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About 3.8 to 4 billion years ago Earth suffered what scientists call the late Heavy Bombardment: a mysterious rain of asteroids and comets that pummeled most of its surface. The moon was heavily cratered then too.

Art by Dana Berry; Sources: Stephen Mojzsis, University of Colorado/NASA Lunar Science Institute; William Bottke, Southwest Research Institute (SWRI).

Disturbing the Giants

The Late Heavy Bombardment of Earth may have resulted from a dramatic disturbance of planetary orbits. That led Neptune (foreground) and Uranus to disrupt a belt of comets, and Jupiter the asteroid belt. According to the Nice model (named for the French town where it was conceived), Jupiter, Saturn, Uranus, and Neptune had been born close together in the solar nebula, a disk-shaped cloud littered with rocky and icy debris. As the four giants’ strong gravity sucked in or slung away such debris, their own orbits slowly shifted—until they hit a tipping point.

Art by Dana Berry; Sources: Harold Levison and Dan Durda, SWRI.


The old models called orreries, like this 19th-century one in Northern Ireland, depicted a solar system that ran like clockwork. The real one is more random and sensitive to small changes in gravity: Moving a pencil across your desk today, says one astronomer, can shift Jupiter halfway around its orbit a billion years from now.

Art by Dana Berry; Sources: Harold Levison and Dan Durda, SWRI.


Long ago, somewhere between Mars and Jupiter, two asteroids collided. This two-pound meteorite was probably blasted off the larger one, called Vesta. Jupiter’s gravity later slung it to Earth, where it landed on the Antarctic snow.

Photograph by Mark Thiessen


California’s 230-foot-wide Goldstone antenna makes radar images that reveal an asteroid’s size, speed, and distance—and whether it’s headed near Earth. In February a 130-foot rock passed within 17,200 miles of Earth, closer than some satellites.

Photograph by Mark Thiessen

Comet Dust

In a NASA clean room a scientist examines traps containing dust snared by the Stardust probe as it flew by the comet Wild 2. Each speck left a hair-size track (right) as it plunged into the puffy aerogel at more than 13,000 miles an hour.

Photographs by Mark Thiessen

Our Moon Is Born

The birth of the planets 4.5 billion years ago was extremely violent. They grew to full size by absorbing rival planet embryos in a series of titanic collisions—one of which probably gave Earth its moon (below). The moon’s large size, low density, and other features suggest that it emerged from an explosion of debris after a Mars-size proto-planet struck Earth, vaporizing itself and part of Earth’s rocky mantle (left). According to one recent hypothesis, the moon had a little sister at first.

Art by Dana Berry; Source: Robin Canup, SWRI.

Comet Strikes Jupiter

Tracked by telescopes and broadcast on TV, the impact of comet Shoemaker-Levy 9 in 1994 was vivid proof that the solar system is still violent—and that giant Jupiter shields Earth by vacuuming up comets.

Photographs by H. Hammel, MIT and NASA


Vesta never grew into a planet, but it endured eons of impacts and is now, at more than 300 miles across, the third largest asteroid in the belt between Mars and Jupiter. Six percent of the meteorites that fall on Earth are chips off Vesta.

Photograph by NASA/JPL/UCLA/Max Planck Society/German Aerospace Center/Institute of Computer and Communication Network Engineering

The Solar System’s Attic

During the early chaos of the solar system, Jupiter is thought to have flung trillions of comets and perhaps a few planets into deep space. Only lightly bound to the sun, they now form a spherical cloud, called the Oort cloud, around the solar system we know. In this view from the cloud, the sun and its familiar retinue are a small, bright swirl, and an undiscovered planet looms in the foreground. A new telescope being developed in Chile might reveal such planets.

Art by Dana Berry. Sources: Harold Levison and Dan Durda, SWRI

Cortesy : National Geographic