Space enthusiasts look to the day when ordinary people, as well 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 possible 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(推进)technologies make it a stretch to send probes to distant destinations within the solar system. Spacecraft have to follow multiyear, indirect paths 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 beyond this planet. Some axe radical refinements of current rocket 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 wrote, and virtually all analysts agree that inexpensive access to low-Earth orbit is vital first step, because most possible situations for expanding humankind's reach depend on the orbital assembly of massive spacecraft or other equipment, involving multiple launches.
The need for better launch systems is already immediate, driven, by private and public sector demand. Most commercial payloads(有效荷载) are destined either for the now crowded stationary orbit, where satellites push for elbow room 36,000 kilometers above the equator, or for low-Earth orbit, just a few hundred kilometers up. Low-Earth orbit is rapidly becoming a space enterprise zone, because satellites that close can transmit signals to 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 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