Comet Halley, proud parent of two meteor showers, swings into the inner solar system about every 76 years. At such times, the sun’s heat causes the comet to loosen its icy grip over its mountain-sized conglomeration of ice, dust and gas. At each pass near the sun, the crumbly comet sheds a fresh trail of debris into its orbital stream. It lost about 1/1,000th of its mass during its last flyby in 1986. It’s because comets like Halley are so crumbly that we see annual meteor showers, like the Eta Aquarid meteor shower that’s going on now. Follow the links below to learn more about Comet Halley, the meteor showers it spawns, and about how astronomers calculate the velocities of meteors streaking across our sky.
Comet Halley’s 2 meteor showers. Because Comet Halley has circled the sun innumerable times over countless millennia, cometary fragments litter its orbit. That’s why the comet doesn’t need to be anywhere near the Earth or the sun in order to produce a meteor shower. Instead, whenever our Earth in its orbit intersects Comet Halley’s orbit, cometary bits and pieces – oftentimes no larger than grains of sand or granules of gravel – smash into Earth’s upper atmosphere, to vaporize as fiery streaks across our sky: meteors.
It so happens we intersect Comet Halley’s orbit not once, but twice each year. In early May, we see bits of this comet as the annual Eta Aquariid meteor shower.
Then some six months later, in October, Earth in its orbit again intersects the orbital path of Comet Halley. This time around, these broken-up chunks from Halley’s Comet burn up in Earth’s atmosphere as the annual Orionid meteor shower.
By the way, these small fragments are called meteoroids when in outer space, and meteors when they vaporize in the Earth’s atmosphere.
Meteors in annual showers – made from the icy debris of comets – don’t hit the ground. They vaporize high in Earth’s atmosphere. The more rocky or metallic asteroids are what sometimes hit the ground, and then they are called meteorites.
Where is Comet Halley now? Often, astronomers like to give distances of solar system objects in terms of astronomical units (AU), which is the sun-Earth distance. Comet Halley lodges 0.587 AU from the sun at its closest point to sun (perihelion) and 35.3 AU at its farthest point (aphelion).
In other words, Halley’s Comet resides about 60 times farther from the sun at its closest than it does at its farthest.
It was last at perihelion in 1986, and will again return to perihelion in 2061.
At present, Comet Halley lies outside the orbit of Neptune, and not far from its aphelion point. See the image at the top of this post – for May, 2017 – via Fourmilab.
Even so, meteroids swim throughout Comet Halley’s orbital stream, so each time Earth crosses the orbit of Halley’s Comet, in May and October, these meteoroids turn into incandescent meteors once they plunge into the Earth’s upper atmosphere.
Of course, Comet Halley isn’t the only comet that produces a major meteor shower …
|Meteor Shower||Parent Body||Semi-major axis||Orbital Period||Perihelion||Aphelion|
|Quadrantids||2003 EH1 (asteroid)||3.12 AU||5.52 years||1.19 AU||5.06 AU|
|Lyrids||Comet Thatcher||55.68 AU||415 years||0.92 AU||110 AU|
|Eta Aquarids||Comet 1/P Halley||17.8 AU||75.3 years||0.59 AU||35.3 AU|
|Delta Aquarids||Comet 96P/Machholz||3.03 AU||5.28 years||0.12 AU||5.94 AU|
|Perseids||Comet 109P/Swift-Tuttle||26.09 AU||133 years||0.96 AU||51.23 AU|
|Draconids||Comet 21P/Giacobini–Zinner||3.52 AU||6.62 years||1.04 AU||6.01 AU|
|Orionids||Comet 1/P Halley||17.8 AU||75.3 years||0.59 AU||35.3 AU|
|Taurids||Comet 2P/Encke||2.22 AU||3.30 years||0.33 AU||4.11 AU|
|Leonids||Comet 55P/Tempel-Tuttle||10.33 AU||33.22 years||0.98 AU||19.69 AU|
|Geminids||3200 Phaethon (asteroid)||1.27 AU||1.43 years||0.14 AU||2.40 AU|
How fast do meteors from Comet Halley travel? If we can figure how fast Comet Halley travels at the Earth’s distance from the sun, we should also be able to figure out how fast these meteors fly in our sky.
Some of you may know that a solar system body, such as a planet or comet, goes faster in its orbit as it nears the sun and more slowly in its orbit as it gets farther away. Halley’s Comet swings inside the orbit of Venus at perihelion – the comet’s nearest point to the sun. At aphelion – its most distant point – Halley’s Comet goes all the way beyond the orbit of Neptune, the solar system’s outermost (known) planet.
When the meteoroids from the orbital stream of Halley’s Comet streak across the sky as Eta Aquarid or Orionid meteors, we know these meteoroids/meteors have to be one astronomical unit (Earth’s distance) from the sun. It might be tempting to assume that these meteoroids at one astronomical unit from the sun travel through space at the same speed Earth does: 108,000 kilometers or 67,000 miles per hour.
However, the velocity of these meteoroids through space does not equal that of Earth at the Earth’s distance from the sun. For that to happen, Earth and Halley’s Comet would have to orbit the sun in the same period of time. But the orbital periods of Earth and Halley’s Comet are vastly different. Earth takes one year to orbit the sun whereas Halley’s Comet takes about 76 years.
However, thanks to the great genius, Isaac Newton, we can easily compute the velocity of these meteoroids/meteors at the Earth’s distance from the sun by using Isaac Newton’s Vis-viva equation, his poetic rendition of instantaneous motion.
The answer, giving the velocity of these meteoroids through space at the Earth’s distance from the sun, is virtually at our fingertips. All we need to know is Comet Halley’s semi-major axis (mean distance from the sun) in astronomical units. Here you have it:
Comet Halley’s semi-major axis = 17.8 astronomical units.
In the easy-to-use Vis-viva equation below, r = distance from sun in astronomical units, and a = semi-major axis of Comet Halley’s orbit in astronomical units. In other words, r = 1 AU and a = 17.8 AU.
Vis-viva equation (r = distance from sun = 1 AU; and a = semi-major axis = 17.8 AU):
Velocity = 67,000 x the square root of (2/r – 1/a)
Velocity = 67,000 x the square root of (2/1 – 1/17.8)
Velocity = 67,000 x the square root of (2 – 0.056)
Velocity = 67,000 x the square root of 1.944
Velocity = 67,000 x 1.39
Velocity = 93,130 miles per hour or 25.87 miles per second
The above answer gives the velocity of these meteoroids through space at the Earth’s distance from the sun. However, if these meteoroids were to hit Earth’s atmosphere head-on, that would push the velocity up to an incredible 160,130 miles per hour (93,130 + 67,000 = 160,130). NASA gives the velocity for the Eta Aquarid meteors and Orionid meteors at 148,000 miles per hour, which suggests the collision of these meteoroids/meteors with Earth is not all that far from head-on.
We can also use the Vis-viva equation to find out the velocity of Halley’s Comet (or its meteoroids) at the perihelion distance of 0.59 AU and aphelion distance of 35.3 AU.
Perihelion velocity = 122,331 miles per hour
Aphelion velocity = 1,464 miles per hour
Bottom line: Halley’s Comet spawned two annual meteor showers, the Eta Aquarids in May and the Orionids in October. Plus … where the comet is now, parent bodies of other meteor showers … and Isaac Newton’s Vis-viva equation, his poetic rendition of instantaneous motion.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.