For 85 years, Pluto has been little more than a speck of light captured by telescopes. That changes on Tuesday when NASA's New Horizons spacecraft pays a quick, but action-packed visit to the largest known unexplored world in the solar system.
Hints of what New Horizons will find have been trickling in as the piano-sized probe nears Pluto's domain some 3 billion miles away.
Dark areas spotted on Pluto's surface may be places where hydrocarbons like methane ice have been transformed over the eons by solar ultraviolet radiation. Another theory is that the dark spots are pockets where Pluto's surface ice abates, exposing bare rock.
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"We don't know what we'll find," said New Horizons principal scientist Alan Stern, with the Southwest Research Institute in Boulder, Colo. "Virtually everyplace we've sent a spacecraft on a first reconnaissance mission like this we find out that our notions were flat-wrong."
Like the groundbreaking Pioneer, Mariner and Voyager scouts of the 1960s, 70s and 80s, New Horizons has broad science goals, topped by three key questions: What do Pluto and its co-orbiting mate Charon look like? What are they made of? What's Pluto's atmosphere like?
The answers should flesh out understanding about how the solar system formed. For example, Pluto and thousands of sibling mini-planets in the Kuiper Belt region of the solar system may have assembled much closer to the sun, but were shot out beyond Neptune as Jupiter and the gas giants drifted into their present orbits.
New Horizons may find ice volcanoes on Pluto, similar to what NASA's Cassini spacecraft found on Saturn's moon Enceladus, an indication of possible subsurface water.
That may seem a contradiction for a world with surface temperatures almost 400 degrees below zero Fahrenheit. But Pluto, which is believed to be about 70 percent rock, could be internally heated by the natural decay of radioactive materials.
Once an oddball of the planetary family, Pluto turns out to be king of its own domain, accompanied in its 248-year orbit around the sun by at least five gravitationally tethered companions: Charon - a moon half its diameter - and four known small satellites, Nix, Hydra, Kerberos and Styx. New Horizons will target them all as it punches through the Pluto system for about 30 minutes on Tuesday.
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Closest approach to Pluto will come at 7:49 a.m. ET Tuesday, though time becomes very relative at distances this great: Traveling at light speed, radio signals from New Horizons take about 4.5 hours to reach Earth.
Aside from the time lag, the flyby was designed to maximize the amount of data gathered at the expense of communicating with Earth. Science results and images will be transmitted over the next 16 months, while New Horizons travels toward a Pluto cousin for a possible follow-on mission.