In a few short weeks, the NASA New Horizons spacecraft will finish a long, lonely trek through the outer solar system and zip past Pluto. It will gather the first close-up views of the erstwhile ninth planet and examine, among other things, Pluto’s thin atmosphere by measuring sunlight and radio waves that pass through it.
The 14 July encounter has been a long time in the making. It has taken New Horizons more than nine years to make its way from Earth to Pluto, which was demoted and reclassified by the International Astronomical Union as a “dwarf planet” while the spacecraft was en route.
The US $700 million mission has fared well so far, hibernating for long stretches along the way. But much of New Horizons’ success will hinge on a few short hours around its closest approach, when the probe will thread between Pluto and the orbit of its innermost moon, Charon, pass some 12,500 kilometers above Pluto’s surface, and cross into the shadow cast by the dwarf planet.
“It’s getting busy—exhausting as well,” says mission system engineer Chris Hersman of the Johns Hopkins University Applied Physics Laboratory (APL), which built the craft and manages the mission. Key personnel will have cots in their offices to stay close to the mission as it reaches Pluto, he says.
But with a 4.5-hour communications delay in each direction, New Horizons will be on its own during the most crucial hours. Seven days out, Hersman says, the spacecraft will enter a special “encounter” mode designed by APL to prioritize data collection. If there is a problem, instead of switching into safe mode the spacecraft will shift over to redundant systems to keep data gathering going. Once the spacecraft enters this state, Hersman says, its actions will be set, although the team will be able to shift the spacecraft’s data-taking sequence to earlier or later times, as the exact distance between New Horizons and Pluto becomes clearer.
Some of the spacecraft’s most time-sensitive observations occur during a series of occultations: As it passes behind Pluto, its view of the sun and then the Earth are blocked. Two of the spacecraft’s seven science instruments—an ultraviolet imaging spectrograph called Alice and the radio signal–processing Radio Science Experiment (REX)—will observe the limb, or outer edge, of Pluto as the spacecraft enters and leaves its shadow.
The measurements are expected to yield deeper insight into Pluto’s tenuous atmosphere, whose pressure at the surface is less than one-thousandth of Earth’s. Alice will pick apart the sunlight that shines through Pluto’s upper atmosphere, looking for indicators of absorption that could reveal more about its composition.
In Pluto’s Shadow: Instruments on New Horizons will examine Pluto’s atmosphere by looking at the dwarf planet’s outer edge as it shadows the sun and by watching how radio waves are altered as it shadows Earth.Illustration: Erik Vrielink
REX will be the only instrument used for lower altitudes, Hersman says, where the atmosphere, although still just a wisp of molecules, will be too thick to permit ultraviolet radiation. The instrument, which consists of two circuit boards integrated into the New Horizons’ telecommunications system, will look for radio signals transmitted hours before by antennas at two NASA Deep Space Network tracking stations: the Goldstone complex in California and another outside Canberra, Australia.
Pluto’s atmosphere will refract the waves, slowing them down and altering their phase. If all goes well, the REX team will be able to use this information to measure the temperature and pressure all the way to the surface of the dwarf planet. When New Horizons is in the shadow of Pluto, where it will spend about 10 minutes, REX should also be able to pick up the radio waves given off by the heat of the dwarf planet’s surface, allowing scientists to measure surface temperature. As the probe emerges on the other side, signals from Earth will help better pin down Pluto’s size. REX and Alice will perform the same sorts of occultation measurements on Charon about an hour later, as New Horizons passes into the shadow cast by that moon.
All told, REX’s experiment will be over in a little more than 2 hours, says Ivan Linscott of Stanford University, one of the instrument’s principal investigators. “It’s a one-time measurement,” Linscott says. “I think of it like a Thanksgiving dinner. You [do] all this preparation, you invite all the guests, you have the meal, and then suddenly, it seems, it’s over.”
But New Horizons’ job will continue even after it leaves Pluto far behind. The spacecraft can transmit on average only about 2,000 bits per second of data when it is so far away, Hersman says. With the available radio telescope time on Earth, he says, it will take about 1.5 years for the spacecraft to successfully transmit the 60 gigabits of unique data it collected during the encounter.
During this time, New Horizons may be well on its way to a new target. Later this year, the team hopes to fire the spacecraft’s engines in order to direct it to another target in the Kuiper Belt, the ring of icy objects beyond Neptune that Pluto calls home.
This article originally appeared in print as “Close Encounter of the Ninth Kind.”