An asteroid the size of a house was discovered by NASA last week, which was just in time to announce its near-miss on Wednesday night. Asteroid 2020 GH2 is estimated to be 43 to 70 feet wide according to Space.com. 

NASA’s Planetary Defense Coordination Office assured Earth-dwellers that our planet was safe from destruction, tweeting a video that attempted to put a near-earth pass into perspective. 

Asteroid close approaches happen sometimes, but just how “close” is a close approach? Do this #NASAatHome activity with Dr. Kelly Fast from NASA’s #PlanetaryDefense Coordination Office (PDCO) to find out!

More about NASA’s PDCO: https://t.co/pxKEMkVdRz

Questions? Use #askNASA pic.twitter.com/fkHPLEK17z

— NASA Asteroid Watch (@AsteroidWatch) March 31, 2020

The video depicted the Earth as a basketball, the moon as a tennis ball 25 feet away, and the asteroid responsible for the demise of the dinosaurs as a grain of salt “a couple of blocks away.” But GH2 was not a couple of blocks away. GH2 passed by the Earth at a distance of 220,000 miles, which put it about 16,000 miles closer than the moon. 

NASA is tracking around 20,000 near-Earth asteroids.

Near-Earth #asteroid #2020GH2 will safely zip past our Earth tomorrow, closer than the #Moon. Here it is our image!

➡️https://t.co/JV1OdrAPu3 pic.twitter.com/qXQ8Mee2pl

— Virtual Telescope (@VirtualTelescop) April 14, 2020

NASA is tracking around 20,000 near-Earth asteroids. Any asteroid about 500 feet or larger with an orbit that brings it within 4.7 million miles of Earth is classified as a potentially hazardous asteroid, NASA officials have said. At the moment, scientists have identified more than 20,000 near-earth objects (NEO) and around 40 new ones are being discovered every week. Of the known NEOs, around 5,000 of these are classed as “potentially hazardous.” CNEOS estimated that a cataclysmic collision between an asteroid and the earth that threatens the future of civilization occurs on average once per 100,000 years. But the threat of unseen dangers lurking directly overhead is far more common than previously thought. Over 17,000 near-Earth asteroids remain undetected in our solar neighborhood

Despite the organization’s assurances, threats from outrspace do slip past NASA’s telescopes.  In 2006, a meteor struck in Norway with the blast force estimated at the equivalence of 100–500  tons of TNT, around 3 percent of Hiroshima’s yield. In September 2007, a meteor crashed in southeastern Peru. Many residents became ill, apparently from the noxious gases released from the impact.

In June 2018, the small asteroid 2018 LA was discovered by a ground-based observatory in Arizona eight hours before it impacted in South Africa.

In December 2018, a space rock several meters across entered the atmosphere at 20 miles per second,  exploding 25.6km above the Earth’s surface, with an impact energy of 173 kilotons, 10 times the energy released by the Hiroshima atomic bomb. The explosion went largely unnoticed as it happened over the remote Bering Sea. But the decidedly not-small event went unnoticed by NASA who was informed of the explosive entry from space by the Air Force who caught it on one of their satellites. 

Dr. Lindley Johnson, the planetary defense officer at Nasa, told BBC News that the fireball came in over an area not too far from routes used by commercial planes flying between North America and Asia. He added that a fireball this big is only expected about two or three times every 100 years.

But that prediction seems premature. On 15 February 2013 a meteor approximately 66 ft entered Earth’s atmosphere over Russia at a speed of approximately 40,000 mph. Due to its high velocity and shallow angle of atmospheric entry, the object exploded in an airburst over Chelyabinsk Oblast, at a height of around 18.5 miles, releasing 26 to 33 times as much energy as that released from the atomic bomb detonated at Hiroshima. The object was undetected before its atmospheric entry, in part because its radiant was close to the Sun. Its explosion created panic among local residents. About 1,500 people were injured seriously enough to seek medical treatment and some 7,200 buildings in six cities across the region were damaged by the explosion’s shock wave.

16 hours later, 367943 Duende, an asteroid approximately 100 feet in diameter, passed within 17,200 miles of the earth.

A similar but far more devastating event took place  near the Podkamennaya Tunguska River on 30 June 1908. Known as the Tunguska event, an explosion flattened an estimated 80 million trees over an area of 830 sq miles of forest, and eyewitness reports suggest that at least three people may have died in the event. It is classified as an impact event, even though no impact crater has been found; the object is thought to have disintegrated at an altitude of 3 to 6 miles rather than to have hit the surface of the Earth. The Tunguska event is the largest impact event on Earth in recorded history, though much larger impacts have occurred in prehistoric times. 

In February 2013,367943 Duende, an asteroid approximately 100 feet in diameter, passed within 17,200 miles of the earth. 

On April 29,  1998 OR2, an asteroid half the size of Mt. Everest, is expected to pass by Earth at a distance of 3.9 million miles, more than 15 times that of the moon. 

NASA is beginning to seriously consider the threat of a catastrophic impact as a planet-threatening possibility. In April, scientists and civil authorities from around the world gathered at the International Academy of Astronautics 6th Planetary Defense Conference. in College Park, Maryland. Also, the National Near-Earth Object Preparedness Strategy and Action Plan was published by the White House in June 2018, describing plans for such an eventuality.

The Planetary Defense Coordination Office (PDCO) was established in 2016 to detect any potentially hazardous object. Since the PDCO was established, at least four major impacts have been reported. Only three impact events have been successfully predicted in advance, usually by only a few hours.  Currently, predictions are mainly based on cataloging asteroids years before they are due to impact. This works well for larger asteroids as they are easily seen from a long distance but is ineffective for predicting smaller objects that can still be quite destructive.