PHY

PHYS 105 Unit 3

Near Earth Objects and Past Impacts

Part 1: NEO Impacts

A very large number of meteoroids enter the Earth's atmosphere each day amounting to more than a hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites. The largest found meteorite (Hoba, in Namibia) weighs 60 tons.

The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.

A good example of what happens when a small asteroid hits the Earth is Barringer Crater (a.k.a. Meteor Crater) near Winslow, Arizona. It was formed about 50,000 years ago by an iron meteor about 30-50 meters in diameter. The crater is 1200 meters in diameter and 200 meters deep. About 120 impact craters have been identified on the Earth, so far (see below).

A more recent impact occurred in 1908 in a remote uninhabited region of western Siberia known as Tunguska. The impactor was about 60 meters in diameter and probably consisting of many loosely bound pieces. In contrast to the Barringer Crater event, the Tunguska object completely disintegrated before hitting the ground and so no crater was formed. Nevertheless, all the trees were flattened in an area 50 kilometers across. The sound of the explosion was heard half-way around the world in London.

There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in a million years or so on the average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disastrous consequences. The impact of a comet or asteroid about the size of SL9 hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago. It left a 180 km crater now buried below the jungle near Chicxulub in the Yucatan Peninsula.

Calculations based on the observed number of asteroids suggest that we should expect about 3 craters 10 km or more across to be formed on the Earth every million years. This is in good agreement with the geologic record. It is more difficult to compute the frequency of larger impacts like Chicxulub but once per 100 million years seems like a reasonable guess.

Here are educated guesses about the consequences of impacts of various sizes:

Impactor Diameter (meters)

Yield (megatons)

Interval (years) Consequences

< 50 < 10 < 1 meteors in upper atmosphere most don't reach surface

75 10 - 100 1000 irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area size of city

160 100 - 1000 5000 Iron and stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo)

350 1000 - 10,000 15,000 land impacts destroy area size of small state; ocean impact produces mild tsunamis

700 10,000 - 100,000 63,000 land impacts destroy area size of moderate state (Virginia); ocean impact makes big tsunamis

1700 100,000 - 1,000,000 250,000 land impact raises dust with global implication; destroys area size of large state (California, France)

Data from 'The Impact Hazard', by Morrison, Chapman and Slovic, published in Hazards due to Comets and Asteroids

Procedure:

1. Run this simulator – use questions 1 through 4 as your impactors. Make sure you are outside the blast zone, most of these need to be more than 30 km away to avoid being I the crater!

2. Record the following after you click “Calculate Effects” – making a table is nice and you can always use those table formatting skills when for the office. See, getting your money’s worth from college now, aren’t ya?

Impact Energy Interval Major Global Changes

Final Crater Size

Ejecta

Questions

Impact Variables 1. Meteor Crater,

USA Projectile Diameter: 50 m

Projectile Density: iron (8000 kg/m3)

Impact Velocity: 20 km/s

Impact Angle: 45 degrees

Target Density: 2700 kg/m

2. Ries Crater,

Germany Projectile Diameter: 1500 m

Projectile Density: rock (2750 kg/m3)

Impact Velocity: 20 km/s

Impact Angle: 30 degrees

Target Density: 2700 kg/m3

3. Cheseapeake

Bay, USA Projectile Diameter: 2300-3500 m

Projectile Density: rock (2750 kg/m3)

Impact Velocity: 20 km/s

Impact Angle: 45 degrees

Target Density: 2700 kg/m

4. Chicxulub Projectile Diameter: 17500 m

Projectile Density: rock (2750 kg/m3)

Impact Velocity: 20 km/s

Impact Angle: 45 degrees

Target Density: 2700 kg/m3

5. Research the Chicxulub crater – there is a lot of debate about what happened here 65 million years ago, but it is obvious something big smacked down here. Google Search to get you started, as always, feel free to find your own sources. Just be sure to cite URLs. 100 word summary, please.

Part 2 Near Earth Objects’ Orbits

Background: What is a NEO?

Near-Earth-Objects (NEOs) are small bodies in the solar system (asteroids and short-period comets) with orbits that regularly bring them close to the Earth and which, therefore, are capable someday of striking our planet. Sometimes the term NEO is also used loosely to include all comets (not just short-period ones) that cross the Earth’s orbit. Those NEOs with orbits that actually intersect the Earth’s orbit are called Earth-Crossing-Objects (ECOs).

The Earth’s atmosphere protects us from most NEOs smaller than a modest office building (50 m diameter, or impact energy of about 5 megatons). From this size up to about 1 km diameter, an impacting NEO can do tremendous damage on a local scale. Above an energy of a million megatons (diameter about 2 km), an impact will produce severe environmental damage on a global scale. The probable consequence would be an "impact winter" with loss of crops worldwide and subsequent starvation and disease. Still larger impacts can cause mass extinctions, like the one that ended the age of the dinosaurs 65 million years ago (15 km diameter and about 100 million megatons).

