Comets, often described as cosmic snowballs, are small solar system bodies composed of frozen gases, dust, and rock. These celestial objects typically possess nuclei ranging from a few hundred meters to tens of kilometers in diameter. Comets travel in highly elongated elliptical orbits around the Sun. As a comet approaches the Sun, its icy components warm and undergo sublimation, transforming directly from solid to gas. This process forms a vast, glowing atmosphere called a coma and often produces one or more tails, making the comet visible from Earth.
The Range of Comet Speeds
A comet’s speed is not constant and can vary considerably throughout its journey around the Sun. Comets move fastest when they are closest to the Sun, a point in their orbit known as perihelion. Conversely, their speed is slowest when they are farthest from the Sun, at a point called aphelion. Halley’s Comet provides a clear example of this variation.
At its aphelion, which lies beyond the orbit of Neptune, Halley’s Comet travels at a relatively slow pace of about 0.9 to 1.1 kilometers per second, or approximately 2,000 miles per hour. As it falls towards the inner solar system and nears its perihelion, located inside Venus’s orbit, its speed dramatically increases. During its 1986 passage, Halley’s Comet reached speeds of around 55 kilometers per second, equivalent to about 122,000 miles per hour.
What Makes Comets Change Speed
The primary influence on a comet’s varying speed is the Sun’s powerful gravitational pull. As a comet is drawn inward towards the Sun, the gravitational force accelerates it, causing its speed to increase significantly. Conversely, as the comet moves away from the Sun and travels back into the outer solar system, the Sun’s gravity acts to slow it down. This phenomenon aligns with the principle that objects in orbit move faster when closer to the central body.
Another factor influencing a comet’s speed and trajectory is outgassing, the release of gas and dust as ice sublimates. This process creates a subtle “jet propulsion” effect, acting like tiny thrusters on the comet’s nucleus. While the reaction mass involved is small, these non-gravitational forces can subtly alter the comet’s path and velocity.
How Scientists Measure Comet Speed
Astronomers employ various methods to determine the speed of comets. One common approach involves observing the comet’s position against the backdrop of distant stars over a period of time. By tracking its apparent movement across the sky, scientists can calculate its angular motion. To convert this angular motion into a linear speed, the comet’s distance from Earth must be known.
Additionally, astronomers can utilize the Doppler effect, a principle also used in radar guns, to measure a comet’s radial velocity. This involves analyzing shifts in the light spectrum emitted or reflected by the comet. A shift towards shorter, bluer wavelengths indicates the comet is moving towards Earth, while a shift towards longer, redder wavelengths suggests it is moving away. The magnitude of these shifts provides a precise measurement of the comet’s speed along the line of sight.
Comparing Comet Speeds to Other Objects
To understand the immense speeds of comets, it helps to compare them with more familiar objects. A typical commercial jet aircraft cruises at speeds between 500 and 600 miles per hour, while a rifle bullet can travel from 700 to over 3,000 miles per hour.
Our own planet Earth orbits the Sun at an average speed of approximately 67,000 miles per hour. Asteroids in the main belt typically move at orbital speeds ranging from 38,029 to 55,925 miles per hour. When comets or asteroids impact Earth, their speeds can vary significantly, with asteroids averaging around 18 kilometers per second, short-period comets about 30 kilometers per second, and long-period comets reaching an average of 53 kilometers per second. The fastest human-made object, the Parker Solar Probe, achieves speeds of 430,000 miles per hour as it approaches the Sun.