A comet’s speed depends almost entirely on where it is in its orbit around the Sun. Far out in the cold beyond Neptune, a comet drifts at just a few kilometers per second. As it falls inward toward the Sun, it accelerates dramatically, and by the time it swings through the inner solar system it can be traveling at tens or even hundreds of kilometers per second. The fastest known examples, sungrazing comets that nearly skim the Sun’s surface, can exceed 500 km/s. That enormous range makes “how fast is a comet?” a question with no single answer, but there are patterns behind the variation that are worth understanding.
Why Comets Speed Up and Slow Down
Comets travel on highly elongated orbits, stretched-out ellipses that carry them from the outer reaches of the solar system to the Sun’s neighborhood and back again. The key principle is simple: the closer a comet gets to the Sun, the faster it moves. This is the same rule that governs all orbiting bodies, from planets to spacecraft, and it comes from the way gravitational energy converts to speed as an object falls toward a massive body. A comet at the far end of its orbit, perhaps several hundred astronomical units from the Sun, is barely moving relative to the solar system’s center. As it falls sunward over thousands or millions of years, it picks up speed continuously until it reaches its closest point to the Sun, called perihelion, where it hits its maximum velocity.
At roughly Earth’s distance from the Sun, a comet on a near-parabolic orbit would be traveling at about 42 km/s, which is the Sun’s escape speed at that distance. Short-period comets like Halley’s Comet, whose orbits are somewhat less elongated, reach slightly lower peak speeds at perihelion, in the range of 40 to 55 km/s depending on how close they get. The relationship between distance and speed is steep: halving the distance to the Sun increases speed by about 40 percent, not double. That steep curve is why the numbers get truly extreme for comets that approach within a few solar radii of the Sun’s surface.
Sungrazing Comets and the Fastest Speeds in the Solar System
The comets that reach the highest speeds are the sungrazers, objects whose orbits carry them so close to the Sun that many do not survive the encounter. The largest known family of these is the Kreutz group, a collection of fragments that share a common ancestor comet that broke apart centuries or millennia ago. Over 300 Kreutz sungrazers have been identified, most of them discovered by the SOHO (Solar and Heliospheric Observatory) spacecraft’s coronagraphs since the mid-1990s. Nearly all of these small fragments disappear during their approach to the Sun, vaporized by the intense heat before they can complete their pass.1The Astrophysical Journal. Statistical Investigation and Modeling of Sungrazing Comets Discovered with the Solar and Heliospheric Observatory
A Kreutz sungrazer with a perihelion distance of roughly one to two solar radii from the Sun’s center is moving at somewhere around 400 to over 600 km/s at closest approach. For perspective, that is roughly 1.4 to 2 million kilometers per hour. Even the rare bright sungrazers observed from the ground between 1843 and 1970, which were larger and survived their perihelion passage, reached similarly extraordinary speeds. These are among the fastest objects ever observed within the solar system, far faster than any spacecraft humans have built. The Parker Solar Probe, humanity’s fastest spacecraft, reached about 190 km/s during its closest solar passes, still well below what a sungrazer achieves.
When Comets Arrive from Interstellar Space
Most comets are bound to the Sun. They may travel incredibly far out before looping back, but they are always gravitationally tethered. A few objects, however, arrive from beyond the solar system entirely. These interstellar visitors travel on hyperbolic orbits, meaning they are not bound to the Sun at all and will leave the solar system after a single pass.
The first confirmed interstellar object, 1I/’Oumuamua, was detected in 2017. The second, 2I/Borisov, followed in 2019 and looked much more like a traditional comet, complete with a visible coma and tail. Analysis of their orbits confirms that both have trajectories most consistent with an interstellar origin, though in principle an extremely close stellar flyby could have scattered an Oort cloud comet onto a similar path. That possibility has a very low probability, and there is no evidence of the kind of recent stellar encounter that would be needed.2Monthly Notices of the Royal Astronomical Society. Hyperbolic orbits in the Solar system: interstellar origin or perturbed Oort cloud comets?
Interstellar comets tend to be traveling fast even at large distances from the Sun, because they carry their own velocity from the motion of their parent star system relative to ours. ‘Oumuamua was moving at roughly 26 km/s relative to the Sun even before the Sun’s gravity began pulling it faster. By the time it reached its closest approach to the Sun, about 0.25 AU (inside Mercury’s orbit), it was moving at close to 88 km/s. Borisov, which came no closer than about 2 AU, topped out at a more modest speed but still clearly exceeded what a bound comet at the same distance would show.
