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In the future, space missions may return surface materials from Mars, or other objects, but much of astronomy appears otherwise confined to Earth-based observations augmented by observations from orbiting satellites and long-range space probes and supplemented by theory.
A central undertaking in astronomy is the determination of distances.
(The measured intensity [.) A standard candle can be identified by means of its spectrum or the pattern of regular variations in brightness.There are a few exceptions to this limitation—namely, meteorites (most of which are from the asteroid belt, though some are from the Moon or Mars), rock and soil samples brought back from the Moon, samples of comet and asteroid dust returned by robotic spacecraft, and interplanetary dust particles collected in or above the stratosphere.These can be examined with laboratory techniques to provide information that cannot be obtained in any other way.Within the inner solar system, some distances can now be better determined through the timing of radar reflections or, in the case of the Moon, through laser ranging.For the outer planets, triangulation is still used.
Pluto’s orbit, with a mean radius of 39.5 AU, is sufficiently eccentric that at times it is closer to the Sun than is Neptune.