route. John Hickman identified the principal obstacle to developing space trade as the distances involved, which will reduce all or nearly all trade to the exchange of intangible goods. That threatens the possibility of conducting business in a genuinely common currency and of enforcing debt agreements incurred by governments. Building commercial spaceports Building high-capacity commercial spaceports may require connection with other modes of transportation, such as railroad or sea, which would make spaceports another dimension of national economy. One analysis of commercial, technical, and logistical concerns for an operating spaceport, formulated by the Spaceport Technology Development Office of NASA, is Vision Spaceport. See also Commercial uses of space Helium-3 (although it cannot currently be exploited as an energy source, that may change in the future if fusion power is developed) Space-based solar power Space manufacturing Institutions International Institute of Space Commerce References Further reading John Hickman. "Problems of Interplanetary and Interstellar Trade." Astropolitics. Vol.6, No. 1. (January 2008). pp. 95–104. The Political Economy of Very Large Space Projects by John Hickman Trade Spaceflight economics Trade routes Trade