Near-Earth Objects

Finding Them Before They Find Us

Donald K. Yeomans

• Publisher: Princeton University Press
• Year: 2016
• ISBN:
  9781400883554
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

An insider's look at the science of near-Earth comets and asteroids Of all the natural disasters that could befall us, only an Earth impact by a large comet or asteroid has the potential to end civilization in a single blow. Yet these near-Earth objects also offer tantalizing clues to our solar system's origins, and someday could even serve as stepping-stones for space exploration. In this book, Donald Yeomans introduces readers to the science of near-Earth objects—its history, applications, and ongoing quest to find near-Earth objects before they find us.In its course around the sun, the Earth passes through a veritable shooting gallery of millions of nearby comets and asteroids. One such asteroid is thought to have plunged into our planet sixty-five million years ago, triggering a global catastrophe that killed off the dinosaurs. Yeomans provides an up-to-date and accessible guide for understanding the threats posed by near-Earth objects, and also explains how early collisions with them delivered the ingredients that made life on Earth possible. He shows how later impacts spurred evolution, allowing only the most adaptable species to thrive—in fact, we humans may owe our very existence to objects that struck our planet.Yeomans takes readers behind the scenes of today's efforts to find, track, and study near-Earth objects. He shows how the same comets and asteroids most likely to collide with us could also be mined for precious natural resources like water and oxygen, and used as watering holes and fueling stations for expeditions to Mars and the outermost reaches of our solar system.

CHAPTER 1

Earth’s Closest Neighbors

The dinosaurs became extinct because they didn’t have a space program.

—Larry Niven

Michelle Knapp and Her 1980 Chevrolet Malibu

Let me introduce Michelle Knapp of Peekskill, New York, and her 1980 Chevy Malibu sedan. On a rainy Friday night, October 9, 1992, just before 8:00 PM, Michelle, an eighteen-year-old high school senior, heard a loud crash in her driveway and raced outside to discover the rear end of her automobile had been completely destroyed by a football-sized rock. The twenty-seven-pound projectile had punched completely through the trunk, just missing the gas tank.

As unlikely as it sounds, a fragment of a near-Earth asteroid that had collided with Earth destroyed Michelle’s car. The fiery trail of the initial, Volkswagen-sized, near-Earth asteroid was first seen over West Virginia appearing with a greenish hue and brighter than the full Moon. Due to the forces of the atmospheric resistance, the asteroid fragmented into more than seventy pieces while traveling northeast for more than forty seconds over Pennsylvania and then New York. The only known surviving fragment came to a full stop underneath Michelle’s Chevy Malibu. Countless people, many of whom were watching high school football games that Friday evening, observed the fiery train of fragments in the Pennsylvania and New York skies. Although Michelle’s automobile insurance company refused to pay for her car’s damages, claiming that it was an act of God, she got the last laugh by selling the so-called Peekskill meteorite and the twelve-year-old Chevy to a consortium of three meteorite collectors for $69,000.

FIGURE 1.1. Michelle Knapp and her 1980 Chevy Malibu. After a small asteroid entered the Earth’s atmosphere on October 9, 1992, it caused a spectacular fireball event traveling northeast over parts of West Virginia, Pennsylvania, and New York before coming to rest underneath Michelle’s 1980 Chevrolet Malibu sedan.
Source: Courtesy of John E. Bortle, W. R. Brooks Observatory.

On a daily basis, at least one hundred tons of interplanetary material rain down upon the Earth’s atmosphere, but most of it is in the form of very small dust particles or very small stones. Much of this dust and sand grain–sized material, the debris from active comets, can be seen on almost any clear, dark night as meteors or shooting stars. Larger basketball-sized rocks rain down upon the Earth daily and while they can cause impressive fireball events, our atmosphere prevents almost all of them from reaching the ground intact. Volkswagen-sized asteroids, like the one that fragmented and caused the Peekskill meteorite, strike the Earth’s atmosphere every six months or so on average. At this point, you may be incredulous because you may not have ever seen a fireball and probably not a major fireball like the Peekskill event. But the vast majority of the Earth’s surface is either ocean or unpopulated and besides, how often do you monitor the skies all night? Department of Defense satellites do, however, continuously monitor the skies and detect fireballs, as they look downward, twenty-four hours each day, to provide alerts of possible missile launch events and nuclear explosions.

