JUPITER PLAYS KEY ROLE IN BOMBARDMENT OF EARTH
by Terence Dickinson, SkyNews Editor

The giant planet Jupiter, currently visible as the brightest starlike object in the southwest just after dusk, has played a more important role in the history of the Earth than previously suspected according to recent research by astronomers and geologists. It appears that Jupiter caused a huge asteroid bombardment of the Earth 3.9 billion years ago. Yet, paradoxically, today Jupiter protects the Earth from impacts from other celestial vagabonds, the comets. Jupiter’s immense gravity now acts as an invisible shield by vacuuming up 99 percent of the comets that could strike the Earth.

What led to the new research was Comet Shoemaker-Levy 9 which crashed into Jupiter’s atmosphere in 1994, exploding with the force of a million hydrogen bombs. The explosions left huge black scars in Jupiter’s atmosphere that lasted for months. If a similar comet smashed into the Earth, it would be by far the greatest natural disaster in history.

Astronomers have calculated that a similar impact occurs on Jupiter every few centuries. Its great bulk and enormous mass, equivalent to 318 times the mass of the Earth, makes Jupiter the solar system’s punching bag for errant comets and asteroids. But unlike Earth, Jupiter’s deep atmosphere swallows the intruders, allowing the big planet to effectively shrug off the impacts with no lasting effects. Earth’s relatively thin atmosphere offers no protection against an asteroid hundreds of metres or larger in diameter.

If Jupiter weren’t there to vacuum this stuff up, roughly once every million years the Earth would suffer a comet or asteroid impact similar to the incredibly devastating one that wiped the dinosaurs 65 million years ago.

But according to University of Arizona astronomer Robert Strom, four billion years ago Jupiter wasn’t where it is now. It was farther out, but doing the same thing--vacuuming up solar system debris. There was more debris to collect back then, and the process caused Jupiter to gradually move inward, toward where it is now at five times the Earth’s distance from the sun.

Jupiter’s inward migration displaced the asteroid belt, throwing millions of asteroids throughout the solar system--many of which collided with the Earth and other planets. Astronomers now estimate that during a 50-to-100-million-year interval about 3.9 billion years ago, this cataclysm produced more than 20,000 craters on Earth ranging between 10 kilometers and 1,000 kilometers in diameter. Most of the big craters on the Moon date from this era.

Whatever oceans existed on Earth were vapourized by the impact explosions. However, conditions must have soon stabilized because fossil evidence of the most primitive living organisms can be traced back at least 3.5 billion years.

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JULY NIGHT SKY
by Todd Carlson, Assistant Editor

Observing Highlight: In the early morning night sky of July 20, the waning crescent Moon passes in front of (occults) the Pleiades star cluster. From most of North America, at least one of the major named stars of the Pleiades and several of the fainter stars will be occulted. The event begins at 3:02 a.m. EDT and lasts for 2 hours. Because the night side portion of the 25-day-old Moon will be illuminated by Earthshine (sunlight reflecting off of the Earth), the entire disc of the Moon will be visible during the Pleiades occultation, even though only a thin crescent is lit directly by the Sun.

Jupiter, the largest planet of our solar system, is easily observed shortly after dusk. Appearing as a bright “star” in the southwest, binoculars will reveal up to four of its moons.

From locations outside cities on moonless nights, the Milky Way is visible directly overhead on mid-summer nights. It appears as a hazy patch extending from the overhead region to the southern horizon. Moonlight and light pollution both reduce the ability to view our home galaxy, though moonlight interference is minimal to non-existent the last two weeks of July.

From a dark sky observing location, the naked eye view of the Milky Way is breathtaking. Binoculars enhance the experience even more. At the zenith (overhead), look for the large constellation Cygnus the Swan, also known as the Northern Cross. The hazy ribbon of the Milky Way cuts a swath through the middle of Cygnus, along the long arm of the cross. The density of stars in the Milky Way increases towards the southern horizon, culminating in the constellation Sagittarius, where the centre of our galaxy is located 26,000 light-years away.

Click HERE for an image showing the location of Cygnus and Sagittarius.

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SkyNews Observer's Guide

Taking your own photos of the night sky is easier than you may think. For tips on how to photograph constellations or the northern lights, see the SkyNews astrophotography webpage.
Are you considering buying a telescope but don’t know where to begin?

See the SkyNews Buyer’s Guide for tips, advice and store listings.

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