A third interstellar comet, designated 3I/ATLAS, was identified more recently. Like Borisov, it displayed cometary activity, and researchers detected a small but measurable non-gravitational acceleration during its passage through the inner solar system. Modeling showed this acceleration was consistent with ordinary carbon monoxide-driven outgassing from a nucleus between about half a kilometer and three kilometers across, not requiring any exotic physical properties.3arXiv. Non-Gravitational Acceleration in 3I ATLAS: Constraints on Exotic Volatile Outgassing in Interstellar Comets That finding matters because ‘Oumuamua’s own unexpected acceleration had sparked wild speculation about its nature. 3I/ATLAS suggests that ordinary outgassing can account for these small speed anomalies without anything unusual going on.
How Outgassing Nudges a Comet’s Speed
When a comet approaches the Sun, its ices begin to sublimate, turning from solid to gas and creating jets of material that stream off the surface. These jets act like tiny rocket thrusters, giving the comet small but persistent pushes that alter its trajectory and speed in ways that pure gravity alone cannot explain. The effect is subtle compared to the Sun’s gravitational pull, but over the course of an orbit it can meaningfully shift a comet’s arrival time and path.
The Rosetta mission, which accompanied comet 67P/Churyumov-Gerasimenko for over two years, provided an especially detailed look at this process. Researchers derived the non-gravitational acceleration of 67P directly from the Rosetta spacecraft’s trajectory data and found close agreement between the orbit-derived acceleration and the water production rates measured by instruments aboard the spacecraft. Standard models of sunlight-driven sublimation captured the overall acceleration, though capturing the full picture required accounting for the comet’s irregular shape and how sunlight hit different parts of its surface.4Astronomy & Astrophysics. Outgassing-induced acceleration of comet 67P/Churyumov-Gerasimenko
The phenomenon gets stranger with a class of objects informally called “dark comets.” These are near-Earth objects that show no visible coma or tail, so they look like inert rocks, yet they exhibit accelerations that radiation pressure from sunlight cannot explain. At least seven such objects have been identified, with some showing acceleration components both within and perpendicular to their orbital planes. The leading explanation is that anisotropic outgassing, where gas escapes unevenly from one side of the object due to its spin orientation and orbital geometry, provides small but detectable thrust even without a visible gas envelope.5Icarus. Seasonally varying outgassing as an explanation for dark comet accelerations Dark comets blur the line between asteroids and comets and suggest that outgassing-driven speed changes may be more common in the solar system than previously appreciated.
Giant Planets as Speed Changers
Gravity from the Sun dictates the broad strokes of a comet’s speed, but close encounters with planets, especially Jupiter, can dramatically alter an orbit in a single flyby. Jupiter’s mass is so large that a comet passing within a few tenths of an astronomical unit can have its orbit reshaped entirely, either gaining enough energy to be flung out of the solar system or losing energy and being captured into a shorter-period orbit.
Comet C/1980 E1 (Bowell) is a well-documented case: it reached its present path into interstellar space after a close encounter with Jupiter at 0.23 AU in December 1980. A more recent example, C/2024 L5 (ATLAS), was scattered out of the solar system following an astonishingly close flyby of Saturn at just 0.003 AU in January 2022.6Astronomy & Astrophysics. Ejected from home: C/1980 E1 (Bowell) and C/2024 L5 (ATLAS) In both cases, a single planetary encounter added enough velocity to push the comet beyond the Sun’s ability to pull it back. These comets are now heading into interstellar space, accelerated to speeds that will carry them away from the Sun indefinitely.
The reverse also happens. Comet Lexell is a famous historical example of Jupiter reshaping an orbit inward rather than outward. In 1767, a close encounter with Jupiter reduced Lexell’s perihelion distance from 2.9 AU down to 0.67 AU, bringing it into the inner solar system where it passed Earth at just 0.015 AU in 1770, one of the closest cometary approaches in recorded history. Then a second very close Jupiter encounter in 1779 sent it back outward, pushing its perihelion beyond 5 AU and effectively removing it from view.7International Astronomical Union Colloquium. Ejection of Particles from Comet Lexell: The Gravitational Influence of Jupiter Lexell’s speed at each of those perihelion passages was wildly different: much faster in its tight 0.67 AU orbit than it had been in its original, more distant one. Jupiter essentially turned a slow, distant comet into a fast, close one, and then undid the change a dozen years later.
How Fast Comet Debris Hits Earth’s Atmosphere
You do not have to wait for a comet itself to arrive to observe cometary speeds. Every year, Earth passes through trails of debris left behind by comets, and those tiny particles slam into the atmosphere as meteor showers. The speeds at which these meteoroids arrive depend on both the parent comet’s orbit and the geometry of the encounter with Earth.
