Tuesday, July 21, 2015

How New Horizons data packs reach Earth

The Pluto flyby on Tuesday marked the culmination of a nearly 10-year journey to the unexplored dwarf planet. Now, the New Horizons team of scientists is playing the waiting game as the spacecraft sends back the expansive data sets it has collected.

The wait is expected to last about 16 months. The reason: The data has to travel roughly 3 billion miles.

"With the flyby in the rearview mirror, a decade-long journey to Pluto is over — but, the science payoff is only beginning," said Jim Green, NASA's director of Planetary Science, at a news conference Friday. "Data from New Horizons will continue to fuel discovery for years to come."

The spacecraft, using a 2.1-meter antenna pointed at Earth, transmits the data through a radio signal traveling at the speed of light. The 3-billion-mile journey, though, takes about 4½ hours. The signals are picked up on Earth by NASA's Deep Space Network (DSN).

The DSN is a network of facilities in Barstow, Calif.; Madrid; and Canberra, Australia, that house antennas strong enough to pick up on the weak signals from the spacecraft.

"We use the 70-meter antennas … it's almost a football field wide. They can pick out the whisper of the signal we get and then very precisely detect the information coming back from the satellite," said Chris DeBoy, New Horizons' communications lead engineer.

However, DeBoy said, the signals spread out over the 3-billion-mile journey from the spacecraft to Earth, making them especially weak when they reach Earth. The DSN downlinks between 1,000 to 4,000 bits per second, whereas modern cable modems can process 300 million bits per second.

The lengthy data transmission process forced the New Horizons team to prioritize the order of information they would receive, a process that took place months ago, said Hal Weaver, New Horizons' project scientist.

Weaver said members of the team, led by principal investigator Alan Stern, determined what data would give them the best glimpse at how successful the data collection was and an overview of what the whole data set would look like. The spacecraft will be sending back a compressed, "browse" data set before the end of 2015. Weaver also said the earliest data sent back will include data from the seven different instruments on the spacecraft so each of the different instrument teams can begin work.

Other than the stunning images of Pluto that have already been released, the information the spacecraft will send back includes plasma, dust and atmospheric data.

One aspect the New Horizons team does not have to worry about is the Earth's movement. The spacecraft is programmed to know the date and time, and thus, knows Earth's position in the sky.

"We're actually performing pretty close to the theoretical limit of what you can achieve," DeBoy said. "The DSN is an exquisite resource for the United States, and it's quite a very capable spacecraft that we have."


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