Northern Arizona astronomy: meteor and comet impacts on Earth

June sky chart.

June sky chart.

WILLIAMS, Ariz. — As our solar system formed, all the planets accreted from collisions by smaller objects because of gravitational attraction.

In fact, the Earth has been struck by meteors and comets over its entire lifetime. Even water may have come largely from continual comet impacts. Regular heavy bombardments may have continued until as recently as 4 billion years ago, making it difficult for life to attain a foothold.

While the great majority of meteors currently are composed of mostly small rocks and dust, rather than very large asteroid sized objects, the Earth is constantly gravitationally attracting and accumulating over 100 tons of matter ever day. Most of this is in the form of dust or tiny sand-grain sized pieces that appear in the night sky as the shooting stars we often see during meteor showers.

By comparison, chunks of rock the size of a school bus may strike every century, and asteroids of 150 feet across, like the one that created the three-quarter mile wide Meteor Crater, 40 miles east of Flagstaff, hit on the average of about once every 50,000 years.

Impactors on the order of 2-3 miles across average every 20 million years and can become regional extinction events. The estimated 6-9 mile wide asteroid that struck Earth 66 million years ago creating the 110 mile diameter Chicxulub crater, now hidden on the Yucatan Peninsula and beneath the Gulf of Mexico, coincides with the quick demise of the dinosaurs. This asteroid approached Earth from the southeast. The immediate impact sent a blast wave of super-heated vaporized rock and debris northward at supersonic speed and, within minutes, set the North American continent aflame killing most of life there.

Material was thrown high into the Earth’s atmosphere which darkened the sky and blocked the sun’s energy, greatly impeding plant growth world-wide, and destroying the food chain, a condition that lasted for possibly centuries.

These results led to environmental consequences on a global scale, in which about 75 percent of all plant and animal species became extinct world-wide.

One of the distinct geologic clues supporting this event scenario is a layer of material rich in the element Iridium, a substance relatively rare on Earth, but common in meteors, that is located all around the world between the geologic Cretaceous and Paleogene layers that correspond to 66 million years ago.

This event profoundly changed the course of life on Earth. Had it not happened, evolution may have followed a greatly different path. This was the last of five major world-wide mass extinctions that are found in the geologic record, though not all may have been of meteoric origin.

Of course we observe thousands of impact craters on the moon, and even with the ever changing erosion because of wind, water and tectonic movement, there are just under 200 known impact sites located on Earth.

However, what about more recent events? On the morning of June 30, 1908, over a sparsely populated area in eastern Siberia a large explosion occurred that flattened 770 square miles of forest, the largest impact event ever historically recorded on Earth. Known as the “Tunguska Event”, it is attributed to a meteoroid or comet that exploded in the atmosphere at an altitude of 3 to 6 miles above the surface of the Earth.

Studies have estimated the object’s size as being on the order of 200 to 620 feet, depending on whether the body was a comet, or a dense asteroid. The “Great Daylight Fireball” was an Earth grazing meteor that passed within 190,000 feet of Earth’s surface at 2:30 p.m. Aug. 10, 1972.

This 30-foot object was visible during the day, and was filmed passing over the Grand Tetons, Wyoming, by a park visitor. It headed northward over Alberta, Canada, before it left Earth’s atmosphere having skipped back out into space, like a stone on water. An eyewitness in Montana reported a double sonic boom, and a smoke trail lingered in the atmosphere for several minutes.

The “Chelyabinsk Meteor” was caused by a 60 foot near-Earth asteroid that entered the atmosphere over Russia Feb. 15, 2013 at about 9:20 a.m. The light from the object was brighter than the sun, and visible over 60 miles away. It was observed over a wide area and in neighboring republics.

On account of its high velocity and the shallow angle of entry, it exploded at a height of about 97,000 feet.

Some 7,200 buildings in six cities were damaged by the explosion’s shock wave, with about 1,500 indirect injuries reported.

This was the largest known natural object to have entered Earth’s atmosphere since the 1908 Tunguska Event. It was also the most photographed, since most Russians have car cameras.

Since the late 1990s astronomers been systematically using telescopes to survey large areas of sky to discover and study Near-Earth Objects (NEO), especially those that could impact Earth.

About 14,000 NEOs have been discovered. Some 900 of these are asteroids with a diameter of a half mile or larger, while about 1,700 have been classified as potentially hazardous.

Given the details of their orbits, there are varying probabilities for each striking Earth in the future.

Also, since they often pass close to other celestial bodies, their orbits can change over time. Hence, constant monitoring is prudent to get the most accurate information.

The asteroid Apophis, about 900 feet across, will pass by Earth in 2029; its close approach could shift its orbit to strike in 2036.

Another possibility to be struck by a really huge asteroid is in 2175, by the one-third mile diameter 1999 RQ36, which has a probability of 1 in 2,700.

One called 2009 FD, of about equal dimensions, could strike 10 years later. And, another, 1950 AD, has a strong probability of striking Earth in 2880.

The more we know, and the longer the lead time we get, will allow us to thwart future impacts with less interaction.

The Williams Monthly Star Party — Friday, June 30

Members of the Coconino Astronomical Society, in conjunction with the city of Williams, will host a Star Party from 7:30 to 9:30 p.m. at Glassburn Park, in the natural area west of Rod’s Steakhouse parking lot. Several large telescopes will be on hand to view the Moon, planets and other celestial objects.

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