Junocam Earth Flyby FAQs (updated 10/8/2013)

Q: Where are the images?

A: The official location is here. The raw source images are here. See here for a simulated version of what the images may look like. (Simulation as of 9/28/2013)

Q: When is Juno's Earth Fly-By (EFB)?

A: Wednesday, October 9, 2013. Earth closest approach is at 19:22 UT or a little after noon Pacific Daylight Time (PDT.)

Q: Why is Juno flying past Earth anyway?

A: The fastest practical way to get to Jupiter would be via a direct "Hohmann transfer orbit", which takes slightly under 3 years. However, the rocket used to launch Juno was not powerful enough to do this unless the mass of the spacecraft was significantly decreased. This would mean pruning much of the scientific payload from the spacecraft, which would compromise Juno's scientific goals. Fortunately, there are more efficient, though slower, ways of getting to Jupiter. The method that NASA chose to use brings Juno back to Earth. In 2011, Juno was launched into an elliptical orbit that went only a little past the orbit of Mars. Using a pair of small adjustments about a year later (dubbed the Deep Space Maneuvers), the spacecraft was navigated such that it would return to Earth on October 9, 2013. When the spacecraft returns, its speed and direction will be changed by Earth's gravity, enough to catapult it into an orbit that will reach Jupiter in July 2016.

Q: Why aren't we taking an Earth rotation movie like Galileo?

A: Two reasons. First, Junocam has a much wider field of view and hence a much lower resolution at a given distance than the Galileo camera did (about 66 times lower). At a distance of 10 million km, for example, Earth was about 126 pixels across in the Galileo images, but would only be about 2 pixels across in Junocam's.

Second, Earth won't even be in the field of view of Junocam until less than an hour before closest approach. Because Juno is solar-powered with fixed solar panels, it roughly faces the Sun throughout the Earth flyby, and it won't be repointed for imaging because of all the other spacecraft activities that have to happen. Junocam's field of view is centered 90 degrees from the Sun. So Junocam's imaging is restricted to a few minutes around Moon closest approach and a little over an hour around Earth closest approach.

Q: What are we trying to image?

A: The EFB activity is split up into four categories: Moon imaging, dayside Earth imaging, nightside Earth imaging, and other imaging. The Moon imaging happens first, at about 4 AM PDT on October 9. Since Juno will be fairly far from the Moon, it will be very small in these images (about 30 pixels across.) The Earth segment begins at about 11:51, just before Earth enters the field of view, when we will attempt to image Jupiter, which just happens to be visible at this time. If detected, Jupiter will be a featureless dot in the Junocam images. Next is the dayside Earth imaging, which starts at about noon PDT and lasts about 20 minutes until the spacecraft passes from day to night. In this segment, we expect to see South America and the southern Atlantic Ocean. Now over the nightside, the spacecraft makes its closest approach over southern Africa, and we will take three images trying to see the lights of the major cities of South Africa, the islands of Reunion and Mauritius east of Madagascar, and finally, the nightside limb of the Earth at about 12:30. The last image of the sequence is a dark image looking for radiation effects as Juno passes through the inner Van Allen radiation belt surrounding the Earth; this may give us some useful insight into how the camera will perform in the much stronger radiation belts of Jupiter.

Q: When will the public see the images?

A: We will put the raw images out as soon as we receive them ourselves and reformat them for Web release. When we will receive them is a more complicated question. Near EFB, the spacecraft is sending data at a fairly fast rate, but Junocam gets only about 3% of the total, which comes out to about 5000 bits/second. Each color band of a full-spin image is about 138 megabits, so even with 8:1 compression it will take about an hour for one such image to come down. So we can expect to see the Moon images about four hours after they're taken, or around 8 AM PDT.

Most of the Earth images are compressed less for the best possible image quality, and there are more of them. Also, the data rate goes down quickly after EFB. In the first 24 hours after EFB ends, starting at about 1 PM PDT, we will get a total of about 420 megabits of data, only about half of the volume we expect to take. Sending the rest could take several more days.

Adding to the uncertainty are two more factors. First, our images may compress better than we expect them to, and therefore be smaller and come down faster. Second, these rates assume that all of the other instruments are sending data as fast as they can. If they're not, then we can use their data rate as well, and our images will arrive sooner. The bottom line is that we will just have to wait and see.

Image processing may require some manual labor, which may delay the availability of processed images by a few more hours. But we'll be working on it as fast as we can!

Q: What issues are we worried about for EFB?

A: The most significant is the exposure time of the imaging. We have gone to fairly major lengths to design the camera to perform well in the dim sunlight present at Jupiter. Since Earth is more than 5 times closer to the Sun, the amount of light is over 25 times greater, so we have to take shorter exposures than the camera was really intended for. Also, it's impossible to predict the amount of bright cloud cover that may be present on the day of flyby. For the nightside images, we really aren't sure how well we will see lights, so we are using the longest practical exposure times and will hope for the best.

In addition, this sequence has to be planned months in advance, when the exact times and locations of the targets are not yet certain. So there may be some variation in exactly what we see in each image.

Finally, because the camera was designed for the much colder environment in Jupiter orbit, there is some chance it may overheat during EFB, but we think this is unlikely.

Q: Why can I see color fringing in the merged Junocam images?

A: Proper overlap of the pushframe color strips requires us to know the precise orientation and position of the camera relative to the target, which is determined by processing telemetry from the spacecraft. Before we have this information, there may be some mismatches. We've given you the raw data in case you want to improve upon our "quick-look" processing.

Q: How was the EFB sequence developed?

A: We knew as the mission was being planned that there would be an Earth flyby and the instruments would be operated for it. We did some initial visualizations of what we might be able to see shortly after launch, but we were unsure what constraints we might have: how would the spacecraft be pointed, how much data could we take, how long could we be powered on for, and so on. These constraints started becoming clear to us in early 2013, and beginning in May, we went through three rounds of development and full-up testing on the simulated spacecraft. A few problems were found and fixed, and the image timing, exposures, and data volumes were adjusted. The final sequence was completed at the end of July 2013. Obviously the EFB sequence as a whole is one of the most complex of the mission, with multiple course corrections, instrument operations, possible collision avoidance with objects in Earth orbit, and many other issues; the Junocam sequence is just one tiny piece of the overall puzzle.

Q: Why are the images tall and skinny?

A: Junocam takes a 360-degree mosaic of 82 individual frames as the spacecraft spins. Only a few of these frames contain the target, but we can't tell in advance which ones they will be. The image you see is the entire raw mosaic.