Bentley, August 4 (The Conversation) Asteroids are remnants of rocks left over after the formation of our solar system. There are approximately half a billion dwarf planets with a diameter of over four meters that orbit the sun at a speed of about 30 kilometers per second, which is roughly the same speed as Earth.
Asteroids certainly exist in people’s imaginations, which is why they are shown in many Hollywood films depicting the potential destruction if a large planet were to collide with Earth.
Almost every week, we come across online headlines describing asteroids as “bus-sized,” “truck-sized,” “vending machine-sized,” “half the size of a giraffe,” or sometimes even “the size of a full-grown giraffe.” We also have special designations for dwarf planets like “city killer,” “planet killer,” and “god of chaos.”
Undoubtedly, the dangers posed by dwarf planets are real. It is widely believed that approximately 65 million years ago, a large dwarf planet’s impact on Earth led to a catastrophic event, resulting in the extinction of most dinosaurs.
Beyond the media descriptions, what are the risks based on numbers? How many asteroids can potentially collide with Earth, and how many can we expect to pass by? What is the risk of a direct impact?
Smaller dwarf planets are much more numerous compared to larger ones, and they cause significantly less damage compared to the larger asteroids.
Therefore, Earth experiences frequent but low-impact collisions with smaller asteroids, and rare but high-impact collisions with larger asteroids. In most cases, the smallest asteroids break up significantly when they collide with Earth’s atmosphere and rarely reach the surface.
When a small asteroid (or meteoroid, an object smaller than a dwarf planet) collides with Earth’s atmosphere, it creates a brilliant “fireball” – a long-lasting and bright version of a shooting star or meteor. If any surviving fragment of the object impacts the ground, it is called a meteorite. Most objects burn up in the atmosphere.
How many asteroids pass right by the earth?
A very simple calculation can help you understand how many small asteroids can come close to our planet.
Let’s consider the case of four-meter-sized asteroids. On average, a four-meter asteroid collides with Earth’s surface once a year.
If you double the surface area, you will encounter two such events per year. Earth’s radius is 6,400 kilometers. Doubling the surface area results in a sphere with a radius of 9,000 kilometers. So, approximately once a year, a four-meter asteroid will come within 2,600 kilometers of Earth’s surface – the difference between 9,000 kilometers and 6,400 kilometers.
If you double the surface area again, you can expect around two events per year within 6,400 kilometers of Earth’s surface, etc. This closely matches recent records of close encounters with asteroids.
A distance of a few thousand kilometers is a significant distance for objects the size of a fist. However, in the eyes of the media, most asteroids that pass by are traveling from much farther distances.
Astronomers consider anything that passes close to the Moon – about 300,000 kilometers – as “close.” However, what an astronomer considers “close” is not what an average person would call “close.”
In 2022, there were 126 near-Earth objects (NEOs), and as of 2023, we now have 50.
Now, let’s consider large asteroids with a diameter greater than one kilometer. The same simplified logic can be applied here. For each impact that could pose a threat to civilization, occurring approximately once every five lakh years, we can expect thousands of smaller impacts (closer than the Moon) within that same period.
Such an event is expected to occur in 2029 when the asteroid 153814 (2001 WN5) will pass at a distance of 248,700 kilometers from Earth.
How do we assess threats and what can we do about it?
It is estimated that about 95% of asteroids with a size larger than one kilometer have already been discovered, and ongoing efforts are being made to continuously search for the remaining 5%. When a new asteroid is found, astronomers conduct comprehensive observations to assess any potential risks it may pose to Earth.
The Torino Scale is a method used to categorize and communicate potential impact hazards from asteroids and comets for the next 100 years. It assigns a value ranging from 0 (no hazard) to 10 (certain collision with a large object) to indicate the level of threat posed by a celestial object.
Currently, all known objects have a rating of zero on the Torino Scale. No known object has a rating higher than 4 (a close encounter that poses no unusual level of danger to Earth, with attention by astronomers being merited).
Therefore, instead of hearing about giraffes, vending machines, or trucks, what we really want to know from the media is the Torino Scale rating of an asteroid.
In the end, technology has advanced to a point where if we ever have to deal with a significant number on the Torino Scale, we have the opportunity to take action. Recently, the DART mission impacted an asteroid with a spacecraft, altering its trajectory. In the future, it is commendable that such actions, given enough time, can aid in protecting Earth from potential impacts.