I know I’ve promised other parts to this series, but this one will be quick* and I want to get it out there because it feeds into a lot of varied and various conspiracies related to NASA’s New Horizons mission to the Pluto-Charon system, and I’ve even seen many misconceptions on normal science blogs / websites (not to be named): Where’s the data!?
Deep breath people: It’s coming. Slowly.
*I thought it would be quick, but it turned out to be nearly 2000 words. Oops…
The Slowness of Spacecraft Data Transfer
Every space mission – save for one very recent, experimental one – relays data via radio signal. In other words, light. The amount of power that the spacecraft can muster goes into figuring out the data rate it can sustain. Think of it a bit like this: If you have the Bat Signal, but you were using a flashlight, you’d be lucky if someone could just see the flashlight aimed up at the sky. There’s no way they could see details of a bat cut-out. But if you use a really really bright spotlight, you can see it farther, and you can even stick a detailed bat cutout over its front and you can make out that cutout.
Perhaps a bad analogy, but that’s kinda the idea here: If you have a very strong signal, then you can include a lot of detail really quickly. If you have a weak signal, then the data rate is slower. Oh– better analogy: bad wifi reception. You know you have low signal strength when it gets really slow.
Moving on, the New Horizons REX antenna does not have a huge amount of power. New Horizons launched with less plutonium for power than originally intended, and it needs power for running the spacecraft. It has so little power for the antenna that only the 70 meter dishes in NASA’s Deep Space Network (DSN) are big enough to receive the signal at Earth, which is a paltry 3 * 10-19 Watts. (Compare that with a 100 W light bulb.) To me, first off, it’s amazing that we can even receive that faint of a signal.
But once you get over that amazement, the DSN also has to be able to detect changes in that tiny signal. That’s how we get data. Like blinking your flashlight in Morse code, or putting the Bat Signal stencil up. If we have very little signal strength, we can’t change our signal very quickly, or the DSN may not be able to read it. Change more slowly, then they will.
For planning purposes, we were able to send data at 1296 bits per second. I’m old enough (sigh…) to remember dial-up modems in the 1990s. My family’s first modem was the dreaded 14.4 kbps modem which was painfully slow at pulling up AOL’s e-mail. Or Hamster Dance. But even that was over 10 times faster than New Horizons’ data rate. And, let’s convert it to real things, bytes. There are 8 bits to a byte. 1296 bits per second is only 162 bytes per second. I have a thumbdrive attached to my computer that holds 64 GB, or 64 gigabytes. It would take about 4572 hours, at the average New Horizons download rate, to fill that fairly modest thumb drive. That’s 190 days.
Keep in mind that the spacecraft is still taking data. Keep in mind that there are only 3 70m DSN dishes at the correct latitudes to see the spacecraft, ever, from Earth. Keep in mind that there are other missions out there that need the DSN to communicate with Earth. Keep in mind that 1296 is an average planning bit rate, and while the Canberra and Goldstone dishes get more like 2000 bps, Madrid tends to get less due to the elevation of the spacecraft above the horizon.
So, from the get-go, just from considering the data rate (power requirements on the spacecraft, distance to the spacecraft, and timetable of receiving stations on Earth), one should be able to see that it will take a painfully long time to get the data from the spacecraft.
While we could keep up with the data rate and did a large download a month before encounter (which is why data weren’t taken in late May), there’s no way we could get all the data during encounter very soon after it, which is why the craft flew with two 8 GB storage drives, and it filled up 60 Gb during encounter (see what I did there, switching between bit and byte?).
There’s Other Data Besides Images!
And that’s any kind of data. There aren’t just images and “pretty pictures” that many of us want. There is one B&W camera on the craft, but there’s also a color camera, two spectrometers, a dust counter, two plasma instruments, the antenna itself took data, and there’s basic spacecraft housekeeping and telemetry that says things like, “Yes, I really did fire my thrusters at this time when you wanted me to!”
Basic Download Plan
I can discuss this because the basics have been made public. It’s just not “sexy” like pretty pictures so it’s not that easily findable.
Leading up to encounter, data were prioritized as though we were going to lose the spacecraft at any time, so the most important, “Tier 1” science data were downloaded first. And, critical optical navigation images.
After encounter, the same thing happened, where compression algorithms were used on the data on-board the spacecraft and that lossy-compressed data were sent back to Earth to fulfill as many Tier 1 science goals as possible. That’s how – and why – in the last week we’ve already revolutionized what we know about Pluto. Those first high-res (0.4 km/px) images of the surface were planned out based on Hubble Space Telescope maps of the surface and the spacecraft timing and trajectory to get images that cover different brightness and color patches. (Which takes care of another, minor conspiracy that I’ve seen that claims we “knew” where to point the cameras because the Secret Space Program had leaked us information about what would be interesting.)
