Astronomers have announced two more asteroids with hyperbolic orbits, following the enormous excitement generated by our first interstellar visitor last year. Unlike Oumuamua, both almost certainly originate within the Solar System. However, one is expected to escape the Sun's gravity, providing a glimpse of how we come to get visitors from other stars. Moreover, the discoveries suggest more significant announcements could be on the way.
The discovery of Oumuamua, originally known as A/2017 U1, set off a frantic scramble to learn more about this precious insight into the galaxy beyond our Sun's gravity well. Oumuamua's strange features, such as its extreme length-to-width ratio and color, even sparked theories it could be an alien spaceship, although further study indicates this is unlikely.
Ten days after Oumuamua another object was detected with motion that indicates it will pass once around the Sun and never return. However, its announcement has only been made this week. In the case of A/2017 U7, careful study by astronomers suggests its origins lie in the outer reaches of the Solar System, known as the Oort Cloud, rather than around another star. Interactions with Jupiter's gravity have tweaked its path, giving it enough energy to escape the Solar System. One day, in millions of years' time, it could pay a close visit to some other star.
A/2017 U7 is much larger than Oumuamua at 13-59 kilometers (8-37 miles) across, and will make its closest approach to the Sun on September 10, 2019.
The announcement coincided with that of a third asteroid on a hyperbolic orbit, A/2018 C2. This asteroid also originates in the Oort Cloud, and got an extra kick of speed from the gravity of large planets, but in this case, astronomers expect the burst will be temporary, and further gravitational interactions will keep it orbiting the Sun.
Many comets have been observed with similar orbits. Some, like A/2018 C2, have further encounters that keep them in the Solar System. Others are more A/2017 U7-like and expected to escape. None are thought to have come from elsewhere, instead being Solar System objects given gravitational boosts.
Comets are easy to spot, however, with their long tails advertising their presence. To pick up an asteroid making such a rapid visit to the inner Solar System, and notice its extreme orbit, is much more of an achievement. This is why the latest announcements have astronomers excited, although it is still possible cometary activity will be seen as they get closer. Until six months ago we had not found a single asteroid with an orbit like this; now we have three. This proves new asteroid-tracking equipment is working well, and if any other interstellar visitors come our way, we are likely to find them.