A Messenger Arriving First
On October 19, 2017, astronomer Robert Weryk, using the Pan-STARRS1 telescope at Haleakalā Observatory, Hawaii, spotted an object moving rapidly through the solar system. Its trajectory was not a closed ellipse, but a sharp, open-ended hyperbola, confirming it was not from around here. Designated 1I/2017 U1, it became the first confirmed interstellar object ever detected. The Pan-STARRS team gave it the Hawaiian name ‘Oumuamua, which means "a messenger from afar arriving first".
Initial observations showed a small, reddish object, likely no more than 400 meters long. Its brightness varied by a factor of ten every 7.3 hours, which suggested a highly elongated shape, perhaps ten times longer than it was wide, and that it was tumbling chaotically through space. Based on its color and lack of a visible coma (the cloud of gas and dust surrounding a comet), it was first classified as an asteroid, A/2017 U1. At its closest pass to the Sun on September 9, 2017, it reached a velocity of 87.3 kilometers per second.
An Unexplained Acceleration
As astronomers tracked ‘Oumuamua heading out of the solar system, they noticed something peculiar. Its trajectory deviated slightly from the path dictated by the Sun's gravity alone. The object was accelerating, receiving a small, continuous push away from the Sun. This non-gravitational acceleration was statistically significant, amounting to a total velocity change of about 17 meters per second.
This behavior is common in comets. As they near the Sun, their ice heats up and turns to gas, which jets from the surface and acts like a tiny rocket engine. Powerful telescopes, including the Spitzer Space Telescope, detected no trace of a dusty coma or the typical gases like carbon monoxide or carbon dioxide. This lack of visible outgassing created a scientific puzzle.
Several hypotheses emerged to explain the phantom push. One idea is that the outgassing was composed of molecules invisible to the telescopes, such as molecular hydrogen released from water ice that had been processed by cosmic rays over millions of years. Another theory suggests it could be a "nitrogen iceberg," a fragment chipped from an exo-planet similar to Pluto.
A more debated explanation, proposed by Avi Loeb, suggests solar radiation pressure was responsible. For sunlight to exert enough force, ‘Oumuamua would need to be a very thin object with a large surface area and low mass, perhaps less than a millimeter thick—a structure like an artificial light sail. This hypothesis remains highly contentious, with most scientists favoring a natural explanation for the interstellar visitor's strange path.