A Messenger Arrives
On October 19, 2017, astronomer Robert Weryk spotted a faint streak of light using the Pan-STARRS1 telescope in Hawaii. This object, soon given the designation 1I/2017 U1 and the Hawaiian name ʻOumuamua, meaning "a messenger from afar arriving first," was the first confirmed interstellar visitor to our solar system. It arrived from the general direction of the star Vega, though it did not originate there. Traveling at 26.33 kilometers per second relative to the Sun, its trajectory was strongly hyperbolic, proving it was not bound by the Sun's gravity and was only passing through.
Initial observations revealed a bizarre object. It had a reddish hue, similar to some bodies in the outer Solar System, likely caused by millions of years of exposure to cosmic rays. Its brightness varied by a factor of ten—every 7.3 hours. This variation shows a highly unusual shape, with estimates pointing to an object perhaps ten times longer than it was wide, like a cigar or flat pancake up to 400 meters long. Before being designated the first in a new class of "interstellar objects" (I), it was briefly classified as a comet (C/2017 U1) and then an asteroid (A/2017 U1).
An Unexplained Acceleration
ʻOumuamua reached its closest point to the Sun on September 9, 2017, passing inside the orbit of Mercury at a peak speed of 87.71 kilometers per second. As it moved away from the Sun, astronomers carefully tracked its path. They discovered it was not following the trajectory dictated by gravity alone. The object was accelerating, receiving a slight but persistent push away from the Sun. This non-gravitational acceleration was statistically significant, measured at about 5 x 10⁻⁶ m/s².
The explanation for such a push in comets is outgassing, where solar heating vaporizes ice, creating jets that act like thrusters. However, powerful telescopes, including the Spitzer Space Telescope, were pointed at ʻOumuamua and detected no coma—the typical cloud of gas and dust that surrounds an active comet. This absence of a visible coma makes the outgassing hypothesis problematic. The push was also stronger than expected for a typical comet of its size. This discrepancy between the observed acceleration and the lack of a coma is the central anomaly of ʻOumuamua. Several alternative theories have been proposed, including the outgassing of unseen molecules like hydrogen or nitrogen, or even the idea that the object was extremely thin and light, allowing it to be pushed by the pressure of sunlight itself, much like a solar sail. Having passed beyond the orbit of Neptune in 2022, it is now too distant and faint to observe, leaving the mystery of its true nature unresolved.