Alan Stahler: Largest of all planets |

Alan Stahler: Largest of all planets

Alan Stahler
Guest Columnist

Our moon will sidle up close to Jupiter in the sky tonight, making the giant planet easy to spot.

Jupiter orbits half-a-billion miles out from the sun. Half-a-decade after launch, the spacecraft Juno fired its braking rocket and entered orbit around Jupiter last summer.

Jupiter is not like Earth. It's huge — over 300 times more massive. Composed almost entirely of hydrogen and helium, it's got no solid surface. Earth's weather is energized from above, by the sun; Jupiter's, mostly from below, by the planet's own heat.

In one respect, though, the two planets are similar. Both Earth and Jupiter generate magnetic fields. It's our magnetic field that pulls the compass needle toward north.

The electrons in the spark that jumps between hand and doorknob are electrically charged. Charged particles don't normally feel magnetic fields … unless they're in motion.

The charged particles streaming toward Earth from the sun and from outside the solar system move at a good fraction of the speed of light. Like wind piling sand into dunes, our magnetic field gathers these particles — still moving — into belts of radiation surrounding our planet.

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Earth's radiation belts would kill the astronauts and fry the circuitry in spacecraft, but for a gap between the top of the atmosphere and the bottom of the belts. Within that gap, astronauts can live, electronics function.

Jupiter, too, generates a magnetic field, way stronger than Earth's, that amasses radiation belts more intense than Earth's.

As at Earth, there's a gap between Jupiter's cloudtops and the lowest radiation belt. Juno will dive into that gap, to study Jupiter close-up. To get into the gap, though, the spacecraft must cross the radiation belts.

Juno's highly eccentric (non-circular) orbit takes the spacecraft five million miles out from Jupiter.

Orbiting a planet is a lot like falling — falling both down and sideways, so the spacecraft falls around the planet, rather than into it.

Throw a ball into the air and it leaves your hand fast. But as it rises, it slows, until it stops rising and falls back down, falling slowly at first, then faster and faster.

One weekend (the next-to-last) last April, Juno was five million miles out from Jupiter and began to fall — slowly.

Picking up speed, the spacecraft fell toward Jupiter for nearly a month, until, approaching the planet, she was traveling over 100,000 miles an hour.

That speed would minimize the time Juno spent in the radiation belts. Screaming in at over 100,000 miles an hour, she would thread the eye of the needle, fly through the region between cloud-tops and radiation belt.

Dropping down from above in mid-May, Juno flew over the north pole, crossed the equator, reached the south pole, and headed back out, through the radiation belts, to more-distant, less-radioactive regions of space.

In all of its 53 1/2 day orbit, Juno spent just two hours near Jupiter. But those two hours allowed the spacecraft to see things it could not see from a distance.

NASA last week published the results from Juno's February and March flyovers, and they're seriously messing with what we thought we knew.

More on that next week. Don't forget to look for Jupiter by the moon this weekend.

Al Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at

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