Even with fully automated production, limitations on the number of goods produced would arise from the availability of raw materials and energy, as well as ecological damage associated with manufacturing technologies.[4] Advocates of technological abundance often argue for more extensive use of renewable energy and greater recycling in order to prevent future drops in availability of energy and raw materials, and reduce ecological damage.[4] Solar energy in particular is often emphasized, as the cost of solar panels continues to drop[4] (and could drop far more with automated production by self-replicating machines), and advocates point out the total solar power striking the Earth's surface annually exceeds our civilization's current annual power usage by a factor of thousands.[18][19] Advocates also sometimes argue that the energy and raw materials available could be greatly expanded if we looked to resources beyond the Earth. For example, asteroid mining is sometimes discussed as a way of greatly reducing scarcity for many useful metals such as nickel.[20] While early asteroid mining might involve manned missions, advocates hope that eventually humanity could have automated mining done by self-replicating machines.[20][21] If this were done, then the only capital expenditure would be a single self-replicating unit (whether robotic or nanotechnological), after which the number of units could replicate at no further cost, limited only by the available raw materials needed to build more.[21]