Space probes have several scientific advantages.
Space has fascinated humans and spurred our collective imagination since time began. While the astronomers of the Renaissance began to unlock the secrets of the heavenly bodies, until the 20th century humans could only look to outer space. While humans succeeded in putting people into space, most space exploration is done by unmanned space probes. These probes present a number of issues for government-run space agencies.
Cost
Sending human-made objects into space is always a costly venture. However, in comparative terms, unmanned space probes will cost less than manned missions because the design of the vehicles do not have to accommodate and sustain human life, which includes provisions for breathable air, a livable interior temperature, and the ability to safely re-enter the Earth's atmosphere. Cutting out these additional engineering challenges makes space missions cheaper, which lets the space agency do more missions with a limited budget.
Conditions
Unmanned space probes can go where astronauts could not. These include missions that get close enough to the sun to where heat and radiation levels would kill a human as well as long-term voyages that go farther than a craft could store food to sustain life. These allow such missions as Voyager I and II, which not only visited the outermost bodies in the solar system, but continue to journey out into space, and send data back to Earth, until power or signal strength fail.
Malfunctions
While space probes can undertake missions that would preclude human involvement, they are not perfect. Whereas humans can adapt to changing situations and repair malfunctions, probes can only execute their programming. If this programming is flawed, such as the ill-fated Mars Climate Orbiter that crashed into the Martian surface because two different teams used different measurement systems, then the mission is doomed the moment the probe lifts off. These can lead to costly and embarrassing public failures.
Enthusiasm
While space probes conduct good science and undertake useful missions, they do not capture the human imagination or ignite the same kind of excitement that a human physically exploring space does. Government space agencies are dependent upon the budgetary politics of the moment for funding, and a lack of public interest in space exploration makes space agencies an attractive target to cut. While manned missions are more limited from a scientific standpoint, they are far more effective at capturing the public opinion necessary for funding space exploration.
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