Should You Worry About an Asteroid Hitting Earth?

Star Walk
5 min readJul 18, 2021


Artist’s visualization of the asteroid 2015 TB145

Every now and then, the media produces dozens of scary headlines about rocks from space headed towards our planet. But is there really a big chance that an asteroid could hit Earth in the near future? In this article, you’ll find answers to the most burning questions about potentially hazardous asteroids.

What defines a potentially hazardous asteroid?

To be classified as potentially hazardous, an asteroid must meet two main criteria.

First, it must have a minimum orbit intersection distance (MOID) with Earth of 0.05 AU or less. An asteroid with such an orbit is capable of getting dangerously close to our planet.

Second, it must have an absolute magnitude of 22.0 or less. The smallest asteroids of such brightness are estimated to be 110 to 240 meters in size — it is enough to cause significant local damage in case of impact.

An asteroid’s hazard potential is measured using two scales: the Torino Impact Hazard Scale and the Palermo Technical Impact Hazard Scale. The Torino scale is used to communicate the potential risk of a future asteroid impact to the general public. On this simple scale, an object is assigned a 0 to 10 value based on its collision probability and the kinetic energy of the possible collision. The Palermo scale is a similar but more complex scale that is mainly used by professional astronomers.

How big does an asteroid have to be to damage?

To answer this question, let’s turn to examples.

- The Chicxulub impactor that caused mass extinction on Earth 65 million years ago was about 10 kilometers in diameter. This very asteroid presumably ended the age of the dinosaurs.

- The Tunguska impactor that exploded in the air and flattened 80 million trees of the taiga forest in 1908 was about 100 meters in diameter. This was and still is the largest asteroid impact in recorded history.

- Finally, the Chelyabinsk meteor that entered Earth’s atmosphere in 2013 was about 20 meters in diameter. This asteroid didn’t even reach the terrestrial surface, but the explosion it produced still damaged more than 7,000 buildings. To learn more about these three famous asteroids, watch our video.

You can make a conclusion yourself. Even comparatively small space rocks, like the Chelyabinsk meteor, can cause local damage. Asteroids larger than 1 kilometer can have worldwide effects like causing long-term climate change.

How many potentially hazardous asteroids are there?

As of July 2021, astronomers have detected 2,189 potentially hazardous asteroids, of which 158 are larger than 1 kilometer in diameter. The largest known potentially hazardous asteroid is (53319) 1999 JM8 — it is estimated to be about 7 kilometers in diameter.

We’d like to emphasize that this data doesn’t mean that all of these asteroids will eventually hit Earth — only that they have the potential to do so. None of these asteroids pose any sufficient risk of impact within the next 100 years. You can find more particular info on the NASA CNEOS website. There’s a table there that summarizes all the potential future impact events and provides hazard ratings using the Torino and Palermo scales.

Is there an asteroid coming to Earth in 2021?

You can easily check if any asteroid gets close to our planet soon. Open the Minor Planet Center’s website and find the Close Approaches list in the lower-right part of the main page. It contains all the known asteroids that will pass by Earth within a close range in the next few months. Apart from the asteroid’s name and date of close approach, you can learn its size (in meters) and distance at which it will fly past Earth (in lunar distances). The lunar distance (LD) is the average distance between Earth and the Moon which equals approximately 400,000 kilometers.

If you’re interested in a particular asteroid, use NASA JPL’s Small-Body Database Browser. Enter the asteroid’s name or number and you will get extensive information on its orbit, physical parameters, and discovery circumstances.

How likely is an asteroid to hit Earth?

And now, for the most important question of all: what are the chances that you will experience an asteroid impact in the near future? According to NASA, asteroids larger than 100 meters that can cause local damage hit Earth about every 10,000 years. Space rocks larger than 1 kilometer that can threaten life on our planet come along only once in a few million years.

In other words, there’s a very small chance that you’ll be harmed by an asteroid over your lifetime. Despite that, it’s always good to be prepared. Let’s take a look at the measures astronomers take to guarantee Earth’s safety.

How do we spot near-Earth asteroids?

There are many projects that constantly work on detecting near-Earth objects (NEOs) — asteroids and comets which pass close to Earth’s orbit. Let’s name just a few of such projects.

First, there’s ATLAS — the Asteroid Terrestrial-impact Last Alert System built in Hawaii. It consists of two 0.5-meter telescopes located 160 km apart that survey the sky every clear night. Among other NEOs, ATLAS discovered the famous comet C/2019 Y4 (ATLAS).

Then, there’s Catalina Sky Survey (CSS) located in the Catalina Mountains, Arizona, USA. This astronomical survey focuses specifically on detecting potentially hazardous asteroids and estimating impact risks.

Apart from ground-based telescopes, there’s also a space telescope working on Earth’s orbit. It’s called the Near-Earth Object Wide-field Infrared Survey Explorer or NEOWISE. Currently, NEOWISE is searching for asteroids that could potentially collide with Earth. Among its many discoveries, there’s last year’s naked-eye comet C/2020 F3 (NEOWISE).

In the past ten years, ground-based surveys and NASA’s space telescope have detected thousands of near-Earth objects and made a great contribution to our knowledge about tracking asteroids and comets.

How can we stop asteroids from hitting Earth?

So, what can we do if there’s an asteroid headed for Earth? The good news is we have at least one plausible idea on how to deflect it. The not-very-good news is we’ll need a lot of time to prepare.

The Double Asteroid Redirection Test (DART) is NASA’s mission to test the technologies for preventing an asteroid impact. The target of the mission is asteroid Dimorphos — the moonlet of the binary near-Earth asteroid Didymos. According to the plan, the DART spacecraft will crash into the moonlet (which is about 160 meters in size), thus changing its speed and orbital period. The DART spacecraft is planned to be launched in November 2021.

But even if this mission is a success, there’s still the factor of time needed to build such a spacecraft. A typical space mission of such sort takes several years from approval to launch. This means we must detect a dangerous asteroid years (better, a decade) before it comes close to Earth if we want to make a spacecraft to deflect it in time. That’s why it’s so important to find as many near-Earth objects as possible and calculate their orbits in advance.

Thanks for reading this article! If some of your questions about potentially hazardous asteroids were left unanswered, don’t hesitate to ask them on our social media. We wish you clear skies and happy observations!



Star Walk

Point your device at the sky and see what stars, constellations, and satellites you are looking at 🌌✨