Asteroids near Earth: are we in danger?

Are near-Earth objects a threat to Earth?
Maia Mulko
Artist's impression of the double asteroid 90 Antiope
Artist's impression of the double asteroid 90 Antiope

Source: ESO 

An asteroid is a metallic or rocky body that orbits the Sun within the asteroid belt, a region of space between Mars and Jupiter.

Asteroids near Earth: are we in danger?
Location of the asteroid belt in the solar system

Asteroids are believed to be remnants of the early formation of the solar system. They are often called “minor planets”, although they do not have an atmosphere. They can be the size of a dwarf planet, but they can also be as small as 6 feet (2 meters) in diameter —which is the case of the tiniest asteroid ever found, 2015 TC25.

Most asteroids are not that small. If it were to hit the Earth, a sufficiently large asteroid may have a higher chance of surviving the entrance to our atmosphere and impacting the ground.

Asteroids near Earth: are we in danger?
Meteor Crater, in Arizona, was formed by asteroid debris known as Canyon Diablo meteorite

As you may already know, our atmosphere has five major layers (and several secondary layers) that protect us against asteroids hitting Earth by producing enough friction to heat them up to the point where they disintegrate.

According to NASA, car-sized asteroids burn up in our atmosphere at least once per year. But suppose an asteroid is between around 82 feet (25 meters) and half a mile (1 kilometer) in diameter. In that case, the asteroid may be large enough that it does not completely burn up and has a greater chance of impacting the ground. If the asteroid were more than one-half mile (1 or 2 kilometers) in diameter, the impact could have a global effect.

To date, we have discovered more than 1 million asteroids of all shapes and sizes. Could one of these hit Earth in the near future? Let’s find out.

What are near-Earth asteroids?

As far as astronomers know, there are currently somewhere around 27,000 asteroids in near-Earth orbits right now. These are asteroids that were knocked out of the main asteroid belt, attracted by the gravity of nearby planets, and have orbits that bring them within around 4.6 million miles (7.5 million kilometers) of Earth's orbit around the Sun.

But is that distance large enough for us to be safe? The answer is not on how far the asteroids are from Earth but how closely they approach the Earth as they travel in their orbits. Suppose an asteroid or other celestial body is large enough to reach Earth’s surface, and astronomers predict that its orbit will take it within around 5 million miles (8 million kilometers) of the Earth. In that case, it is considered a Potentially Hazardous Object (PHO).

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Asteroids near Earth: are we in danger?
Asteroid detection procedure

NASA constantly monitors near-Earth objects from its Planetary Defense Coordination Office (PDCO).

When did the last asteroid hit Earth?

The last asteroid of a considerable size impacted Earth's atmosphere was around 65 feet (20 meters) in diameter.

This was the Chelyabinsk meteor, which exploded about 20 miles (30 km) above the city of Chelyabinsk in Russia on February 15, 2013. As the unnamed near-Earth asteroid fell, it was moving at over eleven miles per second. The explosion released the energy equivalent of around 440,000 tons of TNT. Even at that height, the shock wave and debris from the explosion damaged more than 7,000 buildings and produced 1,491 indirect injuries (most from broken glass). The blast, which was stronger than a nuclear explosion, triggered detections by monitoring stations as far away as Antarctica. The meteor was so bright it may have briefly outshone the Sun.

What is the closest near-Earth asteroid?

Asteroids move all the time, so each year, there are one or more different asteroids closest to Earth —closer than other near-Earth asteroids— at a given time.

Asteroids near Earth: are we in danger?
Asteroid 6478 Gault

For example, on March 11, 2022, a small asteroid called 2022 EB5 impacted Earth's atmosphere over the Arctic Ocean off Norway’s Jan Mayen Island. On March 25, another asteroid, 2022 FD1, grazed Earth’s atmosphere, coming only around 5,260 miles (8,470 km) from Earth's surface. That’s closer than many orbiting satellites.

Is an asteroid going to hit Earth in 2036?

Near-Earth asteroid 99942 Apophis is another potentially hazardous asteroid. It has a diameter of 1,210 feet (370 meters), and in 2004, initial observations indicated a high probability that it could hit Earth in April 2029. Later, the highest probability of an impact was moved to 2036. But calculations in 2021 estimated that 99942 Apophis would pass as close as 19,600 miles (31.500 km) but should miss the planet.

Then, what asteroid is most likely to hit Earth?

Bennu and 1950 DA are two asteroids that are often believed to hit Earth within the next hundred years. Let’s see:

  • Asteroid 1950 DA was discovered in 1950. It has a size of 0.68 miles (1.1 kilometers) in diameter. According to NASA, it has an impact probability of 1 in 300 in the year 2880. If it impacts Earth, it will most likely crash into the ocean, producing massive tsunamis and floods in coastal areas.
  • Asteroid Bennu, on the other hand, has a diameter of 1,722 feet (525 meters). If it collided with Earth, which NASA calculates could occur between 2175 and 2199, it could cause massive destruction on a planetary scale. However, it is also estimated that there is only an impact probability of 1 in 1,750.

NASA is continually compiling impact risk data with an automated collision monitoring system called Sentry. Currently, asteroid 2010 RF12 has one of the highest chances of Earth impact: 1 in 21 or 4.7 percent (1 in 16) on September 5, 2095. During this time, the asteroid is expected to fly past Earth from a distance of roughly 1,060 miles away. However, this asteroid is only 23 feet (7 meters) in diameter, so a highly damaging impact event is unlikely.

What if we’re wrong?

NASA has actually mapped only about 40 percent of the largest near-Earth asteroids. It has the Planetary Defense Coordination Office (PDCO) to anticipate close approaches to Earth and possible impact events. Still, it is also investigating the possibility of deflecting asteroids and other objects that pose a threat to Earth before they reach the surface.

Asteroids near Earth: are we in danger?
DART impact pillars

In November 2021, NASA launched the Double Asteroid Redirection Test (DART), the first planetary defense mission against near-Earth objects. The plan is to crash a space probe into the small asteroid satellite Dimorphos (the minor-planet moon of a binary system with 65803 Didymos as the primary asteroid) to evaluate if a spacecraft impact can divert or obliterate a dangerous asteroid coming too close to Earth. The collision is scheduled to occur between September 26 and October 2, 2022.

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