§ 8.3.8 Asteroid Enterprises - Sample Return Missions to Enable Asteroid Mining
In a paper presented at a conference in 1996, Willoughby et al. (paper reference) report on an asteroid sample return mission analysis, as work sponsored primarily by the NASA Lewis Research Center.
Their analysis of a sample return vehicle launched on a present day Delta II 7925 rocket (chosen for its reliability - and among the class of "small" launchers it is the largest) shows the sizes of samples that could be returned to Earth's surface from various asteroids near Earth - up to more than 600 kilograms (over 1200 pounds)! Their estimate of the cost to produce and operate the first vehicle over its life cycle is US $345 million, and extra copies would cost $223 million.
(However, note that when the others looked into another copy of the U.S. government's NEAR probe, more or less, they cut the total cost to well under $100 million. When estimating the cost, one must look at private vs. government bureaucratic/programmatic costs and efficiencies.)
The mission design of Willoughby et al. was conservative. Off the shelf hardware was used as much as possible. After the design was completed and the weight of the vehicle was estimated, 15% was added to its dry weight (including an extra thruster in case one failed) and a 10% propellant margin was added. By improving the vehicle design, the vehicle gets smaller and the returned sample gets even larger. If the sample is not returned to Earth but just retrieved to Earth orbit (e.g., for mining and processing experiments in space), the sample increases further.
The vehicle thrusters use an electric ion drive propulsion system using Xenon propellant such as that used on Hughes satellites for several years, and on the NASA Deep Space 1 probe which launched in October 1998 and will rendezvous with an asteroid in July 1999. It was assumed that the Delta II rocket would launch the entire vehicle to Earth escape velocity before the much more efficient electric ion drive thrusters kicked in. However, just launching the vehicle above Earth's atmosphere and letting the ion thrusters lift it up and away from low Earth orbit would greatly increase sample return capability. (Hughes currently markets its electric ion drive engines as an upper stage in order to be able to increase satellite size.)
Using a larger launch system instead of the Delta II, for example the Atlas IIAS, the Ariane, the Space Shuttle, or a Russian rocket, the return sample gets even larger.
No asteroid sampling system exists, and this was by far the main design uncertainty:
"Several types of sampling systems were considered: projectile penetrators, a tether system with a screw conveyor, and landers with arms or excavators. The tether system seems lightest and simplest. Its mass varies with sample size and depth. Almost 120 kg of this sampling apparatus can be left at the asteroid.
"Our tether sampler was given a cursory review by Honeybee Robotics (of the Champillion comet sampling team) and found reasonable. The slow drilling rates pose no risk of loss of volatiles. Future studies should define the sample apparatus in much greater detail, and consider innovative sampling techniques."
The vehicle mass is slightly over 1000 kg, and is released from the Delta II rocket in interplanetary space at a speed of 1 km/sec.
One mission scenario is to the asteroid 1991-VG, an asteroid whose spectrum indicates it is probably of desirable composition. During the best time to visit the asteroid, the round trip time is 2.5 years, including a 90 day stay. This is the 600 kg sample return mission. However, if we visit this asteroid at the worst possible time in its orbit relative to Earth, an absolute worst case scenario, the return sample is still around 350 kg (roughly 700 pounds) but the round trip time increases to over 6.2 years using gentle ion drive engines.
A few other asteroids were also considered in the study, as well as Mars' moon Deimos (75 kg in a 4.6 year mission, best case).
When the vehicle returns, it could be reused for another mission rather than launching up another vehicle, after replacing the excavation hardware that was left behind (unless it is returned, too).
There is vast room for improvement in this conservative study, but the authors put forth a good baseline reference mission based on conservative assumptions.