A Unistellar team traveled to Oman in early September to observe the occultation of an 11-mag star by the Trojan asteroid Orus. We succeeded in capturing the event with one of our eVscopes, an achievement in and of itself considering the small and uncertain path of this occultation. This observation is a breakthrough because this detection will be used for another occultation by Orus, coming up soon in Australia. On top of that, this positive detection heralds the arrival of a new tool (the eVscope network) for the study of asteroids, centaurs, and trans-Neptunian objects by occultations.
Lucy is a Discovery-class mission scheduled for launch by NASA in 2021. Its goal is to study primitive asteroids by conducting a flyby of five Trojan asteroids and one main-belt asteroid. Last year, Cathy Olkin, deputy principal investigator of the Lucy mission, sent me a list of occultation events involving those targets. One target, in particular, got my attention because it included a bright star (V=11) and an event observable on Earth from a narrow ~50 km-band stretching from Oman to Brazil—with, however, an uncertainty of at least 200 km (the red line in the figure below).
In the months before the event, I communicated with the Lucky Stars team at the Observatoire de Paris, as well as Marc Buie from the Southwest Research Institute (SWRI), in hopes of getting more refined and accurate predictions of the event. On August 15, Marc sent me an email with a new forecast that he had calculated, which combined a precise position for Orus using the Gaia DR2 catalog, Pan-Starrs, and several observations from SWRI robotic telescopes. The good news was that the uncertainty had decreased and that the band would pass through Oman.
The eVscope and its network are not just an innovative way to enjoy the dark sky, but also a great instrument that anybody can use to conduct observations that support professional astronomical research. To demonstrate the potential of the recently manufactured eVscope, we decided to send a Unistellar team to Oman to observe this event. Arnaud Malvache, president and CTO, Laurent Marfisi, CEO, and Alexis Martin, AI software engineer, volunteered for the trip and flew to Oman with two eVscopes a few days before the occultation.
As predicted by Marc Buie, on the night of September 6, we located one station in Khalil and a second one at Mussannah. We used a prototype of the eVscope’s occultation mode that was implemented and tested over the summer in our onboard software. This mode records consecutive frames at short cadences (200 ms) a few minutes before and after the predicted time of the event. It then determines the exact time when those frames were recorded using its phone connection.
A posteriori analysis of the data revealed that the station located in the middle of the predicted path observed a blinking of the star that lasted ~4 s (see video below). In contrast, the one located at the edge did not see any change in brightness, implying that the occultation had indeed happened as predicted.
I presented this result at the EPSC/DPS conference in Geneva in early October, which also allowed me to meet some of the SWRI team and show them the eVscope in action. We also talked about the future of occultation and how we could continue to support Lucy in the future.
From the Oman data, we calculated a projected size of 54.8 km for the asteroid, which agrees with the size estimates of Orus reported elsewhere. More importantly, we now have an excellent estimate of Orus’ position and can better predict the path of its future occultations.
This is breakthrough information that will be put to use very soon. On November 4, Orus will occult a 12-mag star in northern Australia. Analysis by my colleague Josselin Desmars, an astronomer at the Observatoire de Paris, clearly shows improvement in the prediction of the path thanks to our successful occultation. That path has now shifted north by ~100 km, and more importantly, the margin of error (in red in the figure below) is reduced substantially.
Marc Buie and his team will attempt to observe this occultation by Orus. This time, they will deploy fourteen stations across the band to guarantee that multiple stations detect the occultation, which in return will give astronomers information about the shape of the Orus asteroid and the presence of moons if any. We are eager to see their results.
We’re preparing to implement Occultation Mode in the eVscope’s software to facilitate observation of these events and allow all eVscope users to use it to become citizen astronomers. Our calculations have shown than more than forty occultations are observable every day on Earth using the eVscope.
In partnership with IOTA, Obs. de Paris, NASA/SWRI and other organizations, we will prepare future occultation campaigns focusing on interesting small Solar System bodies such as distant centaurs and trans-Neptunian objects, large asteroids in the main-belt, and, of course, targets of future space missions. For instance, on December 29, there is a very interesting occultation by Leucus, another Lucy mission target. The error bars are still significant (several thousand km), but new observations may reduce it drastically, and by then, we should be ready to catch this challenging but rewarding event. With our small but mighty network of eVscopes, each of us will able to help NASA plan and deploy the next Discovery mission. How cool is that?
Acknowledgements: First of all, thanks to the fantastic Unistellar team, especially Arnaud Malvache, Laurent Marfisi, Alexis Martin, Emmanuel Arbouch, and Thomas Bellier who contributed to the success of this campaign with their time and energy. We are grateful for the help of Mohammed Alajmi, one of our backers in Oman, who guided our local team and showed them the beauty of his country and its hospitality.
