How do you talk to a spacecraft that’s three billion miles away circling around Pluto? Very slowly.

It’s a challenge NASA is dealing with right now. By now you’ve heard about New Horizons, the spacecraft launched in 2006 to take close-up shots of what was once the most distant planet in our solar system. So far New Horizons have sent back some of the best images we’ve ever had of Pluto and its moon Charon.

NASA New Horizons has an 83-inch antenna that is sending radio waves at 1,000 to 2,000 bits per second. It takes them 4.5 hours to travel 3 billion miles back to earth.

Pluto is really, really far away though so getting data back and forth between NASA and New Horizons is a pretty slow process. New Horizons is equipped with two radios and an 83-inch (2.1m) antenna that it uses to broadcast radio waves back to earth. It takes about 4 and a half hours for the waves to get from the spacecraft to earth.

The signals are so weak that NASA has to use some of its largest antenna dishes on earth to capture the data. The Deep Space Network (DSN) of antennas are positioned across the world, including in California, Spain and Australia. There are a lot of other projects that need to talk to other deep space instruments, so New Horizons gets a limited window to send data back to Earth.

Data is being sent at a snails pace too. In 2007 when New Horizons passed by Jupiter, it was able to send data at about 38 kilobits per seconds (kbps). Since it’s arrived at Pluto, the data rate has slowed since it is so much further away now. Data is now being transferred at between 1,000 and 2,000 bits per second.

NASA is sending compressed files that do not include metadata to get images back ASAP, but even doing so, it can take about 45 minutes to download a single image from New Horizons.

NASA will be collecting data through next year as it all makes its way back to earth.

More reading for those interested:

New Horizons operations information

Talking to Pluto is hard!

New Horizons phones home