How a Great Comet Grows Its Tail: The Physics Behind NEOWISE

What you actually saw in July 2020

For about three weeks in July 2020, tens of millions of people in the Northern Hemisphere stepped outside before dawn — and later, after dusk — to see something no living generation had grown up expecting: a genuinely bright comet hanging in the twilight, tail pointing up and away from the horizon. That was Comet C/2020 F3 (NEOWISE), named for the space telescope that spotted it on 27 March 2020.

At its best it reached apparent magnitude ≈ 1.0 — comparable to a bright star like Aldebaran — the brightest comet visible from the north since Hale–Bopp in 1997. Its two tails were plainly separable in photographs and even, under dark skies, to a careful naked eye: a broad, softly curved, golden dust tail, and above or alongside it a fainter, ramrod-straight, bluish ion tail. Long-exposure images stretched the visible tail across many degrees of sky, corresponding to a physical length of tens of millions of kilometres.

The wonder of it is the mismatch of scale. The object producing all this was a dirty iceberg only about 5 km across — smaller than many cities. Everything you saw was not the nucleus at all (far too small and distant to resolve) but the atmosphere and exhaust that iceberg produced as it was cooked by the Sun. To understand a great comet's tail, you have to follow the energy: sunlight in, ice and gas and dust out.

The engine: sublimation, not melting

A comet nucleus is a loosely bound mix of water ice, frozen carbon dioxide and carbon monoxide, other volatiles, dust and organic-rich grains — Fred Whipple's "dirty snowball" from 1950, refined by later spacecraft into something more like an icy, porous rubble pile. Out in the deep cold it is inert. The transformation begins as it falls sunward.

The critical point is that in the near-vacuum of space, ice does not melt into liquid — it sublimates, passing straight from solid to gas. Different ices switch on at different distances:

• Beyond ~5 AU: super-volatiles like frozen CO and CO₂ can already vent, which is why some comets brighten far out.
• Around 2.5–3 AU: water ice — the dominant material — begins to sublimate vigorously as sunlight warms the surface. This is the classic threshold where a comet "turns on" and grows a visible coma.
• Near perihelion (NEOWISE at 0.29 AU): sunlight is roughly (1/0.29)² ≈ 12 times as intense as at Earth, driving furious outgassing.

Escaping gas drags dust with it, inflating a fuzzy cloud — the coma — that can swell larger than a planet, tens of thousands to over a hundred thousand kilometres wide, even though the nucleus sourcing it is city-sized. That gas-and-dust reservoir is the raw material every tail is built from. Crucially, sublimation is a surface process fed by sunlight, so the outflow is strongest on the sunward side and turns off on the night side as the nucleus rotates.

Two tails, two forces

Once material leaves the coma, the Sun pushes it — but through two completely different mechanisms that sort the debris into two distinct tails. This split, worked out over the 19th and 20th centuries, is the heart of comet physics.

The dust tail is pushed by light itself. Photons carry momentum, and when sunlight strikes a micron-sized grain it exerts a tiny outward force — radiation pressure. For small grains this push is a non-trivial fraction of the Sun's gravity, so grains drift outward and, importantly, lag behind the comet as it sweeps along its orbit. Newly released big grains stay near the nucleus; small old grains are blown far downstream. The result is a broad, curved tail that fans out and bends back along the orbital path, glowing yellow-white because it simply reflects sunlight, like dust in a sunbeam.

The ion tail is pushed by the solar wind. Ultraviolet sunlight strips electrons from coma gas, creating ions (CO⁺, H₂O⁺, CO₂⁺). The Sun blows a continuous solar wind of protons and electrons outward at roughly 350–400 km/s, dragging its magnetic field with it. That field drapes around the comet's ionosphere and picks up the fresh ions, flinging them almost radially away from the Sun at speeds that can exceed 100 km/s. Because they move so fast and so nearly straight, the ion tail is narrow and straight, pointing very close to the exact antisolar direction. It glows blue — chiefly the fluorescence of CO⁺ ions re-emitting absorbed sunlight, not reflection.

So the two tails diverge because two different forces of two different strengths act on two different kinds of material. On NEOWISE, the golden curve and the blue spike splitting apart from a common head were textbook.

