Juno's flying by Earth today, and images of the Moon are already on the ground!
Posted by Emily Lakdawalla
09-10-2013 11:25 CDT
The Juno Jupiter orbiter is on the way in to Earth for a gravity-assist flyby today, with closest approach at 19:22 UTC. You can learn more about the plans for the Earth flyby at the "Missionjuno" website. Or you can just watch Bill Nye explain it!
What I'm excited about right now is that they have already released some images from the encounter acquired by JunoCam! Here is an image of the Moon that was acquired just five hours ago, and already processed by amateurs!
This is it for Moon imaging for the Juno flyby. The rest of the images will be of Earth -- except for one of Jupiter -- to be taken just before closest approach. It will be fun to watch the Earth flyby raw images page fill up with images as the encounter proceeds.
JunoCam is an unusual camera, a "pushframe" imager. It's also unusual because its main function is to produce images for use in public outreach. I'm stoked that they released these pictures so quickly. Anyway, here's a slightly technical explanation of how the camera works from the Malin Space Science Systems webpage:
Like previous MSSS cameras (e.g., Mars Reconnaissance Orbiter’s Mars Color Imager) Junocam is a "pushframe" imager. The detector has multiple filter strips, each with a different bandpass, bonded directly to its photoactive surface. Each strip extends the entire width of the detector, but only a fraction of its height; Junocam's filter strips are 1600 pixels wide and about 155 rows high. The filter strips are scanned across the target by spacecraft rotation. At the nominal spin rate of 2 rotations per minute, frames are acquired about every 400 milliseconds. Junocam has four filters: three visible (red/green/blue) and a narrowband "methane" filter centered at about 890 nm.
The spacecraft spin rate would cause more than a pixel's worth of image blurring for exposures longer than about 3.2 milliseconds. For the illumination conditions at Jupiter such short exposures would result in unacceptably low SNR, so the camera provides Time-Delayed-Integration (TDI). TDI vertically shifts the image one row each 3.2 milliseconds over the course of the exposure, cancelling the scene motion induced by rotation. Up to about 100 TDI steps can be used for the orbital timing case while still maintaining the needed frame rate for frame-to-frame overlap. For Earth Flyby the light levels are high enough that TDI is not needed except for the methane band and for nightside imaging.
Methane-filter images will also be taken during Earth flyby. The methane filter detects near-infrared light which is strongly absorbed by methane. At Jupiter, the methane band will be used to detect atmospheric regions with the highest concentrations of methane, which can be used to vertically probe atmospheric structure. On the other hand, Earth's atmosphere does not contain enough methane to heavily affect the methane-band images. What is noteworthy is that chlorophyll (the pigment present in plants that gives them their green color) reflects the color of light picked up by the methane band very well; so instead of detecting methane, the methane band will highlight areas of dense vegetation, cloud cover permitting. In addition, since the methane filter allows significantly less light to pass through than the visible light bands, it will be better-suited to take photographs of the Earth (which is very bright compared to Jupiter due to its proximity to the Sun) than the color bands, as the images taken by the color bands may be too saturated to see clearly.
In addition to the methane images, a few images will also be taken in exclusively one visible color (red, green, or blue).
They also put together a really informative FAQ about the camera, which they've kindly allowed me to repost here:
Junocam Earth Flyby FAQs (updated 10/8/2013)
Q: Where are the images?
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.
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