Astronomers studying the near-Earth asteroid (3200) Phaethon have discovered that the strange blue body reflects light in a mysterious way.

A team of scientists in Japan and South Korea has found that Phaethon, which came closest to Earth in mid-December last year, reflects much less light than they previously thought.

The findings constitute a brand new mystery that scientists hope to unravel in 2022, when the Japan Aerospace Exploration Agency launches its DESTINY mission to fly by the asteroid.

How Phaethon Reflects Light

Astronomers from the National Astronomical Observatory of Japan used the 1.6-m optical and infrared Pirka Telescope at the Nayoro Observatory in Hokkaido to examine how the surface of Phaethon reflects light. The team also includes experts from Seoul National University and the Chiba Institute of Technology in Narashino, Japan.

By studying the polarization of light when it strikes the surface of the asteroid, the researchers observed that Phaethon reflects far less light than most small bodies in the solar system. In fact, at some angles, the light that strikes Phaethon is the most polarized light that astronomers have ever seen in the solar system.

The study adds another facet to the mystery that is the Phaethon asteroid. When astronomers first discovered Phaethon in the 1980s, they were surprised to find that the surface of the asteroid was blue.

Phaethon is also believed to be the parent body of the Geminid meteor shower, which peaks in the first half of December every year. Most meteor showers come from comets, but Phaethon does not display typical comet characteristics.

The asteroid came closest to Earth on Dec. 16, 2017, at a distance of 6.4 million miles. It is not expected to come back in 75 years in 2093. By that time, Phaethon will skim close to Earth at a little more than 1 million miles.

How Polarization Of Light Works

When an object reflects light, a few things come into play. First, the albedo, or the amount of it light the object can reflect, affects the way light interacts with the object. Second, the angle from which light strikes the object also has an effect.

In the new study, which is published in the journal Nature Communications, the researchers goal was to observe how light is polarized when it reflects off the surface of the asteroid.

Visible light is a part of the electromagnetic spectrum, which means the light humans can see is actually made of electric and magnetic waves. These waves can vibrate in random directions or they can be aligned in a single direction. When waves of light are aligned in one direction, it means the light is polarized.

On Phaethon, the waves of light reflecting its surface are highly polarized, which means there is far less light being reflected off the surface of the asteroid.

Mystery On An Asteroid

The researchers are unsure about the mechanisms that could explain why light on the blue asteroid is polarized. Asteroids are blanketed in a rough carpet of cosmic debris. When light hits the surface of an asteroid, the rubble distributes the light so that the waves go off in random directions. One possible explanation is that Phaethon's surface may be darker than expected.

"If the albedo is lower than previously thought, that would reduce the effectiveness of multiple scatterings," explains lead author and assistant professor at the Astronomy Data Center Takashi Ito, "so that strongly polarized light that has only been reflected a single time would dominate."

It is also possible that the surface of Phaethon is made of larger grains. When the asteroid travels close to the sun, it can heat up to as much as 1,800 degrees Fahrenheit. The extreme temperatures create fissures on the surface that are similar to mud cracks on a dry lake bed.

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