From the May 2013 issue

The end of the line for our planet

A marauding near-Earth asteroid could spell doom for civilization long before the Sun starts to bake us a billion or so years from now. As intellectual exercises, however, both fates are worth pondering.
By | Published: May 30, 2013
Earth fate
A billion years from now, the Sun’s increasing luminosity will have boiled off most of Earth’s water. In this view, water exists only in deep ocean trenches, where thermophilic bacteria cling to life. // Lynette Cook
As Martin Rees explained in “Is this our final century?” the threat humanity faces from asteroids is one of the few that scientists can quantify. Astronomers have found 90 percent or more of the really big objects — those with diameters of at least 0.6 mile (1 kilometer) — that potentially could hit Earth, and have started cataloging the smaller bodies that could cause significant damage. Scientists expect to find 90 percent of the asteroids larger than 500 feet (150 meters) across that travel on Earth-crossing orbits by the end of this decade.

Relatively simple physics can explain what would happen when a space rock strikes our home planet. If astronomers know the object’s trajectory (and thus its speed and impact angle), diameter, and density, they can calculate how much energy it will release when it hits. A website application called “Impact: Earth!” ( — developed by planetary scientist Jay Melosh of Purdue University in West Lafayette, Indiana, and his colleagues — lets you assess the effects of any such collision. Start by plugging in values for the impactor’s diameter, density, speed, and impact angle. Next, tell the application whether the object will hit water or rock. Finally, enter your distance from the impact site. If you’re anything like me, this website will provide hours of entertainment as you explore different impact scenarios.

Of course, the fate of humanity in the distant future rests in the hands of the Sun. With time, our star grows bigger, brighter, and warmer. The changes are imperceptible from one year to the next, but over the course of tens or hundreds of millions of years, they add up significantly. Life likely will have to find a new home within 1 to 2 billion years. The July 2007 issue of Astronomy detailed these solar changes in “Earth’s deadly future.”

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