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Arecibo Moon Discovery

Introduction

On May 20 2021 two amateur astronomers, Peter Nosworthy and Dave Gault, discovered a 'moon' of the asteroid Arecibo. Dave and Peter are both are located in the Blue Mountains west of Sydney Australia and made the discovery from their back-yard observatories.

Although the Asteroid and its moon can not be seen directly with amateur telescopes, the discovery was made by the 'occultation' method - watching the light from a distant star blink out as the asteroid and moon moved in front of it.

Below is the plain language FAQ, while other pages on this site contain videos amd more technical information relating to the discovery.


The FAQ

Who Are We?

We are Peter Nosworthy and Dave Gault, two amateur astronomers from the Blue Mountains near Sydney Australia. We mainly use our telescopes observe asteroid occultations. This is when an asteroid (a large rock in space) passes in front of a star and blocks the star's light for a few seconds. These observations help us to learn about the sizes, shapes and motion of asteroids.
Peter Nosworthy
Peter Nosworthy
We both have our own backyard observatories with large telescopes. Dave's observatory is a roll-off-roof type located at Hawkesbury Heights and is called Kuriwa Observatory. It houses a 12-inch telescope. Peter's observatory is a domed type called Hazelbrook Observatory housing an 11-inch telescope.
Dave's Roll-Off-Roof Observatory
Dave's Roll-Off-Roof Observatory
Peter's Domed Observatory
Peter's Domed Observatory


What Happened?

At about 4am on May 20th, we both watched as an asteroid called Arecibo moved in front of a dim star, blocking its light for a few seconds. The asteroid itself was too dim to see, so we could only see the star. It disappeared for about two seconds as we expected. However, about 3 seconds later the star disappeared again for less than a second. This was not expected!

Peter's video recording of the star disappearing twice and the graph of the light level of the star. The first disappearance is caused by the asteroid and the second by its moon.
Peter's video recording of the star disappearing twice and the graph of the light level of the star. The first disappearance is caused by the asteroid and the second by its moon.
Dave's video recording of the star disappearing twice and the graph of the light level of the star.
Dave's video recording of the star disappearing twice and the graph of the light level of the star.

What Does it Mean?

The second disappearance of the star probably means that there is another smaller object close to the asteroid and moving with it. That could only be a 'moon' in orbit around the asteroid. The animation below shows how the star would be hidden by the asteroid and then the moon and how the light from the star would drop to zero as each object hides it.


Is it Really a Discovery?

We could not be 100% sure that we had discovered a moon until someone else could make similar observations to confirm that our result was real and not caused by something else - for example a bird flying in front of the telescope. We therefore used online forums to ask other amateur astronomers to observe more occultations by this asteroid to try to confirm our discovery.

About three weeks after our observations, two other amateur astonomers in California, Richard Nolthenius and Kirk Bender, recorded another occultation as the asteroid moved in front of a different star. They both saw the star disappear twice just like we did. This confirms that the smaller object is real. With four observations on two separate occasions, we can now be almost certain that the asteroid Arecibo really does have a moon.

Here are the videos recorded in California by Richard and Kirk. Both videos clearly show the star disappearing twice.



How big are they?

We know the distance to the asteroid and how fast it is moving. Therefore, the length of time that it hides the star can tell us the size. The observations made in California gave a good measurement of the sizes of both objects. The asteroid is about 24km across and the moon is about 13km.

How Significant is this Discovery?

There are currently more than 800,000 known asteroids. The first asteroid with a moon (sometimes called a binary asteroid), was discovered in 1993 by the Galileo spacecraft. As of 2021 there are over 400 asteroids which are known to be binary.

This discovery adds to that list, but we believe it is the first asteroid moon discovered and confirmed by amateurs and the first to be discovered and confirmed entirely by the occultation method.


What's Next?

We have officially announced the discovery through the Central Bureau for Astronomical Telegrams (see the Publications Page).

We have also submitted a paper describing our methods and conclusions to the The Minor Planet Bulletin. It should be published in early 2022.

We will also continue to encourage others to observe future occultaions by the asteroid Arecibo so that we can more accurately measure the size, shape and orbit of its moon.