Saturn threw a comet out of the solar system at 6,700 mph. Here's how

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Comet A117uUD is about to go interstellar!

In the forground a irregular light blue sphere trailed by a tail of white smoke there is ringed planet behind it
An illustration shows a comet barrelling out of the solar system after an encounter with Saturn. (Image credit: Robert Lea (created with Canva))

Scientists have discovered a comet traveling one way out of the solar system after a close encounter with Saturn. 

The comet, designated Comet A117uUD (A117uUD), was only discovered on June 14, 2024, by the Asteroid Terrestrial-impact Last Alert System (ATLAS). Yet, researchers were able to use 142 observations of the comet to "wind back" its orbit around the sun. This revealed that A117uUD met up with Saturn, the second-largest planet in the solar system, famous for its bright and distinctive rings, in 2022 and was changed forever. 

The meeting with the gas giant put the comet on a highly flattened or elliptical orbit that will fling it out into interstellar space beyond the influence of the sun. Using models to fast forward the path of A117uUD, the team found that it will exit the solar system at a speed of around 6,710 miles per hour (10,800 km/h). That is about four and a half times the top velocity of a Lockheed Martin F-16 jet fighter.

This is only the second solar system comet that we have seen in the process of being launched out of the solar system. The first was Comet C/1980 E1 (Bowell), which was put on a solar system escape path by an encounter with Jupiter on Dec. 9, 1980.

"Our results show that the case of comet A117uUD is similar to that of C/1980 E1 (Bowell), disfavoring an extrasolar origin for A117uUD," the team behind the research write in a paper published in the journal Research Notes of the AAS. "The fact that two ejections after planetary encounter were observed in less than 45 years suggests that such events are relatively frequent."

Related: A comet approaching Earth could become brighter than the stars this fall

Make up your mind. Are you coming or going?

It wasn't always clear that  A117uUD was a solar system body destined to leave its home planetary system. When the team behind this discovery first analyzed the icy space rock, they thought its hyperbolic orbit could indicate that it was actually an intruder into our solar system. 

The first solar system interloper discovered by humanity was the oddly cigar-shaped asteroid 1I/’Oumuamua (’Oumuamua), the moniker of which is a Hawaiian word that roughly translates as "messenger from afar arriving first."

An artist's interpretation of the strange cigar shape of the 'Oumuamua. (Image credit: ESO/M. Kornmesser)

When it was discovered in 2017, ’Oumuamua immediately stood out to astronomers because of its highly unusual shape and the fact that it had no coma (the halo that surrounds comets), or distinctive cometary tail despite appearing to be a strange comet/asteroid hybrid. It was also accelerating away from the sun, something that led to the now dismissed (mostly) claim that this visitor from another planetary system could be an alien spacecraft.

The solution to these unique features was the roasting of ’Oumuamua by cosmic radiation before reaching the solar system, which caused hydrogen to be created and become trapped within its body. That hydrogen was liberated and began to spray out of the space rock once ’Oumuamua reached the relative warmth of our planetary system, thus propelling it.

'Oumuamua is now out beyond the orbit of Neptune, passing through the Kuiper Belt, a ring of icy bodies near the outer edge of the solar system. Not only is it beyond the reach of our telescopes, but the interstellar interloper will never come back toward Earth or the sun.

An artist's impression of what the surface of the interstellar comet 2I/Borisov might look like. (Image credit: M. Kormesser/ESO)

’Oumuamua isn't the only interstellar intruder we have seen enter the solar system, however. Comet 2I/Borisov (2I/Borisov) was the second confirmed interstellar body and the first confirmed interstellar comet found within the solar system.

2I/Borisov was discovered by Crimean amateur astronomer Gennady Borisov on Aug. 30, 2019. This solar system invader really put A117uUD's speed to shame, racing through the solar system at a hair-raising 110,000 mph. That's 150 times the speed of sound, and it leaves a Lockheed Martin F-16 in its dust trail, traveling 75 times as fast as the jet fighter's top speed.

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Like 'Oumuamua, 2I/Borisov was just passing through, briefly fascinating astronomers and the general public alike, as it headed toward a solar system exit, never to return.

The encounter between A117uUD and Saturn impacted the comet's orbit to the extent that the team was unable to reconstruct it prior to the meet-up, but they did enough to assure themselves this wasn't a third extra-solar interloper. 

Maybe one day, the Saturn-ejected comet A117uUD will thrill and entice the astronomers of some distant alien civilization, perhaps billions of years after its discoverers on Earth have vanished.

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Robert Lea
Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

29 Comments Comment from the forums
  • m4n8tpr8b
    The meeting with the gas giant put the comet on a highly flattened or elliptical orbit
    Should be "eccentric" rather than "elliptical" (I guess a braintypo, after all, later you correctly write that the orbit is hyperbolic, not elliptical).
    Reply
  • Torbjorn Larsson
    The article does not explain the "how" of the title. Annoyingly, the paper reference also do not suggest a mechanism but is applying observational data. At a guess the ejections are due to momentum transfer from the planet (gravity assists).

