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NASA's Juno Spacecraft Sends First Images From Jupiter (sciencedaily.com)
An anonymous reader writes: After its patriotic arrival at Jupiter on July 4th, the Juno spacecraft has sent its first images of the planet back to earth via the JunoCam. The visible-light camera aboard Juno was first turned on roughly six days ago after Juno placed itself into orbit. "This scene from JunoCam indicates it survived its first pass through Jupiter's extreme radiation environment without any degradation and is ready to take on Jupiter," said Scott Bolton, principal investigator from the Southwest Research Institute in San Antonio. "We can't wait to see the first view of Jupiter's poles." The color image, which was obtained on July 10th when the spacecraft was 2.7 million miles from Jupiter, shows atmospheric features on Jupiter, including the famous Great Red Spot, and three of the massive planet's four largest moons -- Io, Europa and Ganymede. "JunoCam will continue to take images as we go around in this first orbit," said Candy Hansen, Juno co-investigator from the Planetary Science Institute, Tucson, Arizona. "The first high-resolution images of the planet will be taken on August 27 when Juno makes its next close pass to Jupiter."
The orbit will eventually, after a couple of planned manoeuvres in the coming months, bring Juno as close at ~4500km from the surface of Jupitor. Currently the orbit brings Juno quite far from the planet, out beyond Callisto. Stage 1 was just to get Juno captured by Jupiter's gravity. We can expect better photos in the coming months when Juno gets closer.
This is Juno's first pass around Jupiter, during which it just barely pushed itself into a highly elliptical orbit. It will continue travelling further from the planet until the end of the month, leaving plenty of time for equipment tests like this. The next engine burn to bring Juno into a closer orbit is scheduled for the 19th of October, two orbits from now.
Why would they perform a braking maneouver resulting in orbit outside Callisto?
Juno is planned to have a very elliptic orbit. It will go close to collect data, then move far away to have time to send the data back to earth. They want to spend as little time close to Jupiter as possible to minimize radiation damage.
Also, the larger apoapsis means they can expend less fuel when breaking. If you just want to be captured, you get as close as possible to the planet and break until you're just gravitationally bound, which will result in an orbit with a very large apoapsis and very small periapsis.
If you want to learn about spacecraft design, I strongly recommend "Space Mission and Analysis Design" by Larson and Wertz. It's sort of the "fundamentals" of all aspects of mission design, and they spend all of section 5 ("Space Mission Geometry") on the topic of the different coordinate systems used in celestial navigation, how to gather the data needed to position in them, how to convert from one to another, and what sort of hardware is needed, how to estimate pointing/positioning errors, etc.
We also have a halon fire extinguisher. Its always nice to have a fire extinguisher that kills people around.
The reason the low periapsis is cheapest is because the closer to the planet you burn the more the planet is helping you slow down. There are two prices you pay for that though. The closer you put your periapsis the higher your orbital speed has to be - so lower periapsis decreases the available burn time to do the capture burn in. Depending on your engines, there is a lower limit below which you are going so fast you'll escape before you can slow down enough to be captured (and of course, go too low and you instead of orbiting you'll crash.
Even then your cheapest burn is not a very low orbit, it's a very eliptical orbit with a very low periapsis and an extremely high apoapsis, go as close as you can and burn as little as possible to be captured. My own jool captures look much the same, periapsis close to the atmosphere with the apoapsis barely inside the SOI.
Then you can do tricks like - when you hit ap lower your pe even further, and now use cheaper (but much slower engines) to lower the ap doing multiple burns on multiple orbits - which lets you lower it with engines that could never have managed the initial capture.
I'm not familiar enough with what equipment juno has to speak about the techniques they have in mind but this initial orbit looks exactly like what a typical first capture in KSP looks like. Where we are now is the post-capture cycle as it goes around that initial high-AP orbit the first time. That orbit has a very long period. Around Jool it's quite possible to get an orbit with a period of 20 or 30 years but this one isn't quite that long - it's around 2 months, and as the AP gets lowered to the final orbit in the upcoming maneuvers will ultimately be around 53 days (according to my memory of a report I read)
Unicode killed the ASCII-art *
The short of it is, for any coordinate system you want to define yourself in, you have to gather a few points of reference - the exact sort of points depending on the coordinate system. There's no single fixed system that all spacecraft have to use, but star trackers (usually combined with a gyroscope) are generally a good one for orientation. The "fixed" stars can give you an absolute frame of reference, while the relative position of the sun and the planets relative to them and the passage of time can give you a very good sense of where you are. Transmissions from Earth are often good for precise determination of distance and velocity, which can be done on either end; however, it's always important that the spacecraft be able to find Earth on its own.
We also have a halon fire extinguisher. Its always nice to have a fire extinguisher that kills people around.
One of the things I learned is that the deeper in a planet's gravity well (ie. closer to the surface) I perform the braking maneouver, the less fuel I need to use, and obviously, the lower the periapsis of the resultant orbit.
Right. This is the Oberth effect.
Why would they perform a braking maneouver resulting in orbit outside Callisto?
They performed the braking burn as close to Jupiter as possible, so the peri jove is right over Jupiter's clouds. But it's a highly elliptical orbit, so the apo jove is very high-- thus, the orbit moves from way in to way way out, beyond the moons.
They want to spend as little time close to Jupiter as possible, because being close to Jupiter is very unhealthy.
Partlyy, but actually, the perijove is in below the radiation belts. The highly elliptical orbit is because they want to traverse the whole of Jupiter's magnetic fields-- but also because the highly elliptical orbit is the one that takes the minimum amount of fuel to get into.
They're basically zipping in for a quick pass then retreating to transmit data and prepare for the next pass.
Yep.
http://www.geoffreylandis.com
There is a liquid core that has a very well defined surface.
Unless it's supercritical, right?
Ezekiel 23:20