Space enthusiasts look to the day when ordinary people, as well as professional astronauts and members of Congress, can leave Earth behind and head for a space station resort, or maybe a base on the moon or Mars. The Space Transportation Association, an industry lobbying group, recently created a division devoted to promoting space tourism, which it sees as a viable way to spur economic development beyond Earth. The great stumbling block in this road to stars, however, is the sheer difficulty of getting anywhere in space. Merely achieving orbit is an expensive and risky proposition. Current space propulsion technologies make it a stretch to send probes to distant destinations within the solar system. Spacecraft have to follow multi-laver, indirect trajectories that loop around several planes in order to gain velocity from gravity assists. Then the craft lack the energy to come back. Sending spacecraft to other solar systems would take many centuries. Fortunately, engineers have no shortage of inventive plans for new propulsion systems that might someday expand human presence, literally or figuratively, beyond this planet. Some are radical refinements of current rockets or jet technologies. Others harness nuclear energies or would ride on powerful laser beams. Even the equivalents of "space elevators" for hoisting cargoes into orbit are on the drawing board. "Reach low orbit and you are halfway to anywhere in the Solar System," science-fiction author Robert A. Heinlein memorably wrote, and virtually all analysts agree that inexpensive access to low Earth orbit is a vital first step, because most scenarios for expanding humankind’ s reach depend on the orbital assembly of massive spacecraft or other equipment, involving multiple hunches. The need for better launch systems is already immediate, driven by private and public sector demand. Most commercial payloads are destined either for the now crowed geo-stationary orbit, where satellites jostle for elbow room 36,000 kilometers above the equator, or for low-Earth or bit, just a few hundred kilometers up. Low-Earth orbit is rapidly becoming a space enterprise zone, because satellites that close can transmit signals to desktop or even handheld receivers. Scientific payloads are also taking off in a big way. More than 50 major observatories and explorations to other solar systems’ bodies will lift off within the next decade. The pressing demand for launches has even prompted Boeing’s commercial space division to team up with RSC—Energia in Moscow and Kvaerner Maritime in Oslo to refurbish an oil rig and create a 34,000—ton displacement semi-submersible launch platform that will be towed to orbitally favorable launch sites. The passage is mainly about______.

A. the development of space technology
B. the obstacles and prospects of space transportation
C. the public interests in space travel
D. the growth of space business