Early this morning, NASA rolled out the highest-resolution image of Pluto to date, taken 16 hours before today’s historic flyby. Tomorrow, we’ll receive a new set of images at a resolution ten times higher. And Pluto Christmas is just getting started, because it’s going to take NASA 16 full months to download all the data New Horizons collects this week.
The trip to Pluto might have been record-smashingly fast, but sending data back across 3 billion miles of empty space is anything but. To understand why, let’s take a quick peek beneath the hood at how one of the most advanced scientific reconnaissance missions ever built collects, stores and transmits data.
Running on a mere two to 10 watts of power—roughly as much as a nightlight— each of New Horizon’s seven state-of-the-art scientific instruments is currently busy collecting a deluge of data on the surface composition, atmosphere, and geologic features of Pluto, its moon Charon, and its four smaller moons. This data is being sent to one of two onboard, solid-state, 8 gigabyte memory banks. From there, the spacecraft’s main processor—a radiation-proof 12 megahertz Mongoose V—compresses, reformats, sorts and stores the data on a recorder, which NASA likens to a flash memory card for a digital camera. Once stored and formatted, the precious science and telemetry (aka housekeeping) data is ready for transmission to Earth—it’s being sent in compressed format now, and will be sent in a lossless format later on.
New Horizons communicates with the Earth through a series of four dish antenna. For key scientific data, it’s primarily making use of a large (2.1 meter-wide), high-gain antenna. But the high gain beam is only 0.3 degrees wide, means New Horizons must be pointing straight at the Earth in order for us to receive its signal. That’s why the craft’s comm system also includes a wider-beam (4 degree) medium gain disk, which it can use as a backup in cases when pointing might not be as accurate. The craft’s comm system also includes two broad-beam, low-gain antennas, which were used at the mission’s outset for near-Earth communications but are largely vestigial at this point.
Each of these spacecraft’s antenna broadcasts and receives science and telemetry data and commands over the x-band, a set of frequencies that are widely used by satellites, wireless LANs, and cellphones. The x-band comprises the 8-12 GHz frequency range, falling within the microwave band of the electromagnetic spectrum. The small wavelength of microwaves (compared with longer radio waves) mean that it’s easier to transmit narrow beams of them between two points—for instance, two parabolic dishes situated several light hours apart.
Speaking of those light-hours: It takes our spacecraft’s x-band signal roughly 4.5 hours to travel the 3 billion miles from Earth to Pluto. Which is why, even though New Horizons technically ‘phoned home’ shortly after 4:00 pm ET today, we’ll have to wait until around 9 pm ET tonight to receive its message. (NASA will be livestreaming tonight’s check-in, and you can catch it over at space.io9.com.)
Sunday and Monday, NASA received a series of ‘failsafe’ downlinks—preliminary sets of high-priority data from all seven scientific instruments, just in case something went terribly wrong and New Horizons didn’t survive the flyby. (Again, we’ll have a better idea around 9:00 PM tonight.) But today represents the most critical data-collection window for the New Horizon’s mission, which is why we’ve been mostly in the dark. Whenever New Horizons is downlinking data to Earth, it literally has to pivot to face us, meaning it can’t take new photos. And right now, we want our spacecraft to be gathering as much intel as possible on Pluto and its five (known!) moons.
After the next few hours of round-the-clock data collection, New Horizons will be able to focus more of its time on sending its tantalizing new images and numbers home. And NASA is very eager for that process to get started, because the rate at which data can be transmitted to the Earth is agonizingly slow.
Screenshot from Deep Space Network’s Goldstone receiver, one of three ground-based antennae receiving data from New Horizons.
A series of three high-sensitivity ground-based receivers—collectively known as NASA’s Deep Space Network—are currently downlinking New Horizons’ data at a plodding 2,000 bits per second. To understand just how much distance matters, we can compare this rate with the 38 kilobits per second—slightly slower than the transmission speed for modern computer modems—at which we were downlinking data during New Horizons’ Jupiter flyby in January of 2007 (Jupiter was, at the time, a mere 500 million miles away from us). Practically, what this pre-dialup rate means is that it can take hours and hours for us to receive a single image or a small package of information.
But again, once the critical data collection phase is over, the New Horizons team is hopeful that we’ll be able to downlink somewhat faster. The craft is currently configured in what NASA calls ‘three-axis pointing mode’ (aka, Pluto observing mode), but it’ll transition over to ‘spin-stabilized mode’ after the encounter is over. In spin mode, New Horizons will be pointing itself arrow-straight at the Earth, spinning along its axis for increased stability. As a result, NASA reckons we’ll be able to boost downlink speeds to something in the neighborhood of 4,000 bits per second over the next few days. That’ll help New Horizons send us back a sampling of the key scientific data its collecting right now.
4,000 bits per second may be double our current downlink speed, but downloading planetary science data over 3 billion miles is still quite a bit slower than loading your email on a 56K connection. Hence the reason it’s going to take us an estimated 16 months to send home all the data we collect in the next several days.
It’s worth keeping in mind that if anything DOES go wrong—if there’s a hardware malfunction on the spacecraft, or an issue with the downlink—we’ve got ten full years of Plutonium power to keep sending and resending our data. We can keep trying to phone home even as New Horizons presses forward toward the Kuiper Belt for the (planned) second phase of its mission, where NASA hopes to explore new icy, planetary bodies between 2017 and 2019.
If you’re really geeking out about this stuff, you can go on the Deep Space Network’s website and watch as some of the most sophisticated comm systems on planet Earth literally collect the information, bit by bit, that will allow us to build an entire world where all we had until very recently was a speck of light in the sky.
As eager as I am to learn more about Pluto, I’m kinda okay with the insanely slow downlink. After all, it’s going to take the New Horizons science team much more than 16 months to figure out what it all means. And in the meanwhile, the rest of us get to wait, watch, and slowly soak up the latest fascinating insights as they filter back to us, bit by bit, from the cold, dark and inconceivably distant reaches of our solar system.
Top image via NASA