Over the next 16 months, the data collected during the New Horizons spacecraft’s mid-July flyby of Pluto will find its way back to Earth, a few bits at a time. Already there are surprises in the data so far received. How could there not be? We are seeing a whole new world revealed in detail for the first time in human history. The analyses and debates will surely only have begun once all the data are in.
The image below on the left is from the Hubble Space Telescope, and until July 2015 it was the best we had. The true-color image on the right is from the New Horizons flyby.
I thought it might be useful to write on what we have learned about the small icy worlds of the solar system from our previous explorations. Perhaps a more appropriately humble description would be that this is what we think we know! Each new world forces us to re-examine our assumptions, each new question begets ten more. Such is the nature of science, and the source of its endless fascination for so many of us.
Worlds of Rock, Gas, and Ice
Leaving out a lot of detail, we can think of solar system bodies as being composed of rock, gas, ice, or some combination thereof. The four innermost planets (Mercury, Venus, Earth, and Mars) are relatively small and rocky, and the four outermost (Jupiter, Saturn, Uranus, and Neptune) are mostly gaseous, with ice becoming more prominent as we reach the more distant environs of Uranus and Neptune.
Among the inner planets, Mars has two tiny rocky moons that may be captured asteroids, and the Earth has a rocky moon that is likely the result of a massive collision early in its history. Neither Mercury nor Venus has a moon.
The inner solar system has numerous smaller bodies collectively called asteroids that are mostly rocky. The greatest numbers of these lie between Mars and Jupiter, or co-orbiting ahead of and behind Jupiter in its orbit. Icy bodies wander into the inner solar system from farther out, spewing gas and dust tails as they are warmed by the sun. These are the comets that periodically grace our skies.