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2002 YE25 Binary Asteroid

Introduction

On May 16 2022 two amateur astronomers, Peter Nosworthy and Dave Gault, discovered the binary nature of the asteroid (172376) 2002 YE25. The asteroid was previously thought to be a single object, but this discovery showed that it is probably two smaller objects. Dave and Peter are both are located in the Blue Mountains west of Sydney Australia and made the discovery from their back-yard observatories. The discovery was made by the 'occultation' method - watching the light from a distant star blink out as the asteroid moved in front of it.

Below is the plain language FAQ, while other pages on this site contain 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 8:43pm on May 16th, we both watched as an asteroid called 2002 YE25 moved in front of a dim star, casting its shadow on the ground and blocking the star's light briefly. The asteroid itself was too dim to see, so we could only see the star.

The asteroid was thought to be about 5.7km across, based on satellite measurements, and our two observatories were about 15km apart (across the shadow path). Therefore the asteroid would be too small for its shadow to cover both our obseratories. However, we both saw the star disappear for about half a second! This was not expected.


What Does it Mean?

The occultations at both locations suggests that the asteroid is about 16km across rather than 5.7km as measured by satellite. However, it is not bright enough to be 16km across.

Another possibility is that it is a very long thin object - 16km long but only 3km wide - and it happened to be oriented just right for the ends of the shadow to cross both observatories. However, even an object like this would have to be much brighter than the actual brightness of the asteroid.

The most plausible explanation is that the asteroid consists of two small objects, each about 3km across, which are orbiting each other.

We assumed the asteroid was one object. In that case its shadow would not be big enough to fall on both observatory sites (the yellow pin markers). However, if the asteroid is really two smaller objects, it would explain why both sites saw the star disappear.
We assumed the asteroid was one object. In that case its shadow would not be big enough to fall on both observatory sites (the yellow pin markers). However, if the asteroid is really two smaller objects, it would explain why both sites saw the star disappear.


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 2022 there are over 400 asteroids which are known to be binary.

This discovery adds to that list, but we believe it is the second binary asteroid discovered by amateurs. The first was also discovered by us in 2021.


What's Next?

We officially announced the discovery through the Central Bureau for Astronomical Telegrams. See the Publications Page (CBET). The announcement can be seen here (may require an account to access the document).

We will also continue to encourage others to observe future occultaions by the asteroid 2002 YE25 so that the discovery can be confirmed independently and to more accurately measure the size, shape and orbit of the two objects.