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Normally I’d take some time to talk about how this is the 900^th issue of BAN, which is super cool, but I have even more superer news to tell you about, so I’ll hold off on the self-accolading until Monday’s issue. 901 is cool too.

On July 1, 2025, astronomers with ATLAS, the Asteroid Terrestrial-impact Last Alert System, were alerted that a new object was detected moving across the sky. This was no ordinary space rock though: its trajectory and velocity indicated it was not from our solar system. In other words, it’s coming from another star, across the gulf of interstellar space!

The object was given a provisional designation of A11pl3Z when it was first discovered, but was given the official name 3I/ATLAS once it was confirmed. It’s likely several kilometers across, which is determined by its brightness and distance.

[That image was taken by the Deep Random Survey, a collection of “amateur” astronomers who follow up with asteroid discoveries. I just want to point out this effort to understand 3I/ATLAS is a cooperation between professional and amateur astronomers, which is extremely wonderful.]

At first the orbital calculation (the determination of its path calculated by how it’s moving across the sky over time) was a bit shaky, which is typical; the longer you observe an object the better the prediction gets. But astronomers have a cool trick they can use: once a rough orbit is determined, they can check older images to see if the object was in them but missed. And sure enough, 3I/ATLAS was seen in ATLAS observations from June 25-28 (and by the Zwicky Transient Facility back to June 14) extending the baseline and allowing for better calculations.

The key to this is the shape of the trajectory. Objects that are bound to the Sun gravitationally, like planets, have orbits that are elliptical. We determine the shape of that ellipse in various ways, but a key parameter is called the eccentricity, which determines how elongated the ellipse is. A circle has an eccentricity of 0, and an ellipse has an eccentricity greater than 0 but less than 1. The higher the number gets, the stretchier the orbit. An eccentricity = 1 means the orbit is no longer an ellipse, but a parabola, meaning the object has just enough speed to escape the Sun’s gravity (I’m simplifying here, but close enough; I wrote about this topic in an earlier newsletter and on The Old Blog™).

An eccentricity greater than 1 means the orbit is a hyperbola, which in turn means the objects has more than enough velocity to escape the solar system. It’s possible that objects that are normally bound to the Sun, like asteroids and comets, can get this much energy if they pass close to a planet, but there’s only so much added kick they can get. Also, we can trace the orbit backwards to see if the object got anywhere near a big planet like Jupiter or Neptune, to see if it did get this kind of gravity assist. If it didn’t then it’s likely to have come from outside the solar system.

3I/ATLAS hasn’t come anywhere near any of our planets, and it’s orbit is very, very hyperbolic: it has an eccentricity of about 6 (!!!), which is really really high. It’s moving so rapidly relative to the Sun — about 58 kilometers per second as it entered the solar system, which, holy crap is fast — its path barely bends as it moves past the Sun. It’s interstellar. 

Interestingly, the path of the object takes it through the plane of the solar system at a very low angle (in technical terms, the inclination of the orbit is low), tilted only by about 5°. If the object is truly interstellar that’s a coincidence, but if this object were part of our solar system that wouldn’t be surprising; most objects orbit the Sun in very nearly the same plane. 

The good news is this object was discovered on its way in, so it’ll be around for many months. It’s currently just inside the orbit of Jupiter, about 750 million kilometers from the Sun and 600 million from Earth. It reaches perihelion, closest point to the Sun, in October when it will be just outside the orbit of Mars, about 270 million km from the Sun. Unfortunately it will be on the other side of the Sun from us in the sky then, so we won’t be able to see it. But after that it pulls away, and it reaches perigee, its closest approach to Earth, in mid-December, when it will be roughly 375 million kilometers away. It’ll be up in the evening after sunset then.

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