Construction debris strikes Earth |

Construction debris strikes Earth


In the summer of 2011, an Atlas V rocket lifted off, bearing a spacecraft — Juno — instrumented to study the atmosphere of Jupiter (and named for the thunder god’s wife).

Jupiter orbits hundreds of millions of miles farther from the sun than Earth. From the moment Juno left Earth, the sun has been pulling her back.

The Atlas V launch vehicle could not give Juno enough energy to fight the sun’s pull all the way to Jupiter. Already, Juno is falling back toward the sun.

Knowing the Juno had insufficient energy to reach Jupiter, Juno’s navigators had targeted the spacecraft for a different planet: Earth.

Next Oct. 9, Juno will cross Earth’s orbit, missing Earth itself by barely 300 miles. Juno will pass behind Earth; she will feel Earth tugging on her, pulling her forward, pulling her faster. Earth’s gravitational tug will give Juno enough extra energy — precisely — to reach Jupiter.

Interplanetary navigators have been goosing spacecraft with “gravitational slingshots” for dozens of years. Nature has been using the technique, flinging objects hither and yon, for billions.

Orion, the Hunter, is high in the sky in winter. Hanging from his straight-line belt of three stars is a three-star dagger. Binoculars reveal the middle star of the dagger to be a nebula – a cloud of gas and dust. A small telescope reveals the cloud to be a stellar nursery: Within that cloud can be seen baby stars.

Our solar system, four-and-a-half billion years ago, was born in just such a cloud of gas and dust. Gas and dust collapsed to the center of the cloud to form the sun. Out from the sun, gas and dust bonded to form larger grains of dust … sand … gravel … cobbles the size of your fist … boulders .. mountains … planetesimals … planets.

Jupiter’s massive gravitational pull, however, disrupted the planet-growing process between Mars and itself. Nothing in that region could grow beyond the size of small planetesimals: asteroids. Most of the asteroids were slung either into the sun or out of the solar system. The vast majority of surviving asteroids remain in orbit between Mars and Jupiter: the asteroid belt.

A few asteroids, however, have orbits that carry them, now and again, close to Earth.

Since its discovery last year, astronomers have been watching asteroid 2012 DA14 circle the sun, watching it as it flew past us in a very close flyby, late last month

Unknown to astronomers, another asteroid was also heading our way, coming so close that Earth’s gravity pulled it into our atmosphere, where it exploded over Chelyabinsk, Russia.

Was the almost simultaneous appearance of two asteroids a coincidence, or were they related?

Suppose you’re being pelted by snowballs – who is throwing them? As the snowballs come in, you calculate their trajectory backwards.

Tracing the Chelyabinsk meteor backwards takes us to a very different part of the solar system than does the trajectory of 2012 DA14. Whereas 2012 DA14 has settled into an orbit that never takes it far from the orbit of Earth, Chelyabinsk came here from the asteroid belt.

If we have clear skies next Saturday night, local astronomers will set up their scopes at the junction of SR 49 and the old Downieville Highway, just outside Nevada City, at 7 p.m. The star nursery in Orion and planet Jupiter (and several of its moons) will be prime targets. It’s free, and all ages are welcome.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He teaches classes to students of all ages, and may be reached at

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