FIGURE 1.2. Because of the atmospheric pressure, the small asteroid that created the Peekskill fireball event fragmented into more than seventy pieces in the Earth’s atmosphere. This image was taken from Mansion Park football stadium in Altoona, Pennsylvania. Only a single meteorite was located on the ground.
Source: S. Eichmiller of the Altoona Mirror.

Mr. S. B. Semenov Gets Blown off His Porch

Allow me to introduce Mr. S. B. Semenov, who was an eyewitness to a larger Earth-impacting near-Earth asteroid on June 30, 1908, in a remote region of Russian Siberia called Tunguska.

Mr. Semenov, a farmer, was sitting at a trading post when he noticed what appeared to be a fire high and wide over the local forest. A loud and strong shock ensued, blowing him a few meters off the trading post porch. He noted that the heat from the blast felt like his shirt was on fire, even though he was located about sixty-five kilometers south of ground zero. Although a wide variety of suggestions have been made to explain the Tunguska event, including the absurd notions that the blast was due to a UFO crash or an overzealous signal greeting by aliens, by far the most likely cause of the Tunguska blast was an atmospheric impact by a near-Earth asteroid. Most likely, a thirty-meter-sized asteroid entered the Earth’s atmosphere and reached an altitude of about eight kilometers before the atmospheric pressure in front of the stony object “pancaked” the rock, causing it to explode above the forest floor. A tremendous blast wave then continued to the surface, and an area that spanned some two thousand square kilometers of forest, involving millions of trees, was leveled. But since the stony asteroid itself disintegrated in the air blast explosion and did not reach the ground, no crater was evident on the forest floor and no sizable meteorites were left near ground zero. Current estimates place the energy of the event at about four million tons (four megatons) of TNT high explosives.¹ Considering there are more than a million of these asteroids, thirty meters and larger, in the Earth’s neighborhood, one would expect an asteroid of this size or larger to strike the Earth every few hundred years. Thirty meters, or Tunguska-sized, is about the minimum diameter of an Earth impactor that could cause significant ground damage. In general, smaller stony objects would not be expected to survive passage through the Earth’s atmosphere.

FIGURE 1.3. Mr. S. B. Semenov, eyewitness to the Tunguska blast of June 30, 1908. Although he was sixty-five kilometers distant from the blast site, he was thrown off a porch and related that his shirt felt like it was on fire.
Source: Courtesy of E. L. Krinov.

The Dinosaurs Check Out Early

At the upper end of the near-Earth object sizes are about one thousand asteroids larger than one kilometer in diameter, and an Earth impact by one of these asteroids would be capable of causing global devastation. Fortunately asteroids of this size would not be expected to strike Earth but every seven hundred thousand years on average, and NASA’s ongoing search programs have already found more than 90 percent of this population. None of them represents a credible threat during the next century. The very largest near-Earth objects are as large as ten kilometers in diameter. Sixty-five million years ago, one of them killed much of the land and sea flora and fauna along with almost all of the large vertebrates on land and at sea. Most species were exterminated. Crater evidence for this major extinction event has been found near Chicxulub on the edge of the Mexican Yucatán peninsula. A ten-kilometer impactor could cause a so-called extinction event because it would subject the Earth to global firestorms, severe acid rain, and the darkening of the skies with soot and impact-created debris. The resultant loss of photosynthesis would cause plants to die along with the animals and marine life that depend upon these plants for food. After more than a 160-million-year run, the large land dinosaurs could not survive this impact event because of the complete disruption of their food chain. An Earth impact by a ten-kilometer-sized near-Earth object would create an impact of unimaginable energy—equivalent to some fifty million megatons of TNT. To put that in perspective, that amount of energy would be equivalent to a Hiroshima-type nuclear blast every second for about 120 years! This underscores the statement that while large near-Earth object impacts are very rare, they are extremely high-consequence events capable of ending civilization as we know it.²

One of NASA’s goals is to discover and track the vast majority of the relatively large near-Earth asteroids and comets that would be capable of threatening Earth or causing a local or regional disaster. If we find them early enough, we now have the technology to deal with them. For example, a massive spacecraft could be directed to purposely run into an Earth-threatening asteroid of modest size to slow it down and alter its trajectory just enough so that it would no longer threaten Earth. As it turns out, the dinosaurs became extinct because they didn’t have a space program.

