October 27, 2016 – NASA’s New Horizons mission reached a major milestone this week when the last bits of science data from the Pluto flyby – stored on the spacecraft’s digital recorders since July 2015 – arrived safely on Earth.
Having traveled from the New Horizons spacecraft over 3.4 billion miles, or 5.5 billion kilometers (five hours, eight minutes at light speed), the final item – a segment of a Pluto-Charon observation sequence taken by the Ralph/LEISA imager – arrived at mission operations at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, at 5:48 a.m. EDT on October 25. The downlink came via NASA’s Deep Space Network station in Canberra, Australia. It was the last of the 50-plus total gigabits of Pluto system data transmitted to Earth by New Horizons over the past 15 months.
“The Pluto system data that New Horizons collected has amazed us over and over again with the beauty and complexity of Pluto and its system of moons,” said Alan Stern, New Horizons principal investigator from Southwest Research Institute in Boulder, Colorado. “There’s a great deal of work ahead for us to understand the 400-plus scientific observations that have all been sent to Earth. And that’s exactly what we’re going to do -— after all, who knows when the next data from a spacecraft visiting Pluto will be sent?”
Because it had only one shot at its target, New Horizons was designed to gather as much data as it could, as quickly as it could – taking about 100 times more data on close approach to Pluto and its moons than it could have sent home before flying onward. The spacecraft was programmed to send select, high-priority datasets home in the days just before and after close approach, and began returning the vast amount of remaining stored data in September 2015.
“We have our pot of gold,” said Mission Operations Manager Alice Bowman, of APL.
Bowman said the team will conduct a final data-verification review before erasing the two onboard recorders, and clearing space for new data to be taken during the New Horizons Kuiper Belt Extended Mission (KEM) that will include a series of distant Kuiper Belt object observations and a close encounter with a small Kuiper Belt object, 2014 MU69, on January 1, 2019.
The Data Downlink Challenge
Why did it take more than a year for New Horizons to send back all of the data from the Pluto encounter?
New Horizons carries seven scientific instruments, which collect several types of data. As an instrument makes an observation, data is transferred to a solid-state recorder (similar to a flash memory card for a digital camera), where they are compressed (if necessary), reformatted and transmitted to Earth through the spacecraft’s radio telecommunications system.
A major challenge for the New Horizons mission is the relatively low “downlink” rate at which data can be transmitted to Earth from a distance as far away as Pluto. Much of the data had to be downlinked at 1 kilobit (125 bytes) per second. And the signals over such a great distance are so faint that only the Deep Space Network’s large 70-meter antennas can detect the signal. The Deep Space Network has other missions to track as well, which limits the amount of data that can be collected each day. The team produced a lossy compressed browse data set that could be sent down first. The browse dataset was downlinked in 2015 and the complete dataset was downlinked after that.
Take a minute to learn more about the challenge of sending data from over billions of miles.