Hazelbrook Observatory

Blue Mountains, NSW, Australia

Discovery of Binary Nature of Asteroid 2002 YE25

All Discoveries

On May 16 2022 Dave Gault and Peter Nosworthy observed an occultation of a magnitude 11 star by the asteroid (172376) 2002 YE25. The screenshot below shows the relative cross-path locations of the observers relative to the prediced shadow path. Note that the cross-path distance between Dave and Peter's sites is wider than the expected width of the shadow (Asteroid Path). For this reason we expected that only one of us could observe the star being occulted.

The map (left) also shows that the shadow path (blue lines) is narrow enough to fit between our two observing sites which are at the locations of the pins.

Light Curves

At right are the light curves from our observation. Both curves show clear occultation events. We then had to explain how this could happen given that the asteroid was too small for the shadow to cover both our sites.

Sky Plane Plots

The size of the asteroid measured by the NOEWISE Space Telescope was 5.6km. This is a sky plane plot assuming a spherical object of that size.

Obviously this does not fit our data as the object is too small.

The object would have to be highly elongated to fit out observations as shown here. However this object would be larger than the NEOWISE measurement and it would also show large changes in brightness as it rotates. However, the asteroid does not vary significantly in brightness.

Therefore we conclude that the occultations were caused by two smaller objects, each about 4km in diameter.

This illustrates what we believe happened. Two objects of about 4km each, caused the occultations we observed with one shadow passing over each of our sites - indicated by the yellow pins.

CBET Published

This was enough evidence to be confident we had detected a binary object. We submitted a CBET (Electronic Telegram) announcing our discovery and it was published on 18 July 2022.

Paper Published

We submitted a paper to the Minor Planet Bulletin which was published in January 2025.