ASTR 1210 (O'Connell) Study Guide


Killer Asteroid

Impact of a "planet buster" asteroid
(Don Davis)

"We can never anticipate the unseen good or evil that may come
upon us suddenly out of space."
--- H. G. Wells

A. History

With the arrival of a scientific assessment of sky phenomena, the supernatural cosmic cataclysms imagined in earlier times were cast off as the superstitions that they were. The cosmos became regarded as mostly benign, apart from a few skeptics like H. G. Wells. It was hard to conceive of Earth being threatened by any extraterrestrial event short of a major change in the Sun. But recognition of a huge, unforeseen, genuine cosmic threat from another quarter emerged in the mid-20th century.

Until that time, craters on the Moon and Earth were usually interpreted as having a volcanic origin even if they were not located in volcanically active regions. Recall that until the first spacecraft reconnaissance in the mid-1960s's, there was no direct evidence for craters on other planets (excepting the Moon).

The implications were serious because, although most impacts had occurred in the distant past, there were still millions of interplanetary bodies that could collide with the Earth in the future.

This threat was bad enough, but the main revolution in our view of the importance of impact events for Earth was begun by the publication by Alvarez et al. in Science Magazine (1980) of "An Exterrestrial Cause for the Cretaceous-Tertiary Extinction".

There has been great controversy over the impact/extinction interpretation

But the extraterrestrial proponents have indisputable facts on their side:


The Barringer "Meteor Crater," near Winslow, AZ. 1 mile diameter.
Created by the impact of a 50-m diameter metallic meteoroid about 50,000 years ago.

B. Direct Evidence for Major Impacts on Earth

Ancient Impacts

Continuing Impacts

C. Energetics

Where does the tremendous explosive energy of impactors originate? From their high velocities. In the theory of Newtonian dynamics, there is an energy of motion, or kinetic energy, associated with any moving object. When a fast moving object hits the Earth's surface, this energy of motion is quickly converted into heat and results in a concentrated explosion. Want to see a real impact on these gigantic scales?

Impacts - Real & Movie

D. Potential Impact Scales

E. Impact-Induced Bio-Extinctions

Extinctions The fossil record (at right) shows that 5 great extinctions of lifeforms on Earth occurred during the last 570 million years. These are times where the fossil record abruptly changes character, and many species vanish from more recent rocks. It is now believed that most of these were probably induced by extraterrestrial impacts.

The last great extinction was 65 million yrs ago at the so-called Cretaceous-Tertiary ("K-T") boundary in the fossil record.

An Extraterrestrial Origin for the K-T Event

Earlier Major Impacts

F. Risk Level?

We can crudely estimate the frequency of large impacts from the history of lunar cratering & bio-extinctions on Earth and our knowledge of the number of potential impactors of different sizes:

Adjusting for the larger number of smaller impactors:

  • Click here for a plot of the predicted frequency of impacts as function of size.

    The estimated risk to an individual (e.g. you) of asteroid impacts has declined by a factor of about 30 in the last decade due to improved surveys of potential impactors. The estimated net fatality risk (all impactor sizes) is now a 1/700,000 chance per person per lifetime

    Risk ranking: Astronomers have created the "Torino Scale" (a combination of estimated impact energy with probability of a strike on Earth) to provide a threat index for potential Earth impactors.

    G. Umbrellas

    Various government agencies and private groups are consideration approaches to mitigating the danger of impacts on Earth.

    1. First, we must identify threatening "Near Earth Objects" (NEO's). A number of ground-based and space-based surveys, described here, have identified over 10,000 NEO's. The census is effectively complete for the brighter, larger objects, but is deficient for those smaller but still dangerous objects under 500-m in diameter.

      • "Planet-busting" large asteroids (10-km) are relatively bright and we believe that almost all have already been identified. There are relatively few, and none pose a foreseeable threat.

      • "People-busting" medium objects (1-km): it is estimated there are about 980 objects larger than 1 km in Earth-crossing orbits, of which 90% are now identified. There is no obvious near-term threat in this category.

      • "City-busting" small objects (10--500-m): bad news. Too many (perhaps a million), too faint; search too expensive. We will have to "live" with these. (Tunguska and Chelyabinsk are in this class.) Over 5,000 with diameters larger than 100-m are known, but another 15,000 are expected in that range. A recent study finds that the number of potential city-busting impactors may have been underestimated by as much as a factor of 10.

      • A sobering footnote. The objects found in NEO surveys are asteroids and comets with relatively small semi-major axes (i.e. with orbits near the Sun). Comet nuclei plunging in from the edge of the solar system could easily be in the dangerous impactor category (as was Comet Shoemaker-Levy 9), but we would have no way of detecting them until late in their approach (say at the distance of Saturn). Making detection even harder is the fact that comets can come from all directions in space, whereas asteroids are more confined to the ecliptic plane.

    2. Second, we must develop technologies to eliminate them

      • Best method: a gentle velocity deflection when they are still at large distance from Earth, enough to change their orbit such that they never intersect with Earth's orbit. Trying to break up potential impactors with explosives, as in the movie "Armageddon," could easily create more danger than it prevents.

      • Requires new, though feasible, space technologies, e.g. an asteroid tugboat

    3. Following the asteroid 1997 XF11 public relations debacle, NASA established a Near Earth Object Program to oversee studies of potentially hazardous objects.

    H. Summing Up

    Impacts from fragments of interplanetary material are not only dangerous to us, but they have also had dramatic effects on the development of life on Earth. Based on our (still incomplete) surveys to date, impacts do not present a looming threat to human welfare. However, we must learn to live with them in the long run, and this will require a large investment of money, energy, and brainpower. The sooner we begin to make that, the better.

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    Last modified May 2017 by rwo

    Opening painting copyright © 1998, Don Davis. Tunguska areal map from Clark Chapman/John Pike. Impact frequency plot copyright © Prentice-Hall. Chicxulub map copyright © 2001 Athena Publications. Text copyright © 1998-2017 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.