Asteroid Phaethon less reflective than previously thought

Target of 2022 Japanese flyby is not well understood.
By Laurel Kornfeld | Nov 07, 2018
A near-Earth asteroid that is the parent body of the annual Geminid meteor shower has been found to be less reflective than scientists originally thought.

Discovered in 1983, asteroid Phaethon is the target of the Japan Aerospace Exploration Agency (JAXA) DESTINY+ flyby mission scheduled for launch in 2022. Plans call for the probe to photograph the asteroid and provide data for astronomers about its surface geology.

In a recent study, an international team of astronomers, including some from the National Astronomical Observatory of Japan (NAOJ) observed Phaethon using the 1.6-meter Pirka Telescope at Nayoro Observatory in Hokkaido, Japan. The researchers observed changes in the polarized light reflected by the asteroid at various angles of illumination. They found that Phaethon reflects light at different angles, suggesting its surface reflects less light than scientists thought.

When sunlight is reflected off an object such as an asteroid or planet, the light waves create changes in their electric and magnetic fields. If those changes are aligned rather than random, the light is described as polarized.

Interestingly, at some angles, light reflected by Phaethon is more polarized than that reflected by any other known small solar system objects.

This could indicate that Phaethon, whose surface is covered in loose rubble, is darker than scientists expected. Light hitting an asteroid can be reflected off multiple surface regions before being reflected to an observer.

"If the albedo (percentage of light an object reflects) is lower than previously thought, that would reduce the effectiveness of multiple scatterings, so that strongly polarized light that has only been reflected a single time would dominate," explained Takashi Ido, a researcher at NAOJ.

Phaethon's surface temperature can reach 1,000 degrees Celsius at perihelion, hot enough to start a process known as sintering, which produces coarser and larger surface sand grains.

Unlike most meteor shower parent objects, Phaethon is not a comet. It has been seen ejecting dust from its surface and has an unusual blue color.

Findings of the study have been published in the journal Nature Communications.


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