The fastest annual meteor shower is the Leonids, whose particles hit Earth’s atmosphere at a geocentric velocity of about 71 km/s. The Leonids originate from comet 55P/Tempel-Tuttle, which has a retrograde orbit, meaning it travels around the Sun in the opposite direction from Earth. When Earth runs into debris coming at it head-on like that, the relative speed is extremely high. The Eta Aquarids and Orionids, both associated with Halley’s Comet, arrive at about 66 km/s for the same reason. The Perseids, one of the most popular showers to watch, come in at around 59 km/s.8Monthly Notices of the Royal Astronomical Society. Learning about comets from the study of mass distributions and fluxes of meteoroid streams
At the other end of the scale, the June Bootids arrive at just 18 km/s, and the Taurids at about 28 km/s. These slower showers come from comets whose orbits are more aligned with Earth’s own direction of travel, so the closing speed is lower. The difference is visible in the sky: fast meteors like Leonids tend to produce bright, quick streaks, while slower Taurid meteors linger longer and can produce dramatic fireballs. The full range across well-known annual showers spans roughly 18 to 71 km/s, which gives a practical sense of how much cometary orbital geometry varies from one parent comet to another.
These speeds also carry consequences beyond the visual spectacle. Studies of impact speeds upon Earth confirm that cometary impacts are much faster on average than asteroidal ones. Comets on near-parabolic orbits arriving from the Oort cloud tend to hit at significantly higher velocities, making them more energetic per unit mass. That high speed makes cometary impacts more likely to eject material from a planet’s surface into space, a mechanism that has been discussed in the context of how rocks get transferred between planets.9Planetary and Space Science. Distributions and moments of asteroid and comet impact speeds upon the Earth and Mars
Measuring a Comet’s Speed from Earth
Astronomers determine a comet’s speed primarily by tracking its position over time against the background stars, then fitting an orbital solution. Optical telescopes provide the positional data, but radar adds a layer of precision that is especially valuable for close-approaching comets. Radar observations of comet 103P/Hartley 2 ahead of the EPOXI spacecraft flyby in 2010 corrected the comet’s position by 70 km and its velocity by 120 millimeters per second relative to the best available optical solution at the time.10The Astrophysical Journal Letters. Radar Observations of Comet 103P/Hartley 2 A 120 mm/s correction sounds minuscule, but for a spacecraft approaching at high speed, that kind of error would translate to a significant targeting miss.
For comets too distant for radar, speed is inferred entirely from the shape of the orbit derived from optical observations. The more observations spread over a longer arc of the orbit, the more precise the solution. Interstellar visitors like ‘Oumuamua and Borisov presented a challenge because they were observed over relatively short arcs, making it harder to pin down their trajectories. Even so, the hyperbolic excess velocity, the speed an object carries that is above and beyond what the Sun’s gravity can account for, was clearly measurable for both objects.
What Happens When a Fast Comet Gets Too Close
Speed and proximity to a massive body combine to create tidal forces that can tear a comet apart. Comet Shoemaker-Levy 9 provided the most dramatic demonstration of this in 1992, when it passed close enough to Jupiter to be ripped into a chain of fragments by tidal stress. Analysis of the breakup showed that the comet had effectively zero structural strength by the time it reached its closest point to Jupiter. It was a loosely bound rubble pile held together mainly by its own feeble self-gravity, and once tidal forces exceeded that, the comet stretched and fragmented into more than 20 visible pieces.11Icarus. Size, Density, and Structure of Comet Shoemaker–Levy 9 Inferred from the Physics of Tidal Breakup Those fragments then slammed into Jupiter’s atmosphere two years later at roughly 60 km/s, producing impact scars visible from Earth.
Tidal disruption also affects sungrazing comets. The Kreutz group itself is thought to be the product of a large progenitor comet that broke apart during one or more perihelion passages, with the fragments gradually separating over successive orbits. The combination of extreme speed and intense tidal and thermal stress near the Sun makes sungrazing passages particularly destructive, which is why the hundreds of small Kreutz fragments discovered by SOHO are virtually all one-way trips.
Comets Around Other Stars
Comets are not unique to our solar system. Astronomers have detected signs of comet-like objects transiting other stars, identified through rapid, variable absorption features in starlight that are consistent with gas released by sublimating icy bodies on highly elliptical orbits. The young star PDS 70, already famous for hosting planets still forming within its disk, has shown variable sodium absorption lines with Doppler-shifted radial velocities consistent with the exocomet phenomenon observed in other systems.12Nature Communications. Potential sublimating exocomets around the young star PDS 70
Measuring the speed of an exocomet directly is not yet possible in the way we track comets in our own solar system. Instead, the speed is inferred from the Doppler shift of the absorption features: as a comet-like body transits the star, the gas it releases shows characteristic velocity signatures that change rapidly, on timescales of hours or days. These velocities are consistent with objects falling inward on elongated orbits, much like sungrazing comets here. The detection of exocomets around a star that is only about five million years old suggests that comet-like bodies form early in a planetary system’s life, and the speed-distance relationship that governs comets in our solar system is likely universal, a simple consequence of gravity that should hold anywhere a small icy body orbits a star.