Halley’s Comet is the most famous comet in history, a ball of ice and dust roughly 15 kilometers long that loops around the Sun every 75 to 79 years. It’s the only short-period comet visible to the naked eye, and its predictable return has been documented by observers on Earth for over two thousand years. It last appeared in 1986 and will next reach its closest point to the Sun in 2061.
How Edmond Halley Proved It Was the Same Comet
For most of human history, people assumed each comet sighting was a unique, one-time event. That changed in 1705, when English astronomer Edmond Halley used Isaac Newton’s newly formulated laws of motion to calculate the orbits of several historically recorded comets. He noticed that the comets seen in 1531, 1607, and 1682 followed strikingly similar paths, and he proposed they were actually the same object returning on a regular cycle. He predicted it would appear again in 1758.
Halley didn’t live to see it. He died in 1742, but the comet returned on schedule in 1758, exactly as he calculated. It was the first time anyone had successfully predicted a comet’s return, and the comet was named in his honor. That prediction also served as powerful confirmation of Newton’s gravitational theory, proving it could describe the motion of objects far beyond the planets.
Size, Shape, and a Surprisingly Dark Surface
Until 1986, nobody knew what the core of Halley’s Comet actually looked like. That changed when the European Space Agency’s Giotto spacecraft flew within 1,372 kilometers of the nucleus, transmitting images that revealed its shape for the first time. The nucleus is roughly potato-shaped, about 15 kilometers long and 8 kilometers wide.
One of the biggest surprises was how dark the surface is. The nucleus reflects only about 3 to 4 percent of the sunlight that hits it, making it darker than coal and one of the least reflective objects in the solar system. Beneath that dark crust, the comet is a mix of water ice, frozen gases, and dust. When it gets close enough to the Sun, heat penetrates the surface and vaporizes the ice below, producing bright jets of gas and dust that shoot outward from the nucleus. Giotto also found the first evidence of organic material in a comet, compounds containing carbon that are considered some of the basic building blocks of chemistry in the solar system.
An Unusual Orbit
Halley’s Comet travels in a highly elongated ellipse that takes it from just inside Venus’s orbit at its closest approach to the Sun (perihelion) out past the orbit of Neptune at its farthest point. One full loop takes between 74 and 79 years, with the variation caused by gravitational tugs from Jupiter, Saturn, and the other giant planets each time it passes through the inner solar system.
What makes the orbit especially unusual is that Halley travels in the opposite direction from the planets. While Earth and all the other planets orbit the Sun counterclockwise when viewed from above the north pole, Halley moves clockwise. This retrograde orbit tilts about 162 degrees relative to the plane of the solar system, meaning it’s essentially flipped nearly upside down compared to planetary orbits. This steep angle and backward motion are part of why encounters between Earth and Halley’s debris trail produce two separate meteor showers each year, as our planet crosses the comet’s path at two different points in its own orbit.
Two Meteor Showers Every Year
Over thousands of orbits, Halley’s Comet has shed a long trail of dust and rocky particles along its path. Earth passes through this debris stream twice annually, producing two meteor showers. The Eta Aquarids peak in early May, and the Orionids peak in late October. Both are modest showers, typically producing 10 to 20 visible meteors per hour under good conditions, though the Eta Aquarids can be stronger in years when Earth passes through a denser section of the trail. You don’t need a telescope for either shower. The meteors are simply tiny grains of Halley’s debris burning up in Earth’s atmosphere at high speed.
The 1986 Visit and What We Learned
Halley’s most recent appearance, in 1986, was somewhat disappointing for casual skywatchers. The comet and Earth were on opposite sides of the Sun during the closest approach, so it appeared relatively faint compared to some of its spectacular historical passes. The 1910 appearance, by contrast, was so bright that Earth actually passed through the comet’s tail, triggering widespread (and unfounded) panic about poisonous gases.
But 1986 was a landmark for science. An international fleet of spacecraft, informally called the “Halley Armada,” was sent to study the comet up close. The most successful was ESA’s Giotto, which flew closer to a comet nucleus than any spacecraft had before. As Giotto approached, the rate of dust impacts rose sharply when it passed through one of the active jets streaming from the nucleus. The mission confirmed that comets are “dirty snowballs” (or more accurately, icy dirt balls), with a thin, dark crust insulating a volatile interior that erupts when heated by the Sun.
When It Returns in 2061
Halley’s Comet will reach perihelion on July 28, 2061. The viewing geometry will be significantly better than in 1986, with the comet positioned much closer to Earth during its brightest phase. Predictions suggest it could be a genuinely impressive naked-eye spectacle, potentially rivaling the dramatic 1910 pass that captivated people around the world.
In the meantime, the comet is still out there, currently in the far outer solar system beyond Neptune’s orbit, moving slowly toward its distant turnaround point before beginning the long fall back toward the Sun. You can still experience a piece of it each year by watching the Eta Aquarids in May or the Orionids in October, tiny fragments of the same icy body that Edmond Halley first recognized over three centuries ago.