The passage is mainly about ______.

A．there is no clear destination in space
B．it is too risky and expensive for people to go beyond the orbit
C．the current space propulsion systems can not meet the needs for the long space travel
D．there is not enough energy for spacecraft to gain velocity

A．When traveling in space, spacecraft take indirect route to avoid gravity from other planets.
B．Space engineers are now developing space elevators.
C．It will take the current spacecraft several hundred years to travel to other solar systems.
D．Scientists from different countries are working together to develop new space technologies.