After nine and a half years and three billion miles, NASA’s New Horizons spacecraft zipped past Pluto and its five moons on Tuesday morning. NASA shared the last photo sent back to Earth before the flyby Tuesday, shown here. New Horizons will send back at least one higher-resolution image on Wednesday, but the rest of the photos will remain stuck on the spacecraft for months.
Why? Because sending them back takes lots and lots of time.
Imagine someone speaking to you softly from the other side of a large room. You might be able to barely hear him but to make out the words if he spoke very, very slowly.
NASA's New Horizons spacecraft is at the edge of the solar system, nearly three billion miles away. Its radio transmitter puts out a tiny 12 watts of power, about the same as an LED bulb in a table lamp.
Even with a huge 70-meter radio dish on Earth listening to the faint transmissions, that means New Horizons has to talk very, very slowly.
As New Horizons starts sending its trove of flyby data back on Wednesday, the pace will be at a glacial 1,000 bits a second. That is roughly the rate at which computer modems talked to each other in the mid-1980s.
Photographs taken by the spacecraft's black-and-white camera measure 1,024 pixels by 1,024 pixels, or more than a million pixels in total. Each pixel consists of 12 bits, to record one out of 4,096 possible gray levels. That's more than 12 million bits of information in each picture.
To send that all back at 1,000 bits a second would take 12,000 seconds, or 3 hours and 20 minutes. Images can be compressed, but even if squeezed to a tenth of the original size, one photograph would still take 20 minutes to download.
New Horizons is taking a lot of pictures and collecting a slew of other measurements, too — so much that it would take three months to send all of it back if New Horizons could get exclusive use of NASA's Deep Space Network, the system of radio telescopes that communicates with distant space probes.
But the network is also needed to talk to NASA's many other space probes, so 16 months will pass before scientists can get their hands on all of the data from this week's flyby.
This week, the New Horizons team will retrieve a few choice images and snippets of what they think will be the science highlights.
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