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article imageNear-Earth Asteroid 2015 HD1 narrowly missed Earth on Tuesday

By JohnThomas Didymus     Apr 22, 2015 in Science
Asteroid 2015 HD1, a small and faint asteroid, about 50 feet across, missed Earth by a hair's breadth at about 3 a.m. CDT ( 8 UTC), Tuesday, April 21, hurtling past at just 45,600 miles (73,400 kilometers) or 0.2 lunar distance.
The asteroid's passage at a distance of 45,600 miles places it among the closest Earth-approaching asteroids known.
[NOTE: Above is the podcast from the live coverage of the event by Virtual Telescope Project].
The information that geostationary satellites, used for weather forecasting and communications, orbit at about 22,300 miles above Earth, helps us put into perspective just how close the passage of asteroid 2015 HD1 at 45,600 miles was.
The asteroid was discovered for the first time on Saturday, April 18, three days before it made closest approach to Earth, by astronomers at the Mt. Lemmon Survey in Tucson, Arizona.
It was too faint to see in the sky with unaided eyes even at closest approach to Earth, but amateur astronomers with at least 8-inch telescopes were able to observe it moving slowly in the constellations Hydra, Antlia and Puppis on Tuesday morning, according to Universe Today.
It reached maximum brightness between 1 and 3 a.m. CDT (6-8 UT) for observers in both low northern and southern latitudes, and appeared brightest in the southwestern sky at about 10 p.m. local time for viewers in the West Coast and at about 9 p.m. for sky-watchers in Hawaii.
The image below was captured by the Virtual Telescope Project on Monday evening, April 20, 2015.
Asteroid 2015 HD1
Asteroid 2015 HD1
Virtual Telescope Project
The Virtual Telescope Project ran live views of the asteroid on its website, beginning at 4 p.m. CDT (21:00 UT) on Monday.
However, the small size of 2015 HD1 means that if it were to collide with Earth it would likely break up in the atmosphere into a shower of meteorites.
According to Earthksy.org, an asteroid in the size class of 2015 HD1, about 50 feet across, wouldn't do much damage if it collides with the Earth because most of its original mass would disintegrate due to the heat of friction as it passes into the Earth's atmosphere.
The meteor which broke up over Chelyabinsk in Russia on February 15, 2015 was about the same size as 2015 HD1, and thus provides convenient case study of what might have happened if 2015 HD1 had broken up directly over a human population center.
2015 HD1 would likely have impacted the Earth's atmosphere with a sonic boom and the shock wave of the impact would have caused minor damages in a population center, such as a big city.
For instance, the shock wave generated by the Chelyabinsk air burst broke windows in more than 7,000 buildings in multiple Russian cities and about 1,500 people were injured mostly by flying glass
But Earthsky.org explains that the destructive force of the Chelyabinsk meteor was enhanced by the fact that it entered the atmosphere at a narrow angle and at high speed, creating an air-burst similar to the famous Tunguska meteor of 1908. The much bigger Tunguska meteor flattened thousands of square miles of forest in Siberia.
Photo shows trees felled by the Tunguska meteor
Photo shows trees felled by the Tunguska meteor
Leonid Kulik Expedition
It is remarkable that astronomers at the Mt. Lemmon Survey were able to detect an asteroid as small as 2015 HD1. This became possible only in recent years following increasing focus on smaller asteroids as astronomers came to appreciate the potential dangers of the myriad of small Earth-approaching asteroids.
According to Universe Today, astronomers conducting searches discover several small Earth-approaching asteroids every month.
But we've been lucky so far because none of the asteroids have been found to be in collision course with Earth. However, the fact that scientists have not identified an asteroid in collision course with Earth does not mean there aren't any or that there haven't been. Astronomers say the Earth has sustained multiple impacts in the past and that impacts by bodies such as 2015 HD1 are far more common than we have thought.
The Nuclear Ban Treaty Organization announced in April, 2014 that, since 2000, its monitoring system has detected 26 major "atom-bomb-scale asteroid" explosions in our atmosphere.
Human populations centers have been spared because most impacts occurred over the seas.
Some asteroids that are currently not in a collision course with Earth still pose a danger to Earth because their paths could undergo alteration that brings then in a collision course with Earth in the future.
While the Earth's powerful gravitational force is able to alter the orbital path of asteroids that approach closely, the force is not sufficiently strong to pull them into a collision course as they pass. However, the ability of the Earth to alter the path of a passing asteroid creates complications for astronomers watching Earth-approaching asteroids to predict future impact events. The gravitational pull of the Earth could alter the orbital path of an asteroid such that it comes into a collision course with Earth in a subsequent close approach.
Astronomers have warned that it is only a matter of time for us to find an asteroid large enough to survive the plunge through our atmosphere and pose serious risk to human population centers.
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