Comet C/2024 L5 has left the building.

The solar system, actually. Or it’s about to.

OK, let’s back up. The comet was discovered on June 14, 2024 by the automated sky survey observatory called Asteroid Terrestrial-impact Last Alert System (ATLAS). Measurements of the comet’s orbit showed it was very nearly a parabola. That means it likely came from a long way out. 

The shape of an orbit tells you a lot about the object (read that page for details on how orbits are determined and what they tell us). An ellipse means it’s bound to the Sun gravitationally, like the planets and asteroids. A hyperbola means it’s not bound to the Sun; it’s moving too quickly for the Sun to hold on to it. A parabola is exactly in between; it’s as if the object were dropped into our solar system from infinity with no extra velocity except what it gets from falling toward the Sun.

These shapes can be defined by a single parameter, called the orbital eccentricity (usually written as just e). A circle has an e = 0. An ellipse has an e between 0 and 1, and the closer it is to 1 the more elongated it gets. An e =1 is a parabola, and if e > 1 you have yourself a hyperbola.

At first, C/2024 L5 was found to have an e very close to 1. But in a new paper, a pair of astronomers used newer observations combined with older ones and found the eccentricity is now 1.057733 ± 0.000008 [link to journal paper]. That uncertainty (0.000008) is extremely small, and far too small for the comet to reasonably have an eccentricity less than 1. That means it’s hyperbolic, the Sun can’t hold on to it, and it’s on its way out into interstellar space. 

But wait! There’s more! 

It’s extremely difficult for a comet that starts off bound to the Sun to get enough extra velocity on its own to escape. More likely, the comet gets a kick by passing close to a big planet, like one of the gas giants — a gravitational slingshot that gives it enough extra velocity such that the orbit becomes unbound. Did that happen with L5?

The astronomers ran orbital calculations for the comet backward in time and found an astonishing thing: in January 2022 it passed incredibly close to Saturn, missing the giant ringed world by perhaps as little as 200,000 km! That’s closer than the Moon is from Earth, so that’s CLOSE.

Saturn has 95 times the mass of Earth, and commensurately more powerful gravity. This caused the comet to whip around the planet, changing the shape of the orbit so much that the calculations can’t determine what the orbit looked like before the encounter; even a tiny uncertainty in the position now changes the orbital shape calculation for before the encounter dramatically. 

However, the simulations do show that the most likely eccentricity for the comet’s orbit before the Saturn encounter was around 0.8, which is elongated but still definitely an ellipse. That means it was almost certainly a solar system comet all along, and then got kicked, hard, by Saturn, and is now on a trajectory that will eject it from the solar system entirely.

That’s super cool. We only have hard evidence of two interstellar objects passing through the solar system — 1I/’Oumuamua and 2/I Borisov — but just those two proved that stars lose material to the galaxy pretty often. And here we see our own Sun’s contribution! We don’t know how many comets are in interstellar space, though it’s certainly in the quadrillions at least (there are billions of stars with trillions of icy bodies in their outer reaches each).

And now we can add one more to that number. Well, soon enough; right now C/2024 L5 is in the main asteroid belt between Mars and Jupiter, and will pass the orbit of Neptune in 2039 or so. A few decades later it’ll pass outside the Sun’s heliosphere and be in interstellar space, heading out to the stars, never to return.

CORRECTION (July 30,2024): I made a typo; I originally wrote the comet encountered Saturn in 2024, but it was actually in 2022.

• The Curiosity rover stumbled — literally, it turns out — on a very surprising thing on the surface of Mars: rocks made of nearly pure sulfur! They didn’t look particularly interesting at first because they were covered with the ubiquitous reddish dust, but then the rover moved over one and crushed it, revealing the yellow interior that caught the eyes of scientists on Earth. Turns out the rocks aren’t alone; Curiosity is sitting a huge field of them! The area is known for sulfur-bearing minerals, but why there would be so much uncut sulfur is a mystery. Read more about this remarkable find at the JPL website.
  

• Astronomers and engineers are going to build a newly funded satellite that will go into geosynch orbit (so it appears to hover in the sky) and appear as a faint but precisely calibrated “star”, allowing telescopes to make more accurate brightness measurements. It’s a clever idea; it will be calibrated in a lab so that its lasers will emit a precisely known amount of light. When telescopes observe how bright it appears to them, astronomers can use the data to correct their measurements of actual cosmic objects. It will be too faint to see by eye, and since it’s a single source it shouldn’t be a problem to big surveys. I was pleased to see its named the Landolt satellite, after astronomer Arlo Landolt, who created catalogs of bright stars that were used as “standards” to calibrate brightnesses.

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