The news that a consortium of guys with deep pockets, including movie-maker James Cameron and Google's chief Exec Larry Page, are keenly interested in "mining asteroids" caught my attention this past week. At an event at the Seattle Museum of Flight these hotshots along with others unveiled "Planetary Resources, Inc." and declared its objective to developing a "low cost series of spacecraft to prospect and mine near-Earth asteroids for water and metals" and thereby "bring the natural resources of space within humanity's economic sphere of influence".
According to Peter Diamandias, quoted in an WSJ piece ('Asteroid Mining is Outlined by a Start up', p. C3) yesterday, the "solar system is full of resources, and we can bring them back to humanity".
Maybe, but it'll take a hell of a lot more than big talk, or "beer talk" (as we used to call it in Milwaukee) to do it! Never mind, the company said it expects to launch its first spacecraft into low Earth orbit (between 100 and 1,000 miles up) within two years. Well, that's a start and I will wait to see if they can at least succeed in that humble goal.
A bit further on in the article, things get more down to earth, as co-founder Eric Anderson asserted the company's spacecraft could essentially "catch a ride on rockets that are scheduled to be launched into space thus bringing down the cost." He added that he "believed" it would "take $25-30 million to send a group of around 6 craft to study an asteroid".
Wait, are you guys going to STUDY an asteroid or mine them? If it's only to study, as seems clear now on getting more details of this ride-hitching theory, then it will be decades before any actual attempts at mining take place. I say "attempts" because serious asteroid mining will require powerful rockets at least twice the size of the Saturn V that took Apollo astronauts to the Moon. After all, it's a far different brand of effort to actually land and dredge up then haul back resources, than to fly by a large space rock!
Even for a small, say 250' (75 m) wide rock, mining would hardly be worth it unless you can retrieve at least 10^5 kg of metals, say. And even then, that loot of 100 metric tons of material may only yield 10 metric tons (or less) after refining. (And we won't even think about the energy to do that mining in an age approaching Peak Oil. So unless these guys have nuclear reactor scale energy, say in the exajoule class,, I don't see how they can refine, far less just bring the stuff-ores back).
Consider the job of hauling 100 metric tons (10^5 kg) of residual ore back to Earth. Let this be essentially what we call all the "final mass" - i.e. after the mining rocket has exhausted all its fuel in getting at least that much mining equipment to the rock. Then, we will need at least 1,000 times more fuel mass to do the passage.
Thus: M(i) = 10^8 kg and M(f) = 10^5 kg
To operate as a mining rocket - and not merely "study" asteroids, it will have to be a self-launch. Unless the Russians allow you to ferrry about 400 times to Earth orbit to assemble the mining ship there, you will have to launch from the surface which means achieving escape velocity. That is, about 11, 200 m/s. (Note: Since the shuttle's retirement, the U.S. has no major rocket plans on the drawing boards, given the Orion seems to have been scrubbed....though it may re-emerge in a diminished scale in 10 years.)
The basic rocket equation, which relates the difference in initial and final velocities (v(f) - v(i)) to the mass ratio (M(i)/ M(f)) and the velocity of the exhaust gases relative to the rocket, v', is"
v(f) - v(i) = v' ln (M(i)/ Mf))
Then, if v(f) - v(i) = 11, 200 m/s we find:
v' = [11, 200 m/s] / ln (10^8/ 10^5) = 11,200 m/s / 6.9 = 1, 620 m/s
which will require an enormous amount of thrust. Much more than small commercial -type craft have developed. Or may ever develop! Again, bear in mind this is assuming liftoff with 100 metric tons of mining equipment, drilling, etc. which will be needed. I also assume, very generously, that the ship and crew will be able to use the same equipment to process enough rock resources for fuel to get back to Earth. (If they can do this getting back won't be as traumatic as leaving Earth because the escape velocity from a space rock is negligible even with 10^5 kg of ores aboard the ship).
Indeed, the WSJ piece notes:
"Planetary Resources executives said that after reaching the first asteroid, the company would mine it for water, set up a fuel depot, then mine the asteroid for iron and other metals. By establishing fuel depots, Planetary Resources hopes to cut costs."
Again, the problem is mainly mass: the mass of mining equipment to get the job done on leaving Earth (whether from the surface or near Earth orbit) and the mass of mined material on the return.
Other mentioned objectives appear to border on the preposterous, such as: "trying to capture a smaller asteroid and bring it into orbit around the Moon so it can be studied closer to Earth."
In this case, we are talking of effecting a change in gravitational potential energy,
V = - GMm /r
where r will change from maybe 3.5 x 10^11 m to 1.5 x 10^11(considering the Moon as part of a double planet system with Earth). G = 6.7 x 10^-11 N-m^2/kg^2, the Newtonian gravitational constant, and M = the mass of the Sun (2 x 10^30 kg) and m the asteroid mass, maybe 10^12 kg. The difference, V2 - V1 comes to about 5.1 x 10^20 J.
Simply put, there simply isn't the technology to effect such a massive change in energy and perhaps not for decades, maybe more than a century.
Even a professor of planetary science from MIT, Richard Benzel, quoted in the article - acknowledges that asteroids will eventually be mined for resources and become "operational stepping stones" but that the effort by Planetary Resources "may be many decades ahead of its time".
To put it via understatement!