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First Known Extrastellar Asteroid: Oumuamua


In October, a strangely shaped asteroid shot through our solar system and is now leaving at roughly 95,000 km/hr, soon to leave us far behind.  Calculations say it is roughly thirty meters in diameter and 140-400 m long; it is rotating around its long axis, producing a 10-1 difference in apparent brightness over a 7.4 hr rotational period.  It appears to be made of rock or iron and have the overall density of water, suggesting that it is nearly solid.  Its color is a dark red, attesting to a prolonged bombardment by interstellar cosmic rays.  It is guessed that the object has been underway from another solar system for the last 40 million years, giving it plenty of time to acquire its reddish sheen from surface alteration by cosmic rays.

The speed of this object is such that no human-built craft could catch up with it, even if it were launched in a timely fashion.  Our Voyager spacecraft are currently leaving the solar system at roughly 35,000 km/hr, a third of this object’s speed.

This object (asteroid) is unusual in several ways.  First, it entered the solar system from high above the plane of the planets, passed close to the sun and earth, and exited at a very different angle due to the sling-shot effect of passing by the sun.  Second, it is quite elongated, shaped somewhat like a cigar.  Third, it is quite dense and appears not to be outgassing at all– unlike a comet, which releases quantities of gas as it passes the sun and leaves a coma, or shroud and tail of gas behind.  The object is classed as an asteroid, a solid that doesn’t have significant amounts of water or gas.

Speculation as to the origin and nature of this object is intense, but the most likely fact is that it arose in a solar system far away and was naturally created during the formation of that system.  It was probably kicked out of its home system by gravitational interactions with planets and other objects that threw it out of a normal, elliptical orbit into an interstellar journey.  The reason for its odd shape is completely open to discussion.

The object has been given the name Oumuamua, which is Hawaiian for “messenger from afar arriving first.”

Videos of the object can be found at and an article in New Scientist is at

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