Are any NEOs predicted to hit the Earth?

There are many more small NEOs than large ones. Astronomers estimate that there are approximately 1000 Near Earth Asteroids (NEAs) larger than 1 km in diameter, and perhaps a million larger than 50 m in diameter (the threshold for penetration through the Earth's atmosphere). The largest NEAs are less than 25 km in diameter. There are probably many more comets than NEAs, but they spend almost all of their lifetimes at great distances from the Sun and Earth, so that they contribute only about 10% to the census of objects that strike the Earth.

As of the end of 2010, astronomers had discovered more than half of the larger Near Earth Asteroids (diameter greater than 1 km). None of the known asteroids is a threat, but we have no way of predicting the next impact from an unknown object.

We don’t know when the next NEO impact will take place, but we can calculate the odds. Statistically, the greatest danger is from an NEO with about 1 million megatons energy (roughly 2 km in diameter). On average, one of these collides with the Earth once or twice per million years, producing a global catastrophe that would kill a substantial (but unknown) fraction of the Earth’s human population. Reduced to personal terms, this means that you have about one chance in 20,000 of dying as a result of a collision. Such statistics are interesting, but they don’t tell you, of course, when the next catastrophic impact will take place—next year or a million years from now.

Who is searching for NEOs?

Several teams of astronomers worldwide are surveying the sky with electronic cameras to find NEOs, but the total effort involves fewer than 100 people. The most productive NEO survey is the LINEAR search program of the MIT Lincoln Lab, carried out in New Mexico with US Air Force and NASA support. The LINEAR team, which operates two search telescopes with one- meter aperture, discovered more NEOs in 1999 and 2000 than all other searches combined. Other active survey groups include the NEAT search program in Hawaii, carried out jointly by the NASA Jet Propulsion Lab and the US Air Force; the Spacewatch survey at the University of Arizona, funded by NASA and a variety of private grants, the LONEOS survey at Lowell Observatory in Flagstaff Arizona, supported by NASA grants, and the Catalina Sky Survey in Tucson Arizona, also supported by NASA. Other searches in the US, France, Japan and China also contribute to discovery of NEOs, while additional astronomers (many of them amateur astronomers) follow up the discoveries with supporting observations.

What is the government doing about it?

NEO impacts are the only major natural hazard that we can effectively protect ourselves against, by deflecting (or destroying) the NEO before it hits the Earth. The first step in any program of planetary defense is to find the NEOs; we can’t protect against something we don’t know exists. We also need a long warning time, at least a decade, to send spacecraft to intercept the object and deflect it. Many defensive schemes have been studied in a preliminary way, but none in detail. In the absence of active defense, warning of the time and place of an impact would at least allow us to store food and supplies and to evacuate regions near ground zero where damage would be the greatest.

The US Congress has held hearings to study the impact hazard (in 1993 and 1998), and both NASA and the US Air Force are supporting surveys to discover NEOs. In 1998 NASA formally initiated the Spaceguard Survey by adopting the objective of finding 90% of the NEOs larger than 1 km diameter within the next decade (that is, before the end of 2008). In 1998 NASA also created a NEO Program Office, and it is expected that at least $3 million per year will be spent on NASA-supported NEO searches and orbit calculations. Other governments have expressed concern about the NEO hazard, but none has yet funded any extensive surveys or related defense research. A private Spaceguard Foundation based in Europe promotes NEO surveys internationally, and further interest on an international basis is provided by the International Astronomical Union and the United Nations.

Procedure:

Impact Databases – check these out to answer the questions below.

• Nice atlas of Terrestrial Impact Structures (with maps and images) • NASA’s NEO Program Webpage (nice interactives here)

• Simulation of a big comet impact by Sandia National Labs • Impact (movies and images) of an asteroid near NYC • Impact (movies and images) of a simulated comet impact in the ocean

Questions

6. What is the largest impact structure/crater still evident on the Earth’s surface? How large was it, when did it hit us, and how often does something that large hit the Earth?

7. What kind of uncertainty is there in all asteroid orbit predictions? Give the address of webpages that support your statements.

8. What could we do if we identified a potential "world-killer" in advance? Again, include webpage addresses of 2 different solutions by reputable sources, please. Not some guy giving the asteroid a nudge with his "rainbow aura" or “cosmic thunderbolts”.

9. If the smallest of the asteroids/meteors in Par 1 Table 1 hit the Earth, what would the effects be on human civilization? Discuss thoughtfully.

10. Did the press over-react? Read 5 different stories from this web search of news stories about the sudden appearance and fly-by of 2001 YB5. In 100 words, do you think that the threat of these NEOs is being overstated or overblown? Include the URL of all 5 webpages.