Monday 11 August 2014

The nature of Asteroid (175706) 1996 FG3.

Asteroid (175706) 1996 FG3 is an Apollo Family Near Earth Asteroid (i.e. an asteroid with an orbit that crosses that of the Earth, but which is further from the Sun than the Earth for most of its orbit), which was discovered on 24 March 1996 by Robert McNaught at Siding Spring Observatory. The designation 1996 FG3 implies that it was the 82nd asteroid (asteroid G3) discovered in the second half of March 1996 (period 1996 F), while the longer designation (175706) implies that it was the 175 706th asteroid ever discovered (asteroids are not given this longer designation immediately to avoid duplicate sightings and sightings of other, non-asteroid objects). It has an 395 day orbit tilted at an angle of 2° to the plane of the Solar System, which takes it from 0.69 AU from the Sun (69% of the distance at which the Earth orbits the Sun, slightly inside the orbit of Venus) to 1.42 AU (142% of the distance at which the Earth orbits the Sun, slightly outside the orbit of Mars). 

The calculated orbit of (175706) 1996 FG3. JPL Small Body Database Browser.

(175706) 1996 FG3 is a binary asteroid, with a secondary body with 28% of the main body’s mass orbiting the main asteroid every 16.14 hours. The main body is thought to be a complex C-type asteroid (expand) with a mass density of 1.4 g per cm3, which orbits on its access every 3.6 hours. It has an estimated equivalent diameter of 1.9 km (i.e. it is estimated that a spherical body with the same volume would be 1.8 km in diameter), and is thought to be an oblate ellipsoid with a primary access about 20% longer than its secondary access. 

Series of Arecibo images of (175706) 1996 FG3, showing the movement of its satellite in November 1998. NASA/JPL.

(175706) 1996 FG3 has been selected as a backup target for the proposed MarcoPolo-R sample return mission (expand) making understanding the nature of the surface of the asteroid a priority for planetary scientists.

In a paper published on the and on the arXiv database at Cornell University Library on 21 January 2014, and accepted for puplication in the Monthly Notes of the Royal Astronomical Society, LiangLiang Yu of the Purple Mountain Observatory of the Chinese Academy of Sciences and the Graduate School of Chinese Academy of Sciences, and Jianghui Ji and Su Wang, also of the Purple Mountain Observatory, describe the results of a study of (175706) 1996 FG3 intended to better understand the nature of the body.

Yu et al. used observations of (175706) 1996 FG3 made by the Mid-Infrared Spectrometer and Imager at Boston University on 1 May 2009 and by the European Space Agency's Very Large Telescope Imager and Spectrometer on 2 May 2009 and 19 January 2011 to build up an accurate model of the shape of the asteroid from the variations in the amount of light it reflected. This method allows for the construction of an accurate shape model of a body, but does not improve our understanding of its size.

Derived shape model of 1996 FG3 shown from four view angles, i.e., north pole (top left), south pole (bottom left), equator (top right and bottom right), respectively. Yu et al. (2014).

The coordinate system adopted in the model, where O is the center of the asteroid, and z-axis is the spin axis. Yu et al. (2014).

Having done this, Yu et al. proceeded to calculate the thermal inertia (tendency to retain or lose heat) of the surface of (175706) 1996 FG3, which was divided into 2040 facets for this purpose. The thermal inertia of a surface is a good proxy for its makeup and roughness, with finer grained regoliths having lower thermal inertia values; for example the dusty regolith of the Moon has an average value of about 50 Jm¯²s−0.5K¯¹, while the course, sandy regolith of the asteroid Eros has an value of 100−200 Jm¯²s−0.5K¯¹).

Global surface temperature distribution obtained from ATPM at the aphelion and perihelion, respectively. The colour bar indicates the range of temperature, where red for high temperature and blue for low temperature. Yu et al. (2014).

Yu et al. calculated that (175706) 1996 FG3 has a low thermal inertia, less than 100 Jm¯²s−0.5K¯¹, implying a covering of fine-grained regolith. On the other hand, the asteroid appears to have a rough surface, which would tend to suggest a covering of coarser sediment. Yu et al. suggest that this may indicate a rubble-pile structure for the asteroid, with a surface covered in rock fragments and sand, but with a finer layer of dusty regolith between 5 and 20 mm thick covering this.

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