The movement of asteroid Phaethon imaged on Dec. 25, 2010 with the 37 cm F14 Cassegrain telescope of Winer Observatory, Sonoita (MPC 857) by Marco Langbroek. The image is a stack of 4 images of 150s exposure each, spaced 15 minutes apart. Image credit: Marco Langbroek
Discovered in 1983, the near-Earth asteroid Phaethon is an intriguing object, primarily due to its unusual orbit. Recently, an international team has conducted a detailed study of this unique space rock, deriving a shape model that characterizes the orbit, spin state, and thermophysical parameters of the asteroid.
Phaethon is an Apollo asteroid, though it is also suspected to be a member of the Pallas family of asteroids. It approaches the sun more closely than any other named asteroid—its perihelion is only 0.14 AU (21 million km). The scientific community believes that this object is the parent body of the Geminid meteor shower seen on the sky every mid-December. For this reason, researchers are trying to detect any activity around this asteroid associated with dust particles being ejected from its surface. However, detecting the activity from this object has always been challenging, because of its close approaches to the sun and also due to its small elongation.
“Phaethon’s elongation is rather small, thus its light curves have small amplitudes of about 0.1 magnitude,” Josef HanuÅ¡. Shape modeling requires photometric data that sample various observing geometries as it is necessary to look at the asteroid from different directions. Thus, data spanning decades are usually required. Therefore, HanuÅ¡ and his team gathered a total of 55 dense-in-time light curves of Phaethon obtained between 1994 and 2015. They have also acquired their own 29 new light curves with various telescopes worldwide.
“Data from each apparition are equally important and they all contributed to the shape model determination. We used light curves obtained with 9 different telescopes, and all of them were important. The largest and most impressive telescope we used was the Uni of Hawaii 2.2m telescope on Mauna Kea,” HanuÅ¡ said. Based on these light curves, the scientists were able to derive a unique shape model of Phaethon. According to their estimates, the sidereal rotation period of this asteroid is about 3.6 hours and its size is about 5.1 km. The team also managed to measure the asteroid’s thermal inertia, geometric visible albedo and the macroscopic surface roughness.
“We contributed with reliable orbital, physical and thermophysical characterization. Thus we now know the past dynamical evolution of Phaethon’s orbit and its rotation axis direction, its size, rotation period, spin axis orientation, reflectance like the geometric visible albedo of the surface, and its thermal inertia, which corresponds to the resistance of the material to the temperature changes,” HanuÅ¡ noted. These values presented in the new study are consistent with previous estimates.
In Dec 2017, it will make a close approach to the Earth at 0.069 AU (~10M km). During this event, the asteroid will be observed with multiple techniques and a large amount of data will be obtained about this intriguing space rock. For now they would like to reveal the true nature of the asteroid’s composition and shed more light into the proposed Phaethon-Geminid-Pallas relationship.
http://phys.org/news/2016-05-astronomers-intriguing-near-earth-asteroid-phaethon.htmljCp
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