The technology to repair and upgrade existing habitats, a practice widespread on Earth, is not likely to be developed under short term exploration goals. If exploration takes on a multi-administration or international character, the prospects for space architecture development by the inhabitants themselves will be broader. Private space tourism is a way the development of space and a space transportation infrastructure can be accelerated. Virgin Galactic has indicated plans for an orbital craft, SpaceShipThree. The demand for space tourism is one without bound. It is not difficult to imagine lunar parks or cruises by Venus. Another impetus to become a spacefaring species is planetary defense. The classic space mission is the Earth-colliding asteroid interception mission. Using nuclear detonations to split or deflect the asteroid is risky at best. Such a tactic could actually make the problem worse by increasing the amount of asteroid fragments that do end up hitting the Earth. Robert Zubrin writes:  If such a crew is to be summoned to a distant asteroid, there may be less risky ways to divert the asteroid. Another promising asteroid mitigation strategy is to land a crew on the asteroid well ahead of its impact date and to begin diverting some its mass into space to slowly alter its trajectory. This is a form of rocket propulsion by virtue of Newton's third law with the asteroid's mass as the propellant. Whether exploding nuclear weapons or diversion of mass is used, a sizable human crew may need to be sent into space for many months if not years to accomplish this mission. Questions such as what the astronauts will live in and what the ship will be like are questions for the space architect. When motivations to go into space are realized, work on mitigating the most serious threats can begin. One of the biggest threats to astronaut safety in space is sudden radiation events from solar flares. The violent solar storm of August 1972, which occurred between the Apollo 16 and Apollo 17 missions, could have produced fatal consequences had astronauts been caught exposed on the lunar surface. The best known protection against radiation in space is shielding; an especially effective shield is water contained in large tanks surrounding the astronauts. Unfortunately water has a mass of 1000 kilograms per cubic meter. A more practical approach would be to construct solar "storm shelters" that spacefarers can retreat to during peak events. For this to work, however, there would need to be a space weather broadcasting system in place to warn astronauts of upcoming storms, much like a tsunami warning system warns coastal inhabitants of impending danger. Perhaps one day a fleet of robotic spacecraft will orbit close to the Sun, monitoring solar activity and sending precious minutes of warning before waves of dangerous particles arrive at inhabited regions of space.