Planet Jupiter. Credit: NASA, ESA, A. Simon-Miller (GSFC),
I. de Pater, M. Wong (UC Berkeley)

Opposition is the best time to see a planet like Jupiter: it's at its largest visual size, it's nowhere near the glare of the Sun, and it's in the sky all night long. Score on three counts—how many things in life work out so that the best of all worlds occur at the same time?

The Opposition of Jupiter occurs about once each year, the timing governed mostly by Earth's relatively speedier orbit around the Sun. Earth takes one year to revolve, while Jupiter takes closer to 12 years. So, opposition actually happens every year plus one month; in the time Earth has taken to come back around to the previous point of opposition, Jupiter has moved along one twelfth of its orbit, so it takes Earth an extra month to "catch up."

This year, the date of the Opposition of Jupiter is September 21, at which time the planet will be only 3.95 Astronomical Units away—about 591 million kilometers (369 million miles). Sounds faraway—and it is—but Jupiter's size helps make up for this. With a diameter of 142,984 kilometers (over 11 times that of Earth), even at this distance there's a lot to see, even through a small telescope.

I remember observing Jupiter through my first telescope—a 4-inch reflector. Jupiter's four "Galilean" moons are easy to see even through a telescope that small, but I could also make out, just barely, the parallel streaks across Jupiter's face that are formed by its cloud belts…ah, my young eyes then! At Chabot, we'll be observing Jupiter in the weeks ahead—come up and take a look through some very large telescopes, Friday and Saturday nights!

Right now, Jupiter rises after 9:00 PM, but by mid-September it will rise closer to 7:30 PM. We should be picking it up toward the end of September, and then onward into the Autumn.

Current Event from Jupiter: Another "big whack" observed, on August 20th 2010. Any planet is potentially subject to the occasional impact by a large object—an asteroid or a comet. Even Earth. But, it appears, Jupiter has been hit by comets and/or asteroids rather frequently of late (three times in the last 13 months), leading some to suggest that this kind of thing may go on more often that at once thought.

So as the recent clustering of the planets Venus, Mars, and Saturn ride off into the sunset to our west, we can now turn the other way and welcome the return of Jupiter, and maybe give our thanks to this huge debris-vacuuming gravity trap of our solar system that may have taken some cosmic bullets for us….

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Three views of Jupiter before, during, and after the disappearing act by its four large moons. Credit, Conrad Jung, Chabot Space & Science Center

What am I talking about? About a week ago a relatively rare alignment of Jupiter and its four Galilean moons—Io, Europa, Ganymede, and Callisto—made for a brief time in which the moons disappeared, hidden behind and in front of their massive parent planet. For that brief time, Earth, Jupiter, and all four Galileans coincided on a nearly perfect line.

The event took place late in the evening on September 2nd, a little after 10:00 PM. Ganymede (the Solar System's largest moon) and Europa (the "snowball" with the probable deep liquid water oceans under its icy crust) crossed in front of Jupiter's disk, and the other pair, Io (the volcano moon) and Callisto passed behind it.

It's not uncommon for one of these moons to be out of view for a time when you aim a telescope at Jupiter. Even Galileo, on his first telescopic look at Jupiter, saw only three of them.

The disappearance of two or three of them at once is more rare, however, and a vanishing act by all four only happens a few times in a lifetime—every century, there are about 20 such alignments. The last such event prior to last week's was back in the 1980's; the next one won't happen until 2019. This event was not only observed on September 2nd by Chabot Space & Science Center astronomer Conrad Jung, but also in 1913 by then Chabot Observatory director Charles Burckhalter.

When Galileo took his newly made telescope and became the first person in history to look at Jupiter through the new invention, he saw three star-like points of light positioned around Jupiter, roughly on a common line that passed through the planet. At first he thought they might be stars, but on subsequent nights he observed that not only did these "stars" follow Jupiter's own movement through space, they changed position relative to each other. This led to Galileo's hypothesis that these were satellites in orbit around Jupiter.

The rest is history (oh, and lifelong house arrest for Galileo for suggesting that there was something in the Universe that didn't revolve directly around the Earth…).

I'm sure that if Galileo had first looked at Jupiter on one of these rare nights and saw no moons, he would certainly have discovered them the next time he looked at Jupiter—so maybe it wouldn't have changed the unfolding of historical events much. But I wonder which would have been more surprising to him: seeing the moons on the first look, or observing them to appear out of nowhere after the initial observation of a solitary Jupiter….

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Hot spot created by impact on Jupiter, taken by NASA's Infrared Telescope Facility in Hawaii. Picture credit, NASA.

Evidently, the aftermath of some kind of collision on Jupiter was spotted by an amateur astronomer in Australia that Monday morning. He spotted a dark marking near the planet's South Pole, and alerted NASA. NASA in turn turned its large infrared telescope in Hawaii onto the scene of the crash.

There glowed the thermal footprint of the likely impact, the affected area roughly the size of the Earth. Had this impact taken place on Earth instead, the results would have been catastrophic. Fortunately this was Jupiter, half a billion miles away and large enough to absorb the impact without lasting effects. (And, owing to the fact that Jupiter is a gaseous planet with no solid surface, it would quickly heal from the trauma, not unlike that liquid-metal Terminator from the second movie of the same name.)

A significant event? Yes, in fact. But that's not all…

Rewind 15 years to July 20th, 1994, the middle of the week during which twenty-something fragments of the broken comet Shoemaker-Levy 9 were in fact colliding with Jupiter… An amazing coincidence? Yes; the two events likely have nothing to do with each other. So, then, a common event, if we're seeing two of them in the span of only 15 years? Well… not really.

When the string of fragments of Shoemaker-Levy 9 hailed down on Jupiter, it was the first time in history that humans had observed actual impacts on a Solar System body (other than perhaps the Sun–but as it turns out comets hitting that huge target are not uncommon). The Shoemaker-Levy 9 impacts, and the one on July 20th this year, left highly visible marks that lasted for days. The amateur astronomer who discovered the recent scar did so with a relatively small 14.5" backyard telescope! So, if this sort of impact were a common event, even if the impacting comets or asteroids were never seen, the gashes they leave in Jupiter's atmosphere ought to be spotted from time to time.

Impacts—on Jupiter, Earth, and all the bodies of the Solar System—do occur, and the smaller the impacting object, the more frequently they happen. For a planet like Earth, on average a chunk of rock a few meters across enters our atmosphere about once a year, and often burns up completely or explodes before hitting the ground. A 50 meter object, again on average, is likely to strike Earth once in a century. A one-kilometer object impact averages every few hundred thousand years, and a multi-kilometer sized asteroid or comet similar to the one that wiped out the dinosaurs and which would cause global catastrophe—well, the last one of that size struck ground 65 million years ago.

As for Jupiter, being a larger target than Earth, having a much stronger gravitational pull, and being close to the asteroid belt—well, Jupiter's impact statistics should probably involve higher frequencies than Earth.
In fact, impacts like the one on July 20th are happy events for us; every time Jupiter is hit by a large object, that's one less object in the Solar System that could potentially hit the Earth in the future. So, on July 20th, Jupiter took another bullet for us.

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