Comet Tails Explained: Ion Tail, Dust Tail and Anti-Tail

Two tails, two materials

NASA says comets have two tails: a dust tail and an ion tail. An ion is an atom or molecule that carries an electric charge. The ion tail is made of charged gas that leaves the coma, the cloud around the nucleus. The dust tail is made of tiny solid grains, released as the comet's ices turn to gas and carry dust along. The overview of comet anatomy explains where both come from.

The colors show up clearly in a Planetary Society image of Comet Hale-Bopp, where the gas tail appears blue and the dust tail appears white. In NASA's picture of Comet Lulin from February 2009, the ion tail is bluish and the dust tail is yellow, with a green coma that NASA links to glowing carbon gas.

The two tails also differ in weight. NASA's spaceflight primer says the ion tail is much less massive than the dust tail. That difference explains most of what follows.

Why tails point away from the Sun

Two things push on a comet's material. The first is the solar wind, a stream of charged particles that flows outward from the Sun all the time. The second is sunlight itself, which presses very slightly on anything it hits. NASA says sunlight pressure and the solar wind push coma dust and gas away from the Sun, sometimes forming a long, bright tail.

The comet's motion plays no part in aiming the tail. The Sun does the pushing, so the tail points along the line from the Sun through the comet. On the way in toward the Sun, the tail trails the comet. After the comet rounds the Sun and heads back out, the tail points ahead of it, so the comet moves tail-first. The same swing happens to every comet at every visit, including a famous short-period one such as the comet in the guide to Halley's return.

A tail also needs warmth to exist. A comet far from the Sun is a dark, frozen body with no tail. The tail grows as the nucleus heats up and gas and dust pour out.

Why the dust tail curves and the ion tail stays straight

The ion tail is light, so the solar wind accelerates it hard. NASA's primer says it ends up looking like a nearly straight line pointing away from the Sun. NASA's note on Comet NEOWISE, which became visible in July 2020, says its blue ion tail points directly away from the Sun, pushed by the charged solar wind. The tail's fine structure comes from uneven release of ions at the nucleus and from changes in the solar wind.

Dust grains are heavier. Sunlight pushes them away, but only slowly. NASA explains that the dust tail curves because the heavier particles resist the push of light and keep traveling along an orbit around the Sun. The result is a broad, curved, fan-like tail that often lags behind the straight line.

Real tails can be more complicated than this tidy picture. NASA reported that satellite data on Comet NEOWISE showed a dust tail and possibly two separate ionized-gas tails. Its dust tail also had an unusual wavy pattern. NASA says the cause of those stripes is not fully understood, though they are likely tied to rotating streams of grit released as ice melts on the nucleus, which is about 5 km (3 miles) wide.

What an anti-tail is

Now and then a spike seems to point toward the Sun, opposite to the main tails. Observers call it an anti-tail. It looks like a tail pointing the wrong way, but it is not a third kind of tail.

The usual explanation is perspective. Dust shed by a comet spreads along its orbit in a thin sheet. When Earth passes close to the plane of that sheet, the sheet is seen edge-on, and the part of it trailing the comet can appear to reach toward the Sun. Nothing is pushing that dust toward the Sun. The effect is about where the observer stands.

NASA's note on Comet Lulin shows how much viewing angle matters. In early 2009 the comet sat almost 180 degrees around the sky from the Sun, so it was up nearly all night. From Earth's position, NASA says, its ion tail and dust tail appeared to point in opposite directions. The tails themselves had not changed. Only the view had.

How long a tail can grow

NASA says comet tails can extend for millions of kilometers from the head. For Comet NEOWISE, NASA described dust and ion tails reaching hundreds of thousands to millions of kilometers. The Hyakutake tail crossed by Ulysses was far longer still.

That crossing was an accident. Ulysses carries ten instruments but no camera. Its magnetometer saw changes in magnetic field direction that matched what comet tails cause, and a second instrument detected ions typical of comets. Two separate teams found the signal independently. The odd readings lasted only a few hours, and the spacecraft saw no visible cloud.

Spacecraft have visited several comets directly. A dated list of those meetings is in the timeline of missions to asteroids, comets and dwarf planets. For ground-based sightings of bright tails, see the story of Hale-Bopp, Hyakutake and NEOWISE.

Common mistakes and myths

"A tail trails behind the comet, like exhaust." The tail points away from the Sun. After closest approach, it leads the comet outward.

"There is one tail." Most active comets have two, and they differ in color, shape and material. A photo can show both at once.

"The curved tail shows which way the comet is going." The curve comes from how dust moves under sunlight pressure. A curved dust tail alone is not a reliable arrow for the comet's direction.

"An anti-tail is a jet shooting toward the Sun." It is a viewing-angle effect, seen when Earth lines up with the comet's dust sheet.

"Tails are dense, solid streamers." They are thin. Ulysses flew through the far end of Hyakutake's tail without a camera or any warning, and the crossing showed up only in its data.

Frequently asked questions

Why is the ion tail blue and the dust tail white or yellow?

The ions in the gas tail glow blue. The dust tail does not glow. It looks white or yellow because the grains reflect sunlight.

Does a comet always have a tail?

No. A comet far from the Sun is a dark, frozen body with no tail. Tails grow as the nucleus warms and gas and dust escape.

Why does a photo show more tail than my eyes do?

A camera collects light over seconds or minutes, and the faint blue ion tail shows up in long exposures. The eye sees less. Binoculars help, but never point binoculars or a telescope at the Sun without a proper solar filter made for that purpose. The guide to seeing a comet has tips for planning a night out.

How long is the longest comet tail on record?

Researchers who studied the Ulysses data said Comet Hyakutake's tail reached about 500 million km (over 300 million miles), the longest recorded. Visible tails are much shorter.