Citizen science discovery during a stellar occultation: Asteroid (5457) Queen’s has a moon!
On September 4, Jan Mánek in the Czech Republic discovered a moon of the asteroid (5457) Queen’s during a stellar occultation. The discovery was confirmed just two weeks later, on September 20, by Serge Dramonis in Greece during a second occultation.
Following analysis of the data by specialist Christian Weber and his team from the European branch of the International Occultation Timing Association (IOTA/ES), the Central Bureau for Astronomical Telegrams of the International Astronomical Union published the discovery in a telegram on November 16th:
The evaluation of the data has shown that Queen’s itself is about 25km x 16km “big” and the moon has a diameter of about 2km.
IOTA/ES has created a website with a press kit for this discovery, which is available at the following link: https://www.iota-es.de/queen.html.
Two things motivated me to write this blog post about my experience of this event. On the one hand, I was lucky enough to also observe a positive occultation during the first observation campaign on September 4, and on the other hand, all three positive observations during the first campaign were recorded with the DVTI+CAM, a video camera specially optimized for stellar occultations, which I co-developed (https://dvticam.com).
I would be happy if this article could motivate other people interested in astronomy to take part in the exciting astronomical field of observing stellar occultations.
The vast majority of asteroids are often not visible to the naked eye even in larger telescopes due to their small size (maximum diameter of a few hundred kilometers, usually much less). The asteroid (5457) Queen’s, for example, had an apparent brightness of just 16.9 magnitudes during my observation. The faintest stars that we can see with the naked eye on a dark night have an apparent magnitude of about 6 magnitudes. On the logarithmic brightness scale, Queen’s was therefore over 20,000 times fainter than the faintest stars visible to the naked eye. Even through the fast 50cm f/5 Newtonian reflector telescope at Bülach Observatory which I used to record the star occultation, an object of this brightness is barely visible to the naked eye.
One way of finding out more about the size, shape and possible moons of asteroids is to send probes past them. For example, the NASA probe Lucy recently visited the asteroid (152830) Dinkinesh. Two small moons were discovered:
Since, according to Wikipedia, well over a million asteroids are already known in our solar system and several thousand new ones are discovered every month, it is of course impractical to study them all using probes. The observation of stellar occultations by asteroids, especially in the context of observation campaigns with several observers, makes it possible not only to determine the orbit and shape of the asteroids with very high precision, but also to discover their moons, and sometimes the occulted stars turn out to be double star systems.
The observers who are ideally distributed along the shadow path each see the star occultation at a different angle (parallax), similar to a solar eclipse where a central, total, partial or no eclipse of the sun can be observed depending on the location on earth.
On the night of September 4, as part of an observation campaign, several stations in Switzerland, the Czech Republic and Poland were registered to observe the occultation of the 12.4 magnitude faint star UCAC4 498–000768 in the constellation “Pisces”.
At my location in the Bülach observatory, the star was 39° above the south-eastern horizon. The sky was reasonably clear and the moon rising in the east did not significantly interfere with the observation. Instead of an eyepiece, a sensitive digital video camera with integrated generation of precise time stamps is mounted on the focuser of the 50 cm Newtonian reflector telescope, which is connected to a computer on top of the ladder via USB. Due to the long exposure time, the camera’s operating LED stands out in red (slightly to the right of the center of the image).
The dark gray rectangle with the red crosshairs on the screenshot is the preview of the video camera. The exposure time of the star occultation camera is set to 20 milliseconds so that the star to be occulted is still just clearly visible in the center of the crosshairs. During the star occultation, the star goes out on the screen, only to reappear shortly afterwards. The shorter the exposure time, the more precisely the time of disappearance and reappearance can be determined. The time stamp, which is integrated into the image directly in the camera, can be seen at the bottom of the preview image. The camera provides the exact start and end time of the exposure for each image. In order to obtain an exact time, the camera is synchronized with multiple navigation satellite constellations (GPS, Glonass, Galileo, Baidu) via an antenna.
Shortly before the occultation, the tension increases. The recording of the video file for the subsequent evaluation of the light curve is started and the star is carefully observed. In this case, the star disappeared briefly once and reappeared after just under a second, as you can see in this video of the recording software that I took with my smartphone:
https://x.com/aschweiz77/status/1698842354205995402?s=20
Each observer evaluates their image as soon as possible after the event. Various software tools support the evaluation by deriving a light curve from the brightness of the star (software packages Tangra and PyMovie) and help to determine the exact start of the disappearance and re-emergence of the star (software packages AOTA in Occult and PyOTE).
The data collected is entered into the SODIS database in a standardized format. Each observation is checked by a country coordinator and then merged across countries. In the case of the asteroid (5457) Queen’s, Christian Weber from IOTA/ES carried out this comprehensive evaluation. The graph below shows the summarized data.
The individual observations appear as chords in the shadow profile. My recording from Bülach (Canton of Zurich, Switzerland) is marked with the number 1 and just caught the edge of the asteroid. Stefan Meister was in Eglisau (Zurich, Switzerland) a little deeper in the shadow and saw a longer occultation (chord 5). Finally, Jan Manek (6, 7, 8) near Ondrejov (Czech Republic) caught both the asteroid and the moon. The observers Michael Rottenborn, Daniel Antuszewicz and Jiri Kubanek were unable to see an occultation. Nevertheless, these “negative” observations also help, namely by setting a limit on the size of the asteroid.
During a second occultation by the asteroid (5457) Queens on September 20, Serge Dramonis in Greece again observed a double extinction of the star and thus confirmed the discovery of the moon.
What fascinates me so much about observing stellar occultations is that the equipment available to amateur astronomers is totally sufficient. It’s not necessary and only possible in very few cases to see the asteroid itself, but it is enough to be able to recognize the star to be occulted. I often observe at home from the backyard with my 10.7cm Borg refractor. Thanks to the high sensitivity of the DVTI+CAM, I can capture stars with an apparent magnitude of less than 13 magnitudes. In addition, the observation doesn’t require a completely clear sky and is also possible in moonlight. In addition, there is of course the moment of suspense as to whether the star really disappears at the time of the occultation and you can report a “positive” occultation, or whether you have to report a “negative” result.
I hope that this blog post has motivated some of you to try observing a stellar occultation. If, for example, you enjoy astrophotography and the conditions are not perfect, this would be an exciting alternative. The easiest way to find out which star occultations are taking place at your location is to use the “Occult Watcher” software, which you can download from here. You can also find individual star occultations on the IOTA/ES website under “Call for observations”.
Thank you for reading!
PS: You can find all my star occultations on my website (http://aschweiz.ch/occultations/). I also post star occultations “live” on X (formerly Twitter) from time to time, including this star occultation of Queen’s on September 4 (https://twitter.com/aschweiz77/status/1698815607737667682). Follow me if you are interested in this topic!
