Cosmic Peanut Points to Ancient Mystery

Artistic representation of the Trojan asteroid Hektor showing its dual shape Hektor and its tiny moon.  Credit: H. Marchis & F. Marchis
Artistic representation showing the Trojan asteroid Hektor and its tiny moon. Credit: H. Marchis & F. Marchis

Our solar system is full of strange objects. One particular asteroid dubbed 624 Hektor, the largest known “Trojan” asteroid, takes the prize.

SETI Institute astronomers now report that not only does the asteroid have a weird peanut shape and sport its very own moon, but it may also have been born of an ancient, unlikely collision between two icy asteroids.

Trojans are a unique breed of asteroids that are particularly difficult to study because they are quite faint, small, and orbit 60 degrees ahead and 60 degrees behind the planet Jupiter. Jupiter and 624 Hektor follow orbits some 483 million miles (778 million kilometers) away from the sun, roughly five times more distant from the sun than Earth is.

Eight years of painstaking observations, conducted in part with the mighty Keck Observatory located on the summit of Mauna Kea in Hawaii, have finally been able to reveal that a 7.5-mile-wide (12-kilometer-wide) moon orbits 624 Hektor once every three days.

Illustration showing the orbit and location of asteroid Hektor as of Feb 28, 2014. Credit: SkySafari Simulation Curriculum
Illustration showing the orbit and location of asteroid Hektor as of February 28, 2014. Credit: SkySafari Simulation Curriculum

The bizarre, double-lobed shape of the asteroid, combined with the complex orbit of its moon, now suggests to astronomers that this system is a result of an ancient smash-up between two large, icy asteroids during the solar system’s youth. In that era, the solar system was a much more chaotic place, filled with giant planets and their moons on the move.

With orbital and photometric observations in hand, the research team was able to construct computer models to re-create the cosmic accident that spawned the asteroid, billions of years ago.

The massive collision must have fused two primordial asteroids, resulting in the double-lobed appearance of Hektor. The debris thrown out from the impact coalesced to form the tiny moon.

The asteroid study is being published this week in Astrophysical Journal Letters.

See for Yourself

While Hektor and its Trojan cousins are too faint for most backyard telescopes to pick up, there is another, much larger and brighter asteroid making an appearance in the night sky this week.

Measuring some 338 miles (544 kilometers) in diameter, 2 Pallas is one of the largest members of the asteroid belt between the planets Mars and Jupiter, and it shines at seventh magnitude, making it an easy target for binoculars even from bright city suburbs around the world.

Making your hunt easier, on March 2 the space rock will be passing only 3 degrees east of the star Alphard (which can be seen with the naked eye) in the constellation Hydra, the Snake, in the southeastern evening sky.

skychart showing the location of asteroid Pallas near the star Alphard in the constellation Hydra on March2, 2014. Credit: SkySafari by Simulation Curriculum
Skychart showing the location of asteroid Pallas near the star Alphard in the constellation Hydra on March 2, 2014. Credit: SkySafari by Simulation Curriculum

Because many of the stars in the field of view through your binoculars can look the same, the best way to identify an asteroid is by its telltale motion. Sketch the position of about a dozen stars you see on a clear night, to start.

A couple of nights later, observe the same star field once more and make the same sketch. The one “star” that has moved is Pallas.

Follow Andrew Fazekas, the Night Sky Guy on Twitter.

Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.

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