling small asteroids are also being sought. Clearly, this could include missions in which a solar electric drive system is placed on an asteroid, and is supplied periodically by refueling flights. This might allow an asteroid deflection mission to provide thrust to change the orbit of an asteroid for a longer period of time by providing additional fuel to add to that carried on the initial installation flight.
NASA also wants ideas for deflecting asteroids, which would be effective against asteroids large enough to do city-scale damage or larger (greater than 100 m). They intend to concentrate on ways to use the Asteroid Redirection craft for such missions, and are interested in testing pushing slowly on a large asteroid. A proof of concept demonstration of the gravity tractor approach may also be attempted. The gravity tractor maneuver keeps a spacecraft or captured small asteroid a short distance from a larger asteroid, and then relies on their gravitational attraction and the spacecraft drive to pull the gravitationally bound partners out of their original orbit.
In other missions, the Asteroid Redirection craft would be used to investigate the sub-surface structure of asteroids to provide an information base to inform our attempts to deflect asteroids, and may also, for example, place transponders on asteroids to make them easier to track over long periods.
Capture and relocationAsteroid capture systems, naturally, are part of this initiative. Their mission characteristics are essentially the same as those floated earlier in the year, seeking the ability to capture and stop the rotation of an asteroid weighing up to 1,000 metric tons, that is neither too irregular in shape nor spinning too fast.
NASA appears to be asking for versions of the capture approach introduced early this year by Caltech's Keck Institute for Space Studies in Pasadena, which aims to capture a small near-Earth asteroid and tow it into lunar orbit, as illustrated in the video below. Still, making a reality of that rather impressive plan will require considerable innovation. As mentioned earlier, some interest is also being given to methods of snagging a boulder from a larger asteroid.
There is also a category for equipment to help astronauts explore/prospect the redirected asteroids once they are in lunar orbit. This category covers everything from better space suits for spending long periods acting like a miner in microgravity (literally less than a microgravity on such a small asteroid), to specialized sample collection tools and instruments for prospecting (ground-thumping sonar, for example). Because these items could be used on so many future missions, it may serve as a catch-all for neat manned-mission gadgets to be developed during this period when the US has no manned launch capability.
Getting the public involvedFinally, NASA is asking everyone from other governments to individual citizens what this initiative could include that would make them care about it and interested in participating. Reaching out to the sensibilities of the masses may be, as is said in the British Civil Service, a "courageous" decision, given how disappointed many people are in what the US space effort has become. On the other hand, it might just give NASA the clout they need to fight Congress to establish new capabilities in space. The response date of this RFI is July 18, so if you have something to say, do it quickly.