Interstellar objects
3 OF 3Three objects have been seen passing through the Solar System on paths the Sun cannot have put them on, and each was moving too fast at arrival to have started here. They are numbered in order of discovery with an I for interstellar, and the numbering began in 2017 because nothing before then had been caught.
NAME
CLASS
MASS
DISTANCE
MAG
CONSTELLATION
1I1I/'Oumuamua★
Hyperbolic asteroid
—
0.256 AU · Sun
22.08 H
—
2I2I/Borisov★
Hyperbolic comet
—
2.007 AU · Sun
—
—
3I3I/ATLAS★
Hyperbolic comet
—
1.356 AU · Sun
—
—
[
COMMON QUESTIONS
] 9 ANSWERED- What makes an object interstellar?
- Speed, not shape. Every one of these arrived with more velocity than the Sun's gravity could have given it, so it was already moving when it got here — which means it was moving before the Sun had any hold on it at all.
- Are they the same as hyperbolic comets?
- No, and the distinction is the whole point. Hundreds of comets in this catalogue are on formally unbound orbits, but they are the Sun's own, flung outward by a close pass with a planet; they leave at a few hundred metres a second. These three leave at tens of kilometres a second.
- Why is the numbering so recent?
- The first was found in 2017. Objects like these are small, fast and faint, and they are only visible for the weeks around perihelion — surveys that scan the whole sky every few nights are what made them findable, and those are new.
- Where did they come from?
- No parent star has been identified for any of them. Their incoming directions can be traced, but a body drifting for millions of years has passed too many stars for the path to point back to one.
- Do any of them return?
- None. Each passed the Sun once and is on its way out for good, which is why the observing window mattered so much and why two of the three were found on the way out rather than the way in.
- How many interstellar objects are known?
- Three have been confirmed: 1I/'Oumuamua in 2017, 2I/Borisov in 2019 and 3I/ATLAS in 2025. Surveys now coming online are expected to find them at a far higher rate.
- What does the I in 1I stand for?
- Interstellar. The Minor Planet Center created the designation for 'Oumuamua, which fitted neither the comet nor the asteroid numbering, and the number counts them in order of discovery.
- Was 'Oumuamua an alien spacecraft?
- Nothing observed requires that. It brightened and faded like a tumbling rock, showed no radio emission, and its slight unexpected acceleration is what a body venting gas does — the same behaviour comets show, from a surface too faint to see the venting on.
- Why did 2I/Borisov matter so much?
- It was caught on the way in, so it could be watched for months rather than weeks, and it behaved like an ordinary comet. That is a result about the rest of the galaxy: the material comets are built from is much the same elsewhere.