Where is the Data From Juno?

The Juno spacecraft zipped around Jupiter last Saturday, collecting data with its instruments.

It will take days for all the science data collected during the flyby to be downlinked and even more to begin to comprehend what Juno and Jupiter are trying to tell us.” (Scott Bolton quoted)

What’s the deal here?  Why the delay?

Let’s do a little nerding here.  Let’s do a little Internet searching figurin’ to understand ‘where the data is’.

First of all, Juno is orbiting Jupiter, and is now over 600 million km from Earth, about 48 light minutes.   Jupiter’s orbit is five times farther from the Sun than Earth’s, so the giant planet receives 25 times less sunlight than Earth.

From this insolation, the solar arrays generate less than 500 W of power, which must be rationed for all uses, including data transmissions. (This is about as much power as 50 mobile phones, or one kitchen blender.)

Sifting around inexpertly on the Internet, I find that the downlink can do about 18 kbps (which is way less than 1% of your garden variety wifi, no?).

In my brief search, I haven’t found very detailed information about the data transmissions from the instruments. But lets do some back of the envelope calculations.

Suppose a simple instrument grabs 10 readings per second, 64 bits per reading. The data collection period was only a few minutes, lets call it 5000 seconds. 5000 * 10 * 64 = 3.2 million bits. Add some overhead for error checking, headers, etc., call it 3.6 mbits.

At 18 kbs, this would take 2000 seconds to transmit, plus overhead or at least 10 minutes of transmitter/receiver time.   With multiple instruments and multiple readings, there will certainly be several hours worth of data, at least.

Overall, there is 256 MB of memory to hold the data (and everything else), which gives us an idea of how much data can be accumulated waiting for downlink.  Say 180MB, that would be 10000 seconds to transmit, a couple hours to clear memory.

There is other latency moving the data from the instrument’s memory to the transmitter, which takes power and has to be coordinated among the instruments.

The transmissions can only occur when the spacecraft and ground stations are ready (spacecraft visible to ground antenna, spacecraft antenna pointing to Earth, etc.). On the ground, the data is captured and checked, and potentially it may be necessary to try to resend data that is garbled or lost. (All in all, it pretty amazing engineering to get all this to work at all!)

Eventually, various chunks of raw data will be read out from the downlink and stored at processing centers. Overall, this will take days, for sure (as indicated by Bowman).

Sanity check: today we see some PR images from the “JunoCam”, with color images of Jupiter’s poles. There is also imagery from the Infrared imager and hours of radio emissions, which I wasn’t expecting to see.

Image credit: NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

(More sanity check: the IR image is about half a MB in JPG, which suggests that the raw data is a MB or more, so at least 1000 seconds downlink for this image alone. Roughly in line with above calculations.)

So when to we see more results? When the data is analyzed, which I’m sure the science team is working on right now.

I note that there will be another pass in early November, and then every 15 days. So the scientists will be busy with new data coming in even as they try to publish this first batch.

Prediction: There will be papers presenting results at AGU in December and AAS in January.


Space Saturday

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