The near-Earth asteroid Apophis fascinates because it regularly passes close to Earth : for example at 16.8 million km in March 2021, or of course in April 2029 at only 31,000 km from the Earth’s surface (i.e. 12 times closer than the Moon). Its trajectory is therefore closely monitored since its discovery in 2004.
On March 7 and 22, 2021, two stellar occultations by Apophis were observed for the first time, in the United States, at the initiative of J. Desmars (IMCCE/IPSA/Obs.Paris), D. Souami and B. Sicardy (LESIA/Obs.Paris), B.Morgado (Obs. Nacional and LIneA, Brazil) and F. Braga-Ribas (UTFPR, Brazil) in the framework of the ERC Lucky Star project [1], in collaboration with P. Tanga (OCA), K. Tsiganis (Univ. Thessaloniki, Greece), and several American teams of professional and amateur astronomers.
Stellar occultations not only allow us to determine the size and shape of small bodies in the solar system to sub-kilometer accuracies, but also to give extremely precise positions.
Predictions of stellar occultations, however, require a good knowledge of the motion of the body and the position of the star, as well as a dense network of telescopes along the occultation path.
For an object like Apophis ( 380 m diameter only !*), a detection was unthinkable without the precision of the Gaia catalog and the expertise of the Lucky Star team. Moreover, at the beginning of March, radar measurements contributed to further improve the accuracy of the orbit. Finally, the participation of amateur astronomers equipped with mobile telescopes was once again decisive in the success of these observations.
In total, 3 stations recorded the event of March 7 and one station recorded the one of March 22**, which lasted less than 0.1 second. Apophis thus becomes the first object of a few hundred meters to be observed by stellar occultation. The deduced positions are complementary and equivalent in precision to radar observations, with the advantage of being much less expensive.
Detection of the Yarkovsky effect
Beyond ruling out any risk of impact for the next 100 years, these observations have also made it possible to measure very weak accelerations of Apophis’ motion due to the Yarkovsky effect (a very small force due to the thermal emission of the body), the latter having a preponderant role on the dynamics of the object (trajectory and future close encounters with the Earth). The scientific repercussions are thus unprecedented.
The success of these occultations announces a new era in the study of near-Earth asteroids. After the successful stellar occultations by the NEO Phaeton ( 6 km in diameter) in 2019 and the Apophis occultations in March 2021, other occultations by asteroids such as the NEO Didymos, target of the DART (NASA) and Hera (ESA) missions, are now fully conceivable.
* This is equivalent to measuring the size of a one euro coin at a distance of 1,000 km.
** A new detection was successful on April 4, during a campaign in the United States.
[1] ERC Lucky Star : https://lesia.obspm.fr/lucky-star/