But now that we’re more than a week from closest approach, thoughts are turning to what to do next. Originally, a “browse” data set of all the lossy data (only the imagers and spectrometers store lossy-compressed in addition to lossless) were going to be returned first, along with the lossless data from other instruments. That would at least let us at least understand the surface at a lossy JPG quality and for the plasma folks to do their science.
But now people are discussing scrapping that and bringing down the lossless data instead, albeit many times slower because of the larger file sizes.
But, believe it or not, planning of what’s downloaded when is made no more than a few weeks out (except for the closest approach weeks). Right now, we’re working on the late August / September load of commands and deciding what data to bring down in what order.
Each of the four science theme teams (geology geophysics & imaging (GGI), atmospheres, composition (COMP), and particles & plasma (P&P)) puts together a list of their top priorities based on what we’ve seen so far. The Pluto Encounter Planning (PEP) team then sits down and looks at how much they can bring down in what time and puts things in order. The sequencers then take that and try to make it happen in the test computers. Then we iterate. Then it gets reviewed. Extensively. Only then does it get uploaded to the spacecraft to execute.
But besides that priority list, it’s the Principle Investigator who decides how much data each science team gets. For example, while I’m on PEP (it’s what I was initially hired to do), I’ve been adopted by GGI. Wearing my GGI hat, I want images from the LORRI instrument. All the time, and only LORRI. I don’t care what the plasma instrument PEPSSI recorded. But by the same token, the P&P folks don’t care anything about images, they want to know what their instruments recorded as the craft passed through the Pluto system to see how the solar wind interacted with escaping particles from Pluto – or even if it did. (Which it did, as was released in a press conference last Friday.)
So Alan Stern has to make the decision of how to be “fair” to so many competing interests within the large – and broad – science team. So while COMP may want to have 5 DSN playback tracks in a row to bring back just one of their very large spectra data cubes, Alan has to make sure that GGI gets their images and P&P gets their data, too.
The decision was made several months ago that after this initial batch of data – what we saw last week, what we see this week – that all of the “low speed” data will come down in August. That’s housekeeping & telemetry, that’s things like how many dark pixels are in any given LORRI image, it’s the two plasma instruments and data recorded by the antenna and dust counter, and that’s about it. After that, we get back to the imagers and spectrometers, per the balance discussed above.
And since it’s not sequenced, and it’s not public, I can’t tell you any more than that.
So we are, unfortunately, not going to see any new images for practically a month, beyond the two navigation images that should come down tomorrow and Friday.
Due to the nature of this blog, obviously this is going to fuel conspiracies: NASA’s hiding the data, NASA’s manipulating the data, NASA’s [whatevering] the data, etc.
It’s just not true.
I have known for years that these conspiracies about NASA somehow intercepting the data and manipulating it before even us naïve scientists can get our hands on it would be very difficult, but being on this mission has made me realize that it’s even more difficult to somehow support that conspiracy than I had thought.
Literally, as the data are received by the DSN – before it’s even completely downloaded – it’s on our processing servers and in the processing high-cadence pipeline. On Monday morning when we were supposed to get four new images, we were literally sitting in the GGI room hitting the refresh button and marveling over each new line of pixels that we were getting back in practically real-time. To use a religious analogy, it was every Christmas morning rolled into a one-hour marathon of hitting the refresh button.
And we were all there watching — over 20 of us. And other science team members kept coming in to look.
The idea of secretly having one or two people intercepting the data, “airbrushing” things in or out of it, and only then giving it from On High to the scientists just shows how out of touch from reality conspiracists are. (By the way, I use the term “airbrushing” here because that’s how many conspiracists still talk. Obviously, no one is physically airbrushing things anymore — and I doubt anyone younger than 30 even knows what a real airbrush is.)
To sustain the conspiracy, I can only see one of two choices: (1) Either all of us scientists are in on it, in which case it becomes ridiculously large and unsustainable and scientists suck at keeping secrets about exciting new things, or (2) somehow there’s super secret advanced tech that intercepts the spacecraft signal and at the speed of light “airbrushes” things out and retransmits it to the DSN to get into our processing pipeline. Because we know when stuff is supposed to appear on Earth. Because we write the sequence that does it.
Not that I expect this to convince any conspiracy theorist of their folly. The lack of image data for the next month, and the lossy JPG data we have now all contribute to the little anomalies that don’t immediately make sense, and the average conspiracist can easily spin into something that it’s not.