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SYFY WIRE Bad Astronomy

Jupiter, acting all superior

By Phil Plait

This is a cool picture:

What you're seeing is from the NASA/ESA satellite Solar and Heliospheric Observatory, or SOHO. It stares at the Sun all the time, monitoring its activity. This image, from May 3, 2012 is from the LASCO C3, one of the cameras on board. It has a little metal paddle (called an occulter) to block the ferocious light of the Sun; that's the black bar and circle. The white outline is the position of the Sun and its size in the image.

You can see an emerging coronal mass ejection on the left: that's the bulb-shaped thingy. It's actually an incredibly violent expulsion of a billion tons of subatomic particles hurled away at high speed due to the explosive discharge of the Sun's magnetic field... but that's not why I posted this picture.

You can also see streamers coming from the Sun; those are places where particles flow freely into space from the Sun. Basically, the magnetic field of the Sun trails into space in those locations, allowing the wind to escape. But that's not why I'm showing you this picture, either.

Look on the left. See that weird dot with the horizontal line through it? That's Jupiter! The line is not real; it's where the camera got overexposed by the planet (digital detectors -- like your phone camera -- convert photons of light into electrons, and if a source is too bright, the electrons overflow the pixels like water from a bucket. The way the camera works, the electrons flow along the horizontal grid of pixels, creating these lines. This is called "blooming").

Jupiter has been gracing our sky for months, but has been getting further west every night, closing the apparent distance between it and the Sun. It's on the opposite side of the Sun from us, at a distance of almost 900 million kilometers (550 million miles). When two objects get close in the sky, it's called a conjunction. When it's a planet on the far side of the Sun, it's called superior conjunction. Just so's you know.

Anyway, I just think this is neat. Jupiter is roughly one-billionth as bright as the Sun, yet there it is in the picture! And even though SOHO is designed to look at the Sun, Jupiter is so bright it's overexposed. Imagine if the spacecraft moved a bit and the Sun were to peek out from behind the occulter... which can happen. SOHO goes into "safe mode" when that happens, shutting down systems that might get damaged. Every astronomical satellite has contingency plans like that, since it's hard to send a repair service to most of 'em. Generally it's fixable by sending software commands to the spacecraft once the underlying problem has been ascertained.

If you want, SOHO has images online that are updated constantly. Go see what the Sun is doing now! Over the next few days Jupiter will get closer to the Sun, then pass very close to or even behind the disk. LASCO 2, another camera on SOHO that has a smaller field of view but a bit more resolution, should show the moons too when Jupiter moves into its field. I'll post again when that happens. That'll be even neater.

Image credit: NASA/ESA/SOHO

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