Impact of a "planet buster" asteroid on Earth

A. HISTORY

• Based on geological evidence (see below), this paper argued that the major extinction of lifeforms, including dinosaurs, 65 million years ago was produced by an asteroid impact.

• Although threats from extraterrestrial impacts had been speculated about before, this study provided the first direct evidence for a link to worldwide biological extinctions.

• Many geologists & paleontologists argue for other, non-astronomical, mechanisms such as the massive volcanic outbreak at the Deccan Traps which might have caused a sudden extreme greenhouse event.

• Earth moves continuously through a swarm of asteroids, meteoroids, and comet nuclei. E.g. "Apollo," "Aten," and other types of Earth orbit-crossing asteroids.

• ===> Major impacts are inevitable! Question is not IF?, but WHEN?

• The areal density of lifetime impacts on the Earth's surface for craters over 500-m diameter is probably slightly higher than that on the Moon. See this illustration of the impact density.

• New recognition: energy deposited by big asteroids is large enough that destructive effects can be global.

Barringer Crater, AZ. 1 mile diameter.

B. KINDS OF DIRECT EVIDENCE FOR IMPACTS ON EARTH

1. Observed near misses (last 50 years)
   
   Some large meteoroids have been seen passing through our atmosphere. At right is a picture taken Aug 10, 1972 in Grand Teton National Park of a near-miss (click for enlargement). The object is about 10-m diameter and at an altitude of about 55 km, moving at 15 km/sec (33,000 MPH). It approached at such a shallow angle that it skipped off the atmosphere. If it had hit the Earth, it would have had H-bomb equivalent impact energy.
   Click here for a video

   Many observed misses within 2x distance of Moon.

2. Impact geology: shocked & melted rocks or other debris characteristic of sudden high temperatures or pressures. This can be identified even in the absence of a definite crater nearby. Events identified at various locations worldwide
   • E.g. Debris from Chesapeake Bay event: 35 million yrs ago; 56 mile-wide crater; estimate tidal waves 1000 ft high. Click here for map of swamped region.

3. Fossil Craters
   • Ferocious impact history obvious on surface of all other solid bodies investigated so far in Solar System (e.g. Moon).
     
   • Owing to weathering and crust recycling, old craters are mostly erased on Earth's surface (unlike Moon, Mars, etc.)

   • But there are about 150 identified fossil craters. E.g. at right is Manicouagan Crater, Canada. This Space Shuttle image shows a 43 mile diameter annular lake (in winter), part of 62 mile diameter crater structure. Age: 210 million years.

4. Biggest recorded hit: Tunguska, Siberia 1908
   • Energy release: equivalent to ~20 million tons TNT. 1 million tons (1 megaton) is the explosive energy of a typical hydrogen bomb. Tunguska was 1000 x energy of the Hiroshima atomic bomb ("only" 20,000 tons).

   • Flattens area size of Washington, DC.

   • Likely air detonation (altitude 10 km) of stony asteroid, ~30 m diameter

5. Air Force infrared monitoring satellites discover that several Hiroshima-sized events occur in the atmosphere each year.
   • Context: imagine the political firestorm if the Chinese government were doing this!

C. ENERGETICS

The impact energy deposited = the kinetic energy (KE) of incoming object, where
• KE = 1/2 M V²

• V is large!
  Orbital velocity ~ 30 km/sec = 66,000 mph

• Mass is large!
  • M = Density x Volume = Density x 1/6 pi D³, where D is the diameter of the object.

  • For rocks, Density ~ 5 gr/cc

  • ===> Mass ~ 2.5 D³ tons, where D = diameter in meters

• Combine, and convert to equivalent explosive energy in units of tons of TNT. [1 ton TNT = 4 x 10¹⁶ ergs]

===> Impact energy = 250 D³ equivalent tons of TNT. D in meters.
Note very strong dependence on size.
A tiny object with D=4 meters packs the explosive power of the Hiroshima bomb (20,000 tons).
If D=200 meters, a common size of asteroid, energy = 2 billion tons of TNT!

Numerical calculations of impact physics have been made by Sandia Laboratories, among others.

See the images of the impact the Comet Shoemaker-Levy 9 fragments on Jupiter. Fragment "G" deposited about 6 trillion tons of TNT equivalent energy.
For context when you view these pictures, remember that Jupiter is 11 times the Earth's diameter. The movie poster below shows an artist's concept of a much smaller impact on Earth.

D. POTENTIAL IMPACT SCALES

• City Buster: 15-m diam meteoroid ===> 10⁶ tons TNT = 1 Megaton (MT). Serious local consequences, though atmosphere provides partial shield. Hydrogen-bomb scale, but without the radioactivity.

