Visible Spectroscopic Characterization of Eight Binary Near-Earth Asteroids

Introduction

 

Near-Earth asteroids (NEAs) are asteroids with perihelion distances < 1.3 AU, whose orbits bring them close to Earth (Kokhirova and Babadzhanov 2023). They provide a unique sample of Solar System material, often transported to the inner regions after millions of years of gravitational interactions with the planets. 

Binary asteroids or binaries are systems composed of at least two bodies gravitationally bound together (Margot 2015). A substantial fraction of NEAs is found in binary systems: approximately 15% of NEAs larger than 200 m in diameter host a satellite (Pravec et al. 2006; Margot et al. 2002). Given that approximately 11,000 NEAs with these characteristics are currently known, it is estimated that around 1,600 of them could be binary systems. Recent discoveries related to the population of binary asteroids continue to increase. 

 

Context

 

Binary systems can form through various mechanisms: collisions, tidal effects during close planetary encounters, spin-up and subsequent fission due to the YORP effect, or rotational instability (Margot 2015). Each asteroid presents unique physical characteristics, differing in size, shape, chemical composition, density, and surface structure (DeMeo et al. 2009). Taxonomic classification and the mineralogical study of asteroids are essential tools for understanding the heterogeneity of these bodies and tracing their evolutionary history.

Their short distance to the Earth gives more easy access to the study of their different physical and compositional characteristics, and formation mechanisms. These properties are way more difficult to determine in furthest main-belt asteroids. This may deepen our understanding of collisional processes and gravitational interactions and estimate mass and density (Merline et al. 2002 , Margot 2015). These properties, along with the taxonomic classification and mineralogical characterization, increase our knowledge of the asteroid population in the Solar System. 

Beyond their scientific value, binary asteroids have become important in the context of planetary defense, as demonstrated by the Didymos binary system. This system is the primary target of the international collaboration AIDA (Asteroid Impact and Deflection Assessment, Cheng 2015), which includes NASA's DART mission (Double Asteroid Redirection Test; Rivkin et al. 2021) and ESA's Hera mission (Michel et al. 2022).

Aim

 

This work presents the preliminary spectroscopic analysis and taxonomic classification of eight binary NEAs: (137170) 1999 HF1, (539940) 2017 HW1, (85804) 1998 WQ5, (420302) 2011 XZ1, (35107) 1991 VH, (385186) 1994 AW1, (175706) 1996 FG3, and (65803) Didymos.

In this list of targets, three very well known objects appear. Didymos, the asteroid impacted by NASA's DART mission, will be further studied by ESA's Hera mission, which is scheduled to investigate the binary system in early 2027. In addition to Didymos, other relevant targets such as 1991 VH and 1996 FG3 are included in this work. These objects exhibit low delta-V values and may be potential candidates for future space missions. 

 

Methods

 

Observations were conducted as part of the NEOROCKS (NEO Rapid Observation, Characterization and Key Simulation) project (funded by ESA) using visible spectroscopy with the 1.22 m Galileo Telescope (Unipd) and the 1.82 m Copernico Telescope (INAF-OAPD) at Asiago Observatory. Spectra were obtained through low-resolution spectroscopy in the wavelength range from 4500 to 9500 Å. Data reduction was performed using the Image Reduction and Analysis Facility (IRAF) software.

For each object, normalization was computed at 6000 Å, ensuring that the result is less influenced by noise. The spectral slope was computed, and a chi-square comparison was carried out with reference taxonomic curves from Bus and DeMeo (2009). The best-fitting meteorite analogs were identified using the M4AST tool and the RELAB database (see Fig. 1). Figure 1 shows the spectrum of asteroid 1991 VH in cyan and that of asteroid 2017 HW1 in blue, as an example. The two dashed curves correspond to the Cb taxonomic curve, with the associated meteorite analog being a C1 Carbonaceous Chondrite (shown in purple).

 

Results

 

Preliminary results reveal taxonomic diversity among the studied asteroids: two are classified as Cb-type (1999 HF1, 2017 HW1, associated with C1 Carbonaceous Chondrites), one as O-type (1998 WQ5, associated with Anomalous Ureilite Achondrite), one as Cg-type (2011 XZ1, associated with CM Unusual Carbonaceous Chondrite), one as Sq-type (1991 VH), one as Q-type (1994 AW1), one as Cb-type (1996 FG3, associated with C1 Carbonaceous Chondrite), and one as S-type (Didymos, associated with Ordinary Chondrite).

 

Conclusions

 

With this work we will present the preliminary results of the taxonomic and mineralogical classification of eight binaries. This characterization increases the information on the chemical composition of these objects. By improving our understanding of their physical and compositional properties, this study offers some insight into the characteristics of near-Earth asteroids, which may support future research on these binary objects.

References:

• G. I. Kokhirova and P. B. Babadzhanov, Current Knowledge of Objects Approaching the Earth, Solar System Research 57, 467 (2023).
• P. Pravec et al., Photometric survey of binary near-Earth asteroids, Icarus 181, 63 (2006).
• J. L. Margot et al., Binary Asteroids in the Near-Earth Object Population. Science 296, 1445-1448 (2002)
• J. L. Margot et al., Asteroid Systems: Binaries, Triples, and Pairs, Asteroids IV (2015).
• W. J. Merline, Asteroids Do Have Satellites, Asteroids III, W. F. Bottke Jr., A. Cellino, P. Paolicchi, and R. P. Binzel (eds), University of Arizona Press, Tucson, p.289-312 (2002).
• A. Cheng and P. Michel, Asteroid Impact and Deflection Assessment mission: the Double Asteroid Redirection Test (DART), European Planetary Science Congress 2015.
• A. S. Rivkin, The Double Asteroid Redirection Test (DART): Planetary Defense Investigations and Requirements, the Planetary Science Journal, Volume 2, Issue 5, id.173, 24 pp. (2021).
• M. Patrick, The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos, The Planetary Science Journal, Volume 3, Issue 7, id.160, 21 pp. (2022).
• F. E. DeMeo, An extension of the Bus asteroid taxonomy into the near-infrared, Icarus, Volume 202, Issue 1, p. 160-180 (2009).

Figure 1 Spectra of asteroid 1991 VH (cyan) and asteroid 2017 HW1 (blue). The two dashed curves correspond to the Cb taxonomic curve, with the associated meteorite analog being a C1 Carbonaceous Chondrite (purple).