On July 14, NASA’s New Horizons spacecraft is set to fly through the Pluto system. It will glide in at an angle of 46 degrees to the plane of the dwarf planet’s orbit, penetrate the plane, then pass through the shadows first of Pluto and then of its moon, Charon.
After it passes Charon it will pivot around and look backward, to hunt for any rarefied rings around Pluto.
New Horizons will also use sunlight reflected from Charon to image the part of Pluto’s surface now in continuous darkness.
You can follow the action with Commentary from Bill McKinnon, co-investigator on the mission’s science team, on the the New Horizons page curated by NASA’s Jet Propulsion Laboratory. Images will typically appear on the page between one to a few days after the event.
During the month of June, New Horizons will be scanning its upcoming trajectory for debris that might damage the spacecraft.
If any hazards are found, it could divert to an alternative pathway or point its antenna in the ram direction to shield the body of the spacecraft.
Images of Pluto are already sharper than the best that can be made with earth-based telescopes.
New Horizons launched in 2001, back when Pluto was still called a planet and Charon was Pluto’s only known moon.
In the 9.5 years the spacecraft has been traveling the 3 billion miles from Earth to Pluto’s perch at the edge of the solar system, everything has changed.
But Pluto is still the most distant solar system body we’ve ever visited, and one about which we know very little. Watching it turn from a pixelated blob into a real world will be a thrill, says McKinnon.
He expects its geology to be spectacular.
At 4.7 billion miles from the sun, Kuiper Belt Objects are the coldest objects in the solar system. Pluto’s surface temperature is 40 Kelvin, or roughly -400 degrees Fahrenheit, and at that temperature many compounds that are gases on Earth freeze out as ices.
“There could be mountains of water ice with glaciers of nitrogen creeping down their sides,” McKinnon says. “That’s entirely plausible. At -400 degrees F, water ice would be hard and brittle, but nitrogen ice would be squishy and easily melted.”
“We know Pluto has atmosphere, or at least an atmospheric haze,” he says. It’s made up mostly of nitrogen, like Earth’s atmosphere, with traces of carbon monoxide and methane (natural gas). These ‘volatiles’ snow out of the atmosphere in some places and vaporize in others, creating shifting patterns of color and brightness on Pluto’s surface.”
Pluto also has seasons; in fact, it has extreme seasons because it orbits tipped on its side.
At its solstices (when the sun is at its highest or lowest in the sky), a quarter of its surface is in continuous daylight and another quarter is in continuous darkness.
While the New Horizons spacecraft has been in transit to Pluto, Pluto’s atmospheric pressure has doubled or tripled. But at the same time, McKinnon says, we know some of its atmosphere is escaping to space and mixing with the solar wind.
What replenishes the atmosphere if it is continually lost?
“There is either a really thick layer of ice that hasn’t yet been depleted, or a source of continuous supply,” McKinnon says. “And that’s something we’ll actually get to the bottom of when we get there, because there will either be thick ice caps that would take billions of years to erode or just a thin glaze of ices that must be replenished from somewhere.”
“We also want to understand as much as we can what lies beneath the surface,” McKinnon says. “Even though Pluto’s surface is icy, we think it has a rocky core. It may even have a subsurface ocean of water spiked with ammonia that is warmed by radioactive decay in the rocky core.”
New Horizons will also look at Charon and Pluto’s other moons.
Charon seems to be very different from Pluto.
Its surface is covered by patches of ammonia hydrates and water crystals, which couldn’t long withstand bombardment with ultraviolet radiation and cosmic rays. Some scientists have suggested this may mean it is being resurfaced by icy lava flows from cryo-volcanos erupting liquid water with dissolved ammonia, McKinnon says.