• People Buster: 1-km diam asteroid ===> 250,000 MT. No atmospheric shield. Hemispheric-scale effects. At threshold for global effects. Significant fraction of all humans killed.

• Planet Buster: 10-km diam asteroid ===> 250 million MT. global effects. Ejected, vaporized rock and water fill atmosphere ===> global winter ===> major extinction of lifeforms, including virtually all humans.
  Note: you have already seen pictures of Planet Buster-class asteroids: Ida, Eros

E. IMPACT-INDUCED BIO-EXTINCTIONS

• Half of all species (not individuals) of plants and animals vanish

• Dinosaurs eliminated. Creates eco-niche for small mammals (our forebears)

• As first shown by Alvarez et al. in 1980, the sedimentary rock layers at the K-T boundary are abnormally rich in platinum group metals like iridium. Their composition is like meteorites/asteroids, not Earth rocks. Deposition is worldwide. Extraterrestrial origin of extinction now virtually certain.
  
• Estimate required 10-km asteroid hit

• Site probably identified: Chicxulub crater, N. Yucatan. See map at right. Diameter 110 miles. Date (radioactive method) 65 Myr ago. Debris deposits throughout Caribbean area. Image here shows gravity and magnetic anomalies associated with the buried/undersea crater. See Washington Post article for details.

F. RISK LEVEL?

• E.g.: 5 great extinctions in 570 million years implies rate of about one global extinction (planet-buster) event per 100 million years; perp is ~10-km diameter asteroid.

A people-buster event (1-km diam impactor killing 1/4 of human race) will occur once per 150,000 years.
• In human terms, very rare but very serious.

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

G. UMBRELLAS

1. First, must identify threatening Near Earth Objects
   • "Planet-busting" large objects (10-km) relatively bright, already ID'd. Relatively few. No foreseeable threats.

   • "People-busting" medium objects (1-km): estimated over 1500 objects larger than 1 km in Earth-crossing orbits. Fainter; less than 10% ID'd. No obvious near-term threat, but expect 1 impact per 150,000 years. Full census requires dedicated search, 10-20 yrs, few $million per year.

   • "City-busting" small objects (10--250-m): bad news. Too many (millions), too faint; search too expensive. Will probably have to "live" with these.

2. Second, eliminate them
   • Best method: gentle velocity deflection when still at large distance from Earth.

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

3. Following the asteroid 1997 XF11 PR debacle, NASA establishes new office to oversee a census of 1-km & larger asteroids.

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Reading for this lecture:

Seeds, Chapter 25
Study Guide 22
Optional reading:
Essay on Impact Risk (Clark Chapman, 2002; PDF).
Dave Barry column on a "near miss"
Rain of Iron and Ice John S. Lewis (Clemons & SciEngr Lib: QB721.L42.1996)

Reading for next (last) lecture:

Seeds, Chapter 26
Study Guide 23

Web Links:

Big screen drama! Not just one but TWO big budget movies about impacts appeared in 1998: Deep Impact and Armageddon. Former was OK, but Armageddon was hopelessly egregious crap. For comments on the science content of these flix, see Bad Astronomy.
Both movies lifted their plots from: Lucifer's Hammer by Larry Niven & Jerry Pournelle (the best comet-flattens-Earth novel)

Tutorial on Impact Geology (N. M. Short, Federation of American Scientists)
Background on the K-T Extinction & Alternate Interpretations (UC Berkeley)
Volcano-Greenhouse Alternative for K-T Extinction (McClean, VPI)
Earth Impact Database (UNB, Canada)
Terrestrial Impact Craters (info & images, C. J. Hamilton)
Tunguska Event (Wikipedia)
Chicxulub Crater and the K-T Boundary (LPL, U. Arizona)
Near-Earth-Object Program (NASA-JPL)
Asteroid Comet Impact Hazards (NASA-Ames)
Asteroid Impacts and Planetary Ecology (Spaceref.com)
Earth Impact Effects Calculator (Univ. Arizona)
Near-Encounter Public Relations Flaps:
Asteroid 97XF11
Asteroid 99AN10
Asteroid 1950 DA

Closest predicted flyby (as of 2005):
Asteroid 2004 MN4 (Apophis): Will pass 18,600 miles from Earth on April 13, 2029. Impact potential: 850 megatons; will be visible as 3rd magnitude object
Apophis (Wikipedia)
A Call to (Considered) Action on Asteroid 2004 MN4 from the B612 Foundation

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Last modified May 2008 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 © 1998-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.