    In recent news, there is a work that shows how Sun, with the help of Milky Way gravitational disturbances, can trap ejected bodies up to planetary masses https://www.universetoday.com/167929/a-new-study-shows-how-our-sun-could-permantly-capture-rogue-planets/].
    Reply
  • Unclear Engineer
    Both the article and the link in the post don't bother to explain the dynamics involved in such ejections and captures.

    I think I understand the dynamics of ejections, which are similar the "gravity boost" trajectories we have already used successfully to get probes like the Voyagers out of our solar system.

    The capture process seems to be similar to some of our lunar transfer orbits, where the spacecraft is in an orbit around Earth and is near its high point when the Moon "comes by" in its own orbit and provides a greater attraction to the spacecraft than the Earth does, so it begins to orbit the Moon. Substitute "galaxy" for "Earth", and "Sun" for "Moon" and the same thing that we do with a lunar spacecraft might happen with a rogue planet approaching the Sun.

    Of course we also use rocket motors to stabilize and shape our spacecraft's orbit around the Moon. For the Sun capturing a rogue planet or something smaller, the same sort of deceleration process could occur gravitationally due to passage by other planets. That would be just the opposite of the ejection trajectory, with momentum being taken away from the interstellar body to one (or more) of the existing planets.

    And that lost momentum is transferred to the existing planet(s), so if there is a lot of momentum transfer from a large interstellar body, our solar system could get significantly disrupted. Hoping that is an extremely improbable event. But, "trillions of rogue planets" is not really a comforting concept.
    Reply
  • Helio
    It's pretty cool to find one of the great tossers caught in a throw. :)

    The Kuiper Belt and Oort Cloud are comprised of those objects that got a more gentle toss, thus are still in the grips of the solar system.

    The best models, IIRC, demonstrate that the early solar system dynamics is similar to a pin ball machine in all the kicks and tosses, including major orbital migration in and out by Jupiter and Saturn, possibly dancing together. The axial tilt of Uranus seems to reveal it got hit in the head with a fast ball. Then there's that 7 deg. axial tilt in the Sun relative to the ecliptic. Something very dynamic seems to have happened.
    Reply
  • billslugg
    I think something big came through the Solar System early on. It either punched the Sun over by 7° or else it scattered the planets into a new orbital plane. It is not something that can be done from within a system, must be an outside influence. In this case, a big one.
    Reply
  • Unclear Engineer
    Isn't the theory that the Sun was born in some sort of dense cloud of gas and dust that also created multiple other stars in much closer proximity than the Sun has to other stars today?

    If so, there may have been multiple ways for the Sun and/or the Sun's protoplanetary disk to have been tilted by gravitational or physical collisions.
    Reply
  • Questioner
    If a body runs through the trailing part of a moving planet's mass field it transfers inertia/speed to the body & subtracts it (vector energy) from the planet.

    Conversely if a body runs through the leading part of a moving planet's mass field it subtracts inertia/speed from the body and adds it (vector energy) to the planet.

    I suppose the encounter with Saturn's mass didn’t alter the speed and just redirected it for the most part.
    That likely had a miniscule change of direction for Saturn as well.

    That transfer of energy is why i don't think the space elevator notion has validity in physics
    Reply
  • Helio
    billslugg said:
    I think something big came through the Solar System early on. It either punched the Sun over by 7° or else it scattered the planets into a new orbital plane. It is not something that can be done from within a system, must be an outside influence. In this case, a big one.
    Yes, that would be a good guess.

    Is there a model that presents this? The one from Nice, France, a while back, was able to get a fair for all the planets. It shows a lot of dynamic activity but was not able to explain Pluto’s orbit.

    Perhaps, however, the magnetic poles were off by 7 deg. during the formation period that included those strong bi-polar flows, which greatly diminished the Sun’s angular momentum.
    Reply
  • Helio
    Unclear Engineer said:
    Isn't the theory that the Sun was born in some sort of dense cloud of gas and dust that also created multiple other stars in much closer proximity than the Sun has to other stars today?
    Yes, the fragmented cloud seems to have produced a stellar cluster of about 1,000 to 3,000 stars. The IMF (Initial Mass Fuction) reveals their mass distribution. It shows a few very massive stars were likely.

    Unclear Engineer said:
    If so, there may have been multiple ways for the Sun and/or the Sun's protoplanetary disk to have been tilted by gravitational or physical collisions.
    Yes, a simple accretion disk smoothly conveying mass onto the protosun is too simplistic, apparently.
    Reply
  • billslugg
    Pluto is an aberrant. I think Pluto had its own encounter with a drive by shooter.

    Whatever did the tilting of the Sun's axis to the ecliptic did it while keeping all of the planets in the same plane. I don't see how any foreign mass passing through the Solar System could tilt the ecliptic. I think it was the Sun that got tilted. The main problem in either case is the rarity of stars as compared to how big the universe is. Our Sun, if the size of a golf ball, would have it's nearest neighbor 600 miles away. How does one hit the other? I don't see how a collision could occur.

    Perhaps, like you say, it is during star formation that somehow the magnetic fields do some redistribution of existing angular momentum. Sun gets a cockeyed field, its angular momentum goes one way, nascent dust cloud's goes the other way, net angular momentum is conserved.
    Reply