Caltech has an asteroid capture feasibility study. The feasibility of an asteroid retrieval mission hinges on finding an overlap between the smallest NEAs (Near Earth Asteroids) that could be reasonably discovered and characterized and the largest NEAs that could be captured and transported in a reasonable flight time. This overlap appears to be centered on NEAs roughly 7 meter in diameter corresponding to masses in the range of 250,000 kg to 1,000,000 kg. To put this in perspective, the Apollo program returned 382 kg of Moon rocks in six missions and the OSIRIS-REx mission proposes to return at least 60 grams of surface material from a NEA by 2023. The present study indicates that it would be possible to return a ~500,000-kg NEA to high lunar orbit by around 2025.
Illustration of an asteroid retrieval spacecraft in the process of capturing a 7-meter, 500-ton asteroid. (Image Credit: Rick Sternbach / KISS)
The report that follows outlines the observation campaign necessary to discover and characterize NEAs with the right combination of physical and orbital characteristics that make them attractive targets for return. It suggests that with the right ground-based observation campaign approximately five attractive targets per year could be discovered and adequately characterized. The report also provides a conceptual design of a flight system with the capability to rendezvous with a NEA in deep space, perform in situ characterization of the object and subsequently capture it, de-spin it, and transport it to lunar orbit in a total flight time of 6 to 10 years. The transportation capability would be enabled by a ~40-kW solar electric propulsion system with a specific impulse of 3,000 s. Significantly, the entire flight system could be launched to low-Earth orbit on a single Atlas V-class launch vehicle. With an initial mass to low-Earth orbit (IMLEO) of 18,000 kg, the subsequent delivery of a 500-t asteroid to lunar orbit represents a mass amplification factor of about 28-to-1. That is, 28 times the mass launched to LEO would be delivered to high lunar orbit, where it would be energetically in a favorable location to support human exploration beyond cislunar space. Longer flight times, higher power SEP systems, or a target asteroid in a particularly favorable orbit could increase the mass amplification factor from 28-to-1 to 70-to-1 or greater. The NASA GRC COMPASS team estimated the full life-cycle cost of an asteroid capture and return mission at ~$2.6B.
NASA insight/oversight 204 15% of prime contractor costs Phase A 68 5% of B/C/D costs Spacecraft 1359 Prime Contractor B/C/D cost plus fee (10% - less science payload) Launch Vehicle 288 Atlas 551 Mission Ops/GDS 117 10 year mission plus set-up Reserves 611 30% reserves Total 2647
Solar Array Technology
The current state of the art for solar array technology is probably best represented by the solar arrays in use on the largest commercial communication satellites. These satellites use rigid-panel arrays with triple-junction cells and beginning-of-life (BOL) power levels up to 24 kW. At least one commercial satellite manufacturer is now offering a 30-kW BOL capability. A typical rigid-panel solar array has a
specific power of approximately 80 W/kg.
The alternative to rigid-panel solar arrays are flexible-blanket arrays. Flexible-blanket arrays have been flown on the International Space Station (ISS) in a rectangular configuration with 12% efficient single-junction solar cells giving a specific power of about 40 W/kg, and on the Phoenix mission in the circular Ultraflex configuration with 27% efficient solar cells resulting in a specific power of about 110 W/kg.
Ultraflex solar arrays were scaled up by nearly an order of magnitude from 0.75 kW per wing for the Phoenix spacecraft to about 7 kW per wing for the Orion vehicle . The ACR mission concept would need an additional factor of four increase in the Ultraflex solar array power to about 29 kW per wing. The circular configuration of the Ultraflex solar array means that a factor of four increase in power per wing could be achieved by increasing the wing radius by only a factor of two. The inverted metamorphic solar cells with an efficiency of 33% are expected to be flight qualified well in advance of the assumed 2020 launch date for the ACR mission.
Near-Term Application of SEP Technology for Human Missions to NEAs
The development of a 40 kW-class SEP system would provide the valuable capability of being able to pre-deploy several tons of destination elements, logistics, and payloads. Initial estimates identify that approximately 3,100 kg of elements and logistics, along with approximately 500 kg of destination payload, could be pre-deployed in support of a human NEA mission, rather than carried with the crew. This approach would reduce the requirements for the launch vehicles and in-space propulsive elements required to conduct a human mission.