Asteroid Occultations 101

Asteroids1 killed the dinosaurs, flattened Tunguska2, and carved a hole in Arizona3, yet most slide through our solar system almost invisibly. How can you observe these elusive telescope targets as a citizen scientist?

One neat trick for amateur astronomers is to look for asteroid occultations4, which happen when an asteroid passes in front of a star, causing its light to blink or dim. With the annual Asteroid Day public awareness campaign approaching on June 30, the Unistellar Network is here to help you learn more about these mysterious space rocks and how to observe them.

Visit to learn how you can observe asteroids from your backyard!

When Asteroids Hit Earth

Asteroids are scraps of rock and metal left over from our solar system’s formation more than four billion years ago. Studying these planetary building blocks gives scientists a glimpse into solar system history and helps explain how the planets formed.

While asteroids can roam both the inner and outer solar system, most lurk between Mars and Jupiter in the main asteroid belt. Visit to learn more about asteroids!

citizen science astonomy Unistellar
Asteroids most often resemble potatoes, meaning they vary a lot in size and don’t have a very specific shape. Credit: Ron Miller
Citizen Science Varsavia asteroid
Asteroids are relatively small and not very reflective. That’s why spotting them in space is a tricky venture without an occultation. Credit: Ron Miller

On any given day, the chances of an asteroid hitting Earth are low. But history also shows that asteroids can cause enormous damage — just ask the dinosaurs.

That’s why space agencies and citizen astronomers track asteroids as a form of planetary protection. Asteroid Day is a worldwide event that aims to increase awareness about the dangers of asteroids and the importance of tracking them.

Visit to discover and celebrate Asteroid Day!

What is an Asteroid Occultation?

Despite their potential for planetwide destruction, asteroids are tiny compared to other celestial objects. And that’s why asteroid occultations are so important to observing these space rocks.

In astronomy, an occultation5 happens when any object passes in front of a star, blocking at least some of its light. It’s like a miniature eclipse. The nature of this dimming can show scientists how fast, big or far away the object is. 

Just like with solar and lunar eclipses, asteroid occultations aren’t visible everywhere in the world at once. Instead, each occultation is only visible within a narrow path. They also only last for minutes or even seconds at a time.

The good news is that asteroid occultations happen so often that you shouldn’t have to wait too long for one to cross your backyard!

Asteroid occultation light curve
During an asteroid occultation, the star blinks out as the asteroid passes in front of it. You can see the matching dip of light in the chart below the field of stars.

These small viewing zones are also what make individual citizen scientists so important. Together, amateur astronomers form a worldwide network that keeps eyes on the skies, even when occultations don’t happen over major observatories.

Why We Observe Asteroid Occultations

The thrill of catching an occultation can be its own reward. But when citizen scientists combine their observations, like the Unistellar Community often does, it offers valuable insights about asteroids and their orbits. 


The movements of planets and moons are fairly predictable. But because asteroids are so small, it’s surprisingly easy for astronomers to lose track of them without regular observations. Their uneven shapes also mean it can take many observations to figure out an asteroid’s true size. That’s why astronomers keep detailed logs6 of asteroids. But it still takes a worldwide network of professional and citizen scientists to fill in these lists and keep them accurate. 


You can see some of the Unistellar Community’s asteroid occultation success stories on our website, and find out more about how those observations are used by SETI and other research institutions!

How to Observe an Asteroid Occultation

Asteroid Day happens on June 30, but all of June is Asteroid Month. As part of the celebration, Unistellar has put together a special list of asteroid occultations7. Just select your location, and Unistellar will show you a list of occultations visible near you, with dates, times and directions. Clicking on a specific event will show you a map of where the asteroid occultation will be visible. You’ll want to be anywhere between the two orange lines. There are opportunities all over the globe, as the video below shows!

Unistellar is celebrating Asteroid Month by making it easy to spot asteroid occultations and then share your observations with a worldwide network of asteroid hunters.

Planning for Your Observing Run

Once you’ve picked an occultation event to observe, make sure you’re prepared to see it. First, you’ll need some special equipment. Very few occultations will be visible to the naked eye, but a quality telescope and camera will reveal them. If you want to share your observation with astronomers, you’ll also need to record the event. 


As the day approaches, check your weather. Being in the viewing zone won’t matter if it’s too cloudy to see the star your asteroid is photobombing. Pick a site with clear skies and unobstructed views. Then, don’t forget to charge your telescope and empty its memory. At least half an hour before your occultation, make sure you’ve set up your telescope, updated any software you need, and found your star. Remember, even though the asteroid is the prize, you’re going to find it by watching a star and waiting for the asteroid to come to you (or at least your field of view). 


Thankfully, Unistellar’s eVscope takes the cumbersome calibration and guesswork out of this process. The eVscope taps into a database with tens of millions of stars and instantly recognizes objects in its field of view, making it ideal for observing asteroid occultations.


Start recording a few minutes before the occultation, then hang tight! You can watch live through the viewer, but these events are quick and easy to miss in the moment. For more guidance on this step, check out the eVscope tutorial videos.


Afterward, upload your data at home so that researchers can combine your carefully collected observations with those from other citizen astronomers. Together, you can contribute to identifying and tracking asteroids and perhaps ultimately help save the planet from an asteroid collision!