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Visit to a Genesis Rock

by Tom Jones 

Then God said: "Let there be lights in the dome of the sky, to separate day from night. Let them mark the fixed times, the days and the years, and serve as luminaries in the dome of the sky, to shed light upon the earth. – Genesis, Chapter 1. 

We members of the species homo sapiens like to think of ourselves as long-term residents and masters of this planet, the smartest beings between here and Alpha Centauri. But modern humans have been around only 200,000 years, a brief flicker in the history of our world. Even our primitive ancestors became recognizably human just two million years ago.

Asteroid 21 Lutetia, imaged by the European Space Agency probe Rosetta on July 10, 2010.

To us, two million years seems an unimaginably long time, but it is nothing compared to the age of our planet and solar system. Measurements of the decay products of radioactive elements in terrestrial rocks and in meteorites tell us that Earth is approximately 4.5 billion years old. Imagine Earth’s age represented by the length of a soccer field – 100 meters. On that scale, our species’ stay on Earth would measure just 44 millimeters, or 1.7 inches. We “masters of the universe” are the latest of latecomers.

But despite our recent arrival on the solar system scene, we have acquired a few marketable skills. We build machines capable of cruising the depths of deep space. Our robots in particular excel at long voyages, well beyond the International Space Station, past our footprints on the Moon, and into the interplanetary reaches of the solar system. Last week, one of our planet’s robot explorers took a close look at an ancient “mini-planet,” even older than Earth.

The European Space Agency’s Rosetta probe, bound for a rendezvous with a comet in 2014, sped by and photographed the largest asteroid ever visited by a spacecraft, the mysterious 21 Lutetia.

Discovered in 1852, Lutetia is a relic from the earliest days of the solar system. Like most big asteroids, it resides in the main asteroid belt, between Mars and Jupiter. Lutetia is about 130 km across at its longest dimension; it would span the distance between Baltimore and Philadelphia.

Planetary scientists suspect Lutetia was once part of a larger “planetesimal” that grew out of dust and debris, even as the planets were forming. These celestial building-blocks should have coalesced into a small planet in the asteroid belt, but the rapid growth of nearby Jupiter, with its massive gravity, disrupted the birth process. Left behind were millions of fragments, among them Lutetia. Telescopic observations suggest it may be iron-rich, part of the once-molten core of a larger parent body, shattered by a disastrous collision.

Rosetta’s flyby images, taken while streaking past at 15 kilometers (km) per second, still don’t tell us what Lutetia is made of, but some of the probe’s other instruments may help answer that question. What we do see in the initial images from last week is a small world battered and scored by billions of years of impacts from other asteroids and comets. Over the eons, pitiless grinding by micro-meteoroids and larger collisions has blanketed craters and grooves with a thin dust blanket. Landslides have rumbled down crater walls and then sloshed freely up the opposite slope, carrying far in a surface gravity field less than a hundredth as strong as Earth’s.

Some of that ancient material, knocked loose by impacts, migrates to the inner solar system. Larger chunks of bodies like Lutetia become Near-Earth Objects, or NEOs, potential threats to Earth. Smaller debris burns up in our atmosphere as meteors – shooting stars. A few pieces survive that fiery passage to be gathered up as meteorites. Meteorites – and the asteroids they come from – are even older than Earth, with formation ages of some 4.6 billion years ago. Some meteorites preserve in almost unaltered form nearly all the elements found in our star. They are samples of the raw materials that probably gave rise to Earth. Others harbor interstellar minerals and organic material that predate the birth of the Sun.

Future robot spacecraft, perhaps followed soon by astronaut explorers, will help link meteorites to their parent asteroids. Laboratory analysis of these primitive rocks will tell us under what conditions they formed, helping test theories of how the solar system began.

Asteroids like Lutetia harbor the original atoms and molecules that, delivered to Earth, gave rise to bacteria, dinosaurs, and us. If we want to read the earliest pages of the history of the Sun’s family, we must journey to the asteroids, witnesses to the Beginning.

Astronaut and planetary scientist Tom Jones is co-author of Planetology: Unlocking the Secrets of the Solar System. www.AstronautTomJones.com. 

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(The views expressed in this column are those of the author and do not necessarily reflect the positions of Headline Bistro or the Knights of Columbus.)