Juno survives its first Jovian Orbit

On the Fourth of July, NASA/JPL's Juno spacecraft arrived at Jupiter. It's the first probe dedicated to the study of the planet itself: its atmosphere, its internal structure; and of particular interest to me, its magnetosphere.

Our planet is surrounded by a magnetic field, called the magnetosphere, which protects us from most of the nasty charged particles from the sun and interstellar space. This field is generated by the churning of the molten iron and nickel in the earth's core. It can be thought of as a bubble, or cavity, in the solar wind (the continuous stream of charged particles streaming out of the sun in all directions).

One of the characteristics of our magnetosphere is the creation of huge bands of electrically charged particles trapped in orbit high above us. These Van Allen Radiation Belts are a serious health hazard to astronauts who venture beyond low earth orbit (LEO). However, one can safely fly under these radiation belts as long as you like (which is exactly what we've been doing since 1972; no one has gone beyond LEO since Apollo 17.)

This radiation is also a grave danger to electronic circuitry. Satellite and spaceship engineers must ensure that their designs are robust enough for these punishing conditions.

Like us, Jupiter has a magnetosphere -- in spades! On the sun-side, its magnetosphere extends some seven million kilometers (four million miles); on the dark side it extends nearly as far as the orbit of Saturn!

Like Earth, Jupiter has intense radiation belts that can quickly fry sensitive electronics. Previous NASA probes -- Pioneer, Voyager, Galileo, Cassini, and New Horizons -- have done an admirable job of mapping the Jovian magnetosphere, discovering dynamic wonders like a huge flux tube (electrical circuit) connecting the volcanic moon Io with the poles of Jupiter. We've found that the largest moons travel within giant plasma tori, and that there are electromagnetic interactions with Jupiter's ring system. (There's a fine Wikipedia entry under 'Magnetosphere of Jupiter' if you want to know more.)

The above discussion is largely from the perspective of the plane of Jupiter's equator. As we know from our own planet, interesting things happen when all of the magnetic lines of force come together at the magnetic poles. It wasn't until the Ulysses probe flew by in 1992 (in a maneuver intended to give us a look at the sun's poles) that we began to map the magnetosphere at the Jovian poles. Juno is there to explore these polar regions in detail.

To accomplish this, Juno has a very unique flight plan. Instead of flying around the equator (or thereabouts), Juno will be flying in a polar orbit, from one pole to the other. However, the orbits will be highly elliptical. Like a seabird diving for fish, Juno will start far from Jupiter, then dive in quite close to the surface (cloud tops, actually). It will collect its data, then quickly 'get out of Dodge' as its orbit takes it to a safer altitude.

Juno is presently completing its second 53-day 'capture' orbit that takes it to within 4300 km of Jupiter. On October 19, Juno will execute a burn to tighten its orbit, bringing it to within 2000 km of the cloud tops every fourteen days.

While not considered a scientific instrument, Juno has a color camera on board so that we, the taxpayers, can enjoy the view. We are even asked to help choose what JunoCam, as it's called, should be looking at. Amateur astronomers have been busy working with mission planners to get the best use of JunoCam. You can see the results so far at http://www.missionjuno.swri.edu/media-gallery.

Juno's flight path is designed to take it clear of the extremely dangerous Jovian radiation belts. However, a phenomenon called frame-dragging causes the orbit to precess a bit on each pass. Correcting for this effect takes more fuel than Juno can spare. Therefore, each successive orbit takes the probe deeper into danger. How much radiation the craft's electronics can withstand is problematic. A number of instruments, including JunoCam, are expected to fail before the probe is deorbited into the atmosphere in February of 2018.

Meanwhile, we can expect plenty of breathtaking images and valuable data to pour forth from this fascinating mission.

Almanac

The weather has already begun the roller coaster ride between summer and winter. With increasing cloud cover, starry skies are no longer assured; but the chronic forest-fire-smoke should be clearing.

There are no astronomical events in October worth mentioning. However, on October 19th the European and Russian space agencies will attempt to join NASA in successfully landing a probe on Mars. (The Soviet Union made the first soft landing in 1971. However, the achievement was marred when craft died after transmitting for only twenty seconds.) That's also the day when Juno is to execute its critical orbit-changing burn.

Planet-watchers will enjoy seeing Venus slide between Saturn and Antares as the month progresses, taking the place of Mars as it now continues to scoot east across Sagittarius. Note how Mars is dimming and how its color becomes harder to discern as it recedes into the distance. (In fact, we are moving away from Mars on our way toward the other side of the sun).

Meanwhile, starting Oct. 7 Jupiter can be seen rising out the morning twilight as it completes its swing around the far side of the sun.

Sunrise/Sunset: 6:48 a.m./6:38 p.m.

(Oct. 1)