Just What Are Comets, Asteroids, Meteoroids, Meteors, and Meteorites?

In interplanetary space, a large rocky body in orbit about the Sun is referred to as an asteroid or sometimes a minor planet. They are inactive and, unless struck by another nearby asteroid, they do not shed material like their cousins the comets. Comets differ from most asteroids in that they are icy dirtballs. When they approach the Sun, their ices (mostly water ice) are warmed by the Sun so they begin to vaporize and release the dust particles that were once embedded in their ices. Inactive comets that have exhausted their supply of ices near the surface or have their ices covered and insulated by more rocky material are no longer termed comets but asteroids. The only real difference between a comet and an asteroid is that comets, when near the Sun, are actively losing their ices and dust, often causing a highly visible trail of dust and gas, and asteroids are not. Even objects that are icy, like some asteroids and other bodies in the outer solar system, are just classified as asteroids since they do not get close enough to the Sun for their ices to vaporize. They are not active and hence they are not comets. Since the physical makeup of an active icy body (comet) near the Sun can be identical to an inactive icy body (asteroid) that is farther from the Sun, the line between comets and asteroids cannot be clearly drawn.

Small collision fragments of inactive asteroids or the dusty debris from active comets in orbit about the Sun are called meteoroids if their sizes are between ten microns, the width of a cotton fiber, and one meter in diameter. Once a tiny meteoroid enters the Earth’s atmosphere and vaporizes due to atmospheric friction, it emits light that causes a meteor or “shooting star.” Almost all meteors are due to cometary sand- or pebble-sized particles while small asteroids or large meteoroids can cause much brighter fireball events in the Earth’s atmosphere. Fireball events can range in brightness from just brighter than the brightest planets to events that briefly rival the Sun. If a fragment of the impacting body survives its passage through the Earth’s atmosphere and lands upon the Earth’s surface, it is then called a meteorite.

The Near-Earth and Potentially Hazardous Objects

Astronomers refer to the approximate average distance between the Sun and Earth as an astronomical unit (AU), which is a distance of about 150 million kilometers or 93 million miles. Near-Earth objects are simply defined as comets and asteroids that approach the Sun to within 1.3 AU so therefore they can also approach the Earth’s orbit to within 0.3 AU if their orbits are close to the same plane as that of the Earth. The so-called potentially hazardous objects are a subset of the near-Earth objects that approach the Earth’s orbit to within 0.05 AU, which is roughly the distance that a near-Earth object’s trajectory can be gravitationally altered by a single planetary encounter. These objects would have to be about 30 meters in size or larger to cause significant damage at the Earth’s surface.

Although sizable near-Earth asteroids outnumber near-Earth comets by more than one hundred to one, the solid nuclei of comets may provide the very largest impactors like the one that took out the dinosaurs. Cometary debris is also the source of most tiny meteoroid particles and meteors. Many comets generate meteoroid streams when their icy cometary nuclei pass near the Sun, begin to vaporize, and release the dust, sand-sized particles, and fragile clumps that were once embedded in the cometary ices. These meteoroid particles then follow in the wake of the parent comet. When the Earth, in its orbit about the Sun, runs into this dusty debris from active comets, meteor showers can be observed. Sometimes hundreds and even thousands of meteors, or shooting stars, can be seen within an hour when the Earth collides with a particularly dense band of meteoroids. The annual August Perseid showers occur when the Earth runs into small particles from comet Swift-Tuttle while...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Illustrations
• Preface to the Paperback Edition
• Preface
• Acknowledgments
• Chapter 1: Earth’s Closest Neighbors
• Chapter 2: The Solar System’s Origin: The Classical View
• Chapter 3: How and Where Do Near-Earth Objects Form?
• Chapter 4: Near-Earth Objects as the Enablers and Destroyers of Life
• Chapter 5: Discovering and Tracking Near-Earth Objects
• Chapter 6: The Nature of Asteroids and Comets
• Chapter 7: Nature’s Natural Resources and the Human Exploration of Our Solar System
• Chapter 8: Near-Earth Objects as Threats to Earth
• Chapter 9: Predicting the Likelihood of an Earth Impact
• Chapter 10: Deflecting an Earth-Threatening Near-Earth Object
• References
• Index of Asteroid and Cometary Objects
• General Index