New Horizons is Still Only Halfway Through Its Download from Pluto

There are more pictures and data yet to come, as NASA faces a decision on the spacecraft’s future.

New Horizons pluto-haze.jpg
Haze layers appear above the horizon of Pluto in this image returned by New Horizons.

Scientists on NASA’s New Horizons team were beaming earlier this month as they showed off their latest results from Pluto, including evidence for possible lakes of liquid nitrogen in the planet’s past, and ice volcanoes and active glaciers on its surface.

But eight months after the flyby, they’ve still seen only about half of New Horizons’ data.

In December, when the team wrote the papers that appeared in the March 18 issue of Science (academic publishing is not a quick process), the spacecraft had only sent home about a quarter of the data it gathered during its Pluto flyby last July. Since then, another quarter has “landed,” as principal investigator Alan Stern puts it, and mission scientists are busy analyzing it. That leaves half the flyby data—including thousands of images, chemical spectra, and other information—still stored on two solid-state drives aboard New Horizons, awaiting download.

Among the data that have arrived since December are information on Pluto’s atmosphere, better images of its four small moons, and the results of a radar mapping experiment. As for what’s still to come, project scientist Hal Weaver is especially looking forward to his first look at the infrared spectroscopy data from Pluto’s small moons, which will arrive next month. That information should help the team understand why Pluto’s smallest moons appear so unusually bright in the images returned by New Horizon. Scientists had expected the moons to be dark like cometary nuclei, which reflect less than ten percent of the sunlight that hits them. But Nix and Hydra reflect 50 to 90 percent of the incoming sunlight, making them even brighter than Pluto and the large moon Charon.

“That suggested to us that they're probably coated with very bright water ice, almost snowy in terms of how bright it is, but that's an inference just based upon the reflectivity of the surface,” Weaver says. The infrared spectra should tell them whether they’re right.

Also still to come are information on the composition and structure of Pluto’s atmosphere and data that scientists will use to look for rings around Pluto. The last of the data won’t reach Earth until late October.

Every month, the New Horizons science team meets to discuss what to downlink from the spacecraft next. Time on NASA’s Deep Space Network, which receives transmissions from the spacecraft, is limited, so the team has to plan carefully. “We try to do it in priority order, so that the most important stuff was sent last year, and the least important stuff will be at the very tail end,” says Stern.

New Horizons sent the last of the highest-resolution images of Pluto and Charon earlier this month. One hemisphere of the planet is now almost fully covered (in the downloaded data) at a resolution of 250 to 300 meters per pixel, while a 78-kilometer-wide strip across Sputnik Planum—the planet’s bright “heart”— has been mapped at much higher resolution, around 78 meters per pixel. Scientists are assembling the mosaic of images into a single topographic map.

The spacecraft’s solid-state drives still hold lower-resolution images taken when New Horizons was looking back at Pluto after its flyby, which will help astronomers map the far side of the planet and some of its moons.  About 30 percent of the planet may never be mapped, because parts of its polar regions are almost permanently shrouded in darkness.

At the moment, New Horizons is transmitting data at a poky 2,000 bits per second, and by summer that will drop to 1,500—a fraction of one percent of broadband data rates here on Earth. It takes about an hour for an average image file to arrive, from the first bit to the last. That hour-long stream of data takes about five hours to cross the Solar System, and the relay time increases every day as New Horizons moves outward into the Kuiper Belt.

Although New Horizons was built with a long voyage through the Kuiper Belt in mind, its current mission technically ends once it has sent all the Pluto flyby data back to Earth. To investigate the small world known as 2014 MU69 and beyond will require NASA to approve funds for an extended mission. Although that’s expected to happen, it’s not official yet.

New Horizons will fly past 2014 MU69 no matter what. It’s already on course thanks to four thruster burns completed last November. And the argument for an extended mission is compelling.

“There are no other [outer Solar System] missions on the books from any space agency— not just NASA, nobody else is going out there,” says Weaver. “It takes a long time to get to the outer Solar System. As we found out, just going to Pluto took nine and a half years. So you have a working spacecraft and some fantastic science that it can perform still, and you've already spent most of the money on it. It doesn’t make any sense to me not to actually squeeze as much as we can out of this flyby of MU69.”

The New Horizons team will submit the extended mission proposal to NASA by April 15, and the agency will probably make a decision by the end of June.

If the extended mission is approved, New Horizons will pass even closer to 2014 MU69 than it did to Pluto, and will send back data and images. That will be the spacecraft’s last close encounter, but it will image about two dozen other Kuiper Belt worlds from a distance.

Meanwhile, the New Horizons team still has plenty of work to do. They’re gearing up for a series of instrument calibrations in July, which will help ensure that they’re interpreting the flyby data correctly. “You want to do calibrations because the instruments are now ten years old, and we want to know if anything has changed even slightly,” says Stern.