The intriguing TRANSNEO population: leading the way for the DESTINY+ mission

The classical distinction between asteroids (rocky and inert), and comets (ice-rich and active) has been blended in the last 15 years, leading to a more nuanced picture. Both classes are now believed to be simply end-members of a physical and dynamical continuum [1]. In particular, transient objects show the characteristics of both classes (asteroids exhibiting intense activity or comets in asteroidal orbits) and they have often double cometary/asteroidal designation.

We started a project called TRANSNEO, devoted to the physical characterization of TRANSient NEOs (Near-Earth Objects showing characteristics of both classes). We targeted TRANSNEO because: i) their repeated passages around the Sun make it in principle easier to detect a potential activity; ii) their proximity with Earth makes them accessible for observations and future space mission; iii) activity on NEO surfaces has been recently discovered even on apparently inactive places, thus attracting the interest of various space agencies. The study of their physical attributes (colors, spectra, activity…) is very promising, showing intermediate characteristics between asteroids and comets. This could strengthen furthermore the idea of a “continuum of small bodies” paradigm, with TRANSNEOs potentially be the missing link to understand the asteroid-comet transition.

Indeed, one of these extremely intriguing bodies is (3200) Phaethon, which will be the target of the future DESTINY+ mission. This JAXA-financed mission will perform a high-speed (36 km/s) flyby of Phaethon at a distance of approximately 500 km. During the flyby, physical and chemical properties of the dust near Phaethon will be measured with a dust analyzer (DDA) [2]; surface characterization will be conducted with a Telescopic Camera (TCAP) and Multiband Camera (MCAP) to better understand the active asteroid surface compositional variation [3].

Phaethon is a very interesting object. Originally discovered as an asteroid in 1983, over the past three decades the situation regarding its physical nature has become less and less clear. It has been identified as an active asteroid, with recurrent dust ejecting phenomena near its perihelion [4]. Moreover, it has an extremely small perihelion distance (q = 0.14 au) and an unusually large eccentricity (0.89), making it one of the largest (> 6 km) near-Sun and near-Earth objects. Due to the extreme temperatures experienced on Phaethon’s surface, particularly at perihelion (up to 1000 K), it is possible that certain minerals, unstable at high temperatures, become volatile, causing dust ejection at small heliocentric distances [5]. Several mechanisms have been proposed over the years to explain Phaethon’s activity, or they are reasonably plausible based on the extreme thermal environment. Among others: volatilization of some elements [6], thermal fracturing [7], meteoroid collisions [8], radiation pressure [9]. Thermal fracturing should be more efficient for larger rocks (i.e. boulders) present on its surface, and could be more relevant in the northern equatorial regions, where boulders should be more abundant. Finally, it is also possible that small particles are lost via radiation pressure, and transported from northern areas to southern latitudes. Recently, using thermophysical modeling, [10] found evidence of potential different grain sizes on Phaethon surface: the southern hemisphere should be dominated by fine-grained material, while the northern hemisphere is probably abundant in coarse-grained regolith and boulders. 

In order to unveil the potential mechanism beyond Phaethon’s activity and assist the proper design of the DESTINY+ mission (i.e. identify the best regions to investigate during the flyby) we obtained new spectral observations during the last year from TNG, NOT and IRTF. During this window, Phaethon was observed close to the equatorial latitudes, up until the southern hemisphere, while during the previous passages the sub-observer's latitude was much closer to the northern pole. We will present this new data, and compare them with previously available spectra in literature, in order to tackle Phaeton’s variability, and put these results in the wider context of other TRANSNEO characterizations made by our group.

 

References:

[1] Jewitt & Hsieh (2024) in Comets III [2] Kobayashi M. et al. (2018), 49th LPSC, #2050 [3] Ishibashi K. et al. (2018), LPSC 49th, #2126 [4] Jewitt, D. (2013), AJ, 145, 133 [5] MacLennan, E. M. et al. (2021), Icarus, 366, 114535 [6] Springmann, A. et al. (2019), Icarus, 324, 104 [7] Delbo, M. et al. (2014), Nat., 508, 233 [8] Szalay, J.R. et al. (2019), PSS, 165, 194 [9] Bach, Y.P. & Ishiguro, M. (2021), A&A, 654, A113 [10] MacLennan et al. (2022), Icarus, 388, 115226