spaceSpace and Physics

This Weekend Amateur Astronomers Can Help Defend The Planet From An Asteroid


Stephen Luntz

Freelance Writer

clockFeb 18 2021, 12:26 UTC

There is a small, but non-zero, chance that Apophis will hit the Earth in 2068, making it easily the largest asteroid known to pose a threat this century. Image credit Marti Bug Catcher/

If you've ever wanted to contribute to guarding the Earth against a threat from outer space, this Sunday/Monday might be your chance – although you will need to be in the right place and have a good telescope.

The asteroid Apophis is the large object considered most likely to crash into the Earth in the lifetime of anyone alive today. It's still a long shot – one chance in 150,000 according to some estimates, a bit higher than that according to others. However, given that an Apophis collision would do world-wide damage to the climate and potentially wipe out numerous cities, that's still something astronomers are keen to keep an eye on.


The reason we don't know the chances more precisely is partly a reflection of the complexity of factors that can change an asteroid's orbit over 47 years. However, our incapacity to measure Apophis' current position with the precision we'd like also plays an important role. At between 340 and 370 meters (1,115 and 1,214 feet) long, Apophis is big enough to do damage but small enough to make measurements difficult. The loss of the Arecibo telescope, previously the world's best instrument for observations like this, increases the challenge further.

One way to improve our knowledge of asteroid orbits is to watch them when they pass in front of a star, briefly shutting out its light. The exact timing and location at which this event occurs allows astronomers to measure their location more precisely than at other times. Since this event, known as an occultation, is usually visible from a path stretching over thousands of kilometers, data obtained from multiple points along the way can be even more useful. Sometimes in the process, we make unexpected discoveries.

On Sunday night (or Monday morning, depending on your time zone) Apophis will block out HD89707, a 7th magnitude star (slightly too faint for the naked eye) as seen over a route curving from Washington state to the Gulf of Mexico and parts of Western Africa. Although long, the path is very thin, so the chance of it taking in any professional observatories is slim. However, thousands of amateurs with telescopes that may prove suitable will live either in the right place or close enough to make a short trip there. Apophis is far too faint at its current distance to be seen with amateur telescopes, but the dimming as it blots out a star may prove to be a different matter. We've never managed this with an asteroid as small as Apophis, but there is a first time for everything.

The anticipated path of the occultation. Note, owing to the uncertainty about its orbit the project hopes to address, the anticipated path may change slightly before the date. Image credit: Unistellar. 

Telescope maker Unistellar runs a website where they encourage owners of their telescopes to join in observations of occultations by Apophis for science, and by other asteroids for fun.


Unistellar's eVscopes have inbuilt features making it easy to record and upload the event – even if you don't notice the change in brightness, a subsequent analysis may reveal it. Other telescopes can do the same thing if paired with cameras and drives that allow them to follow objects across the sky. Consequently, the pool of people able to join the project is a lot broader than those owning one telescope brand.

Certainty about whether Apophis poses a threat in 2068 won't come until its close approach in 2029 at the earliest. Nevertheless, in such an important quest, every bit of data helps – particularly something we can work with now, rather than waiting. The weather in North America recently has not been conducive to late-night observations, but at least some areas should get relief by Sunday.

spaceSpace and Physics