In 2017 a visitor from another star system raced through ours.
ʻOumuamua was the first object ever confirmed to come from beyond the Solar System. It was spotted in October 2017, already on its way out. It was oddly elongated, had no comet tail, and sped up slightly as it left. Astronomers had only weeks to study it before it faded from view.
On 19 October 2017 the physicist Robert Weryk, using the Pan-STARRS telescope in Hawaii, spotted a faint object about 33 million kilometres from Earth. It had already swung past the Sun about 40 days earlier. Its path was the giveaway: its orbit was so strongly hyperbolic that it could not be bound to the Sun, and no encounter with any planet could have flung it that fast. It had come from interstellar space and was going back there. It was designated 1I, the first of a new class, and named ʻOumuamua, from a Hawaiian word for a scout or messenger.
It is small, somewhere between about 100 and 1,000 metres long and far narrower, and reddish like objects in the outer Solar System. Many models suggest it is more elongated than almost any natural body seen in the Solar System, and it appeared to tumble rather than spin neatly. It approached from roughly the direction of the star Vega and was travelling at about 26 kilometres per second relative to the Sun before our star's gravity pulled it in.
Then came the puzzle. In 2018 astronomers reported that it had accelerated slightly beyond what gravity alone could explain. Comets do this when gases escape from them, but no gas or dust was seen. Suggested explanations include sunlight pushing on it, outgassing of hard-to-spot nitrogen ice from a fragment of a Pluto-like world, or, in a 2023 proposal, hydrogen trapped in ice being released as it warmed.
The Harvard astronomer Avi Loeb raised the possibility that it was an artificial light sail. Other scientists said the evidence was insufficient, and radio searches found no signals. By 2019 most astronomers had concluded it was natural. By mid-December 2017 it was already too faint for even the largest telescopes, so some questions may never be answered.
Source: Wikipedia — 1I/ʻOumuamua · Text summarised from Wikipedia (CC BY-SA 4.0)
