5:15 PM - 7:15 PM
[PPS03-P15] Determination of the Spectral Type of the Near-Earth Asteroid “2024 MK” Based on Polarimetric Observations During Its Close Approach Within the Lunar Distance
Keywords:Asteroid, Near-Earth Asteroid, Planetary Defense, Polarimetric observation, Ground-based observation
With advancements in observational instruments and technology in recent years, the number of discovered asteroids has reached 1.4 million, among which the number of Near-Earth Asteroids (NEAs) with orbits close to Earth is approaching 40,000. However, many asteroids remain undiscovered, and even for those that have been identified, their composition and characteristics are still not fully understood. In particular, since NEAs pose a potential impact threat to Earth, it is crucial to determine their orbits, sizes, and compositions in advance from the perspective of Planetary Defense. This is essential for collision avoidance and for estimating the potential scale of the hazard.
The observation target, 2024 MK, was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on June 16, 2024. Approximately two weeks later, on June 29, it passed about 75 % of the lunar distance, and by July 2, it reached a maximum phase angle (the angle between the Sun, the Target asteroid, and the Earth) of 101 degrees. Given the rare opportunity to observe a large ~140 m diameter asteroid at such close proximity, the observers conducted two nights of visible-wavelength Polarimetric observations using the 1.6 m Pirka Telescope and its MSI imaging instrument [Watanabe et al., 2012], owned by the Graduate School of Science, Hokkaido University, and located in Nayoro, Hokkaido. Observations were conducted on June 29, during its closest approach, and on July 2, when the phase angle reached its maximum. This was the first attempt to perform Polarimetric observations of an asteroid of this size and at such a distance.
The polarization degree obtained from the observations in this study exhibited characteristics similar to those of carbonaceous C-type asteroids, which are relatively rare among NEAs. On the other hand, a previous study that conducted Spectroscopic observations at the same time concluded that 2024 MK is a silicate-rich S-type asteroid, which is more common among NEAs [McGraw et al., 2024]. This study discusses the discrepancy between these results.
The observation target, 2024 MK, was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) on June 16, 2024. Approximately two weeks later, on June 29, it passed about 75 % of the lunar distance, and by July 2, it reached a maximum phase angle (the angle between the Sun, the Target asteroid, and the Earth) of 101 degrees. Given the rare opportunity to observe a large ~140 m diameter asteroid at such close proximity, the observers conducted two nights of visible-wavelength Polarimetric observations using the 1.6 m Pirka Telescope and its MSI imaging instrument [Watanabe et al., 2012], owned by the Graduate School of Science, Hokkaido University, and located in Nayoro, Hokkaido. Observations were conducted on June 29, during its closest approach, and on July 2, when the phase angle reached its maximum. This was the first attempt to perform Polarimetric observations of an asteroid of this size and at such a distance.
The polarization degree obtained from the observations in this study exhibited characteristics similar to those of carbonaceous C-type asteroids, which are relatively rare among NEAs. On the other hand, a previous study that conducted Spectroscopic observations at the same time concluded that 2024 MK is a silicate-rich S-type asteroid, which is more common among NEAs [McGraw et al., 2024]. This study discusses the discrepancy between these results.