Why the tail leads at departure — the antisolar rule

Here is the fact that surprises almost everyone: a comet's tail does not trail behind it like the wake of a boat. Both tails point away from the Sun, regardless of which way the comet is travelling. On the way in, the tail streams behind. At perihelion it sweeps sideways. And on the way out, the comet flies tail-first — the tail actually precedes the nucleus along its path.

This is a direct consequence of the two driving forces. Radiation pressure and the solar wind both originate at the Sun and blow radially outward, so whatever they act on gets swept downwind of the Sun, not downwind of the comet's motion. The comet's own orbital velocity only bends the dust tail slightly (that's the curve); the underlying direction is set by the Sun's position. A useful mental image: the tail is not exhaust from an engine but smoke from a chimney in a steady wind — it always leans the same way no matter how the chimney is carried around.

A few consequences worth internalising:

• The best viewing often comes after perihelion, when the comet has moved into a darker sky and the tail, now leading, catches sunlight against the twilight — exactly NEOWISE's mid-to-late-July window.
• Comet tails can be millions of kilometres long yet almost weightless; the entire visible tail may mass less than a large building, spread across a volume bigger than the Sun.
• The ion tail can disconnect in dramatic "disconnection events" when the comet crosses a boundary in the solar wind where the magnetic field reverses — the old tail is pinched off and blows away while a new one grows.

The numbers behind NEOWISE

NEOWISE was a well-observed test case, so we can put figures on the whole process.

• Discovery: 27 March 2020, by NASA's NEOWISE mission — the repurposed Wide-field Infrared Survey Explorer (WISE) spacecraft, hunting near-Earth objects in infrared.
• Nucleus: about 5 km across, estimated from its infrared signature. That is fairly large as comets go, which helped it survive perihelion intact where smaller sungrazers crumble.
• Perihelion: 3 July 2020 at 0.29 AU (≈ 43 million km) — inside the orbit of Mercury, but far enough out to avoid disintegration.
• Closest to Earth: 23 July 2020 at about 0.69 AU (≈ 103 million km) — never remotely a threat, but close enough to look grand.
• Orbit: a highly eccentric long-period orbit whose period differs before and after perihelion. Its inbound period was roughly 4,500 years, so the last people to potentially see it were in the Bronze Age; planetary tugs at this passage lengthened the outbound period to about 6,800 years, so the next apparition falls around the year 8800.

The energy budget is the punchline. At 0.29 AU the nucleus intercepts sunlight about 12× more intense than at Earth. That heat sublimated water and other ices at a rate that, integrated over the encounter, stripped a layer meters deep off the sunward face and released dust that WISE/NEOWISE and ground observers tracked for weeks. Each such passage is lossy: a comet is a fundamentally temporary object, shedding a fraction of its mass every orbit. NEOWISE will make thousands more of these before it either fizzles into an inert rubble core or breaks apart entirely.

Common misconceptions and how we know

A great comet invites myths. A few worth correcting:

• "The tail trails behind the comet." No — it points away from the Sun, as argued above. This was understood in principle even before spacecraft; the German astronomer Ludwig Biermann in the 1950s inferred a fast particle stream from the Sun precisely because ion tails always aim antisolar and respond too quickly to be explained by light alone — an early prediction of the solar wind, confirmed by the Mariner 2 probe in 1962.
• "Comets burn up like meteors." No — nothing is on fire. The glow is reflected sunlight (dust) and fluorescence (ions). The physical process is cold sublimation and photon/particle pushing, not combustion.
• "A comet is basically a fireball flying through the sky in real time." No — despite orbital speeds of tens of km/s, comets are so distant that they appear nearly fixed among the stars over a night, drifting only slowly from evening to evening.
• "NEOWISE nearly hit Earth." No — it passed 0.69 AU away, over 100 million km, about 270 times the Moon's distance.

Our confidence in the tail mechanism rests on more than theory. Spacecraft have flown into comets directly — Giotto imaged Halley's nucleus in 1986; Stardust returned dust grains from comet Wild 2 in 2006; Deep Impact excavated comet Tempel 1 in 2005; and above all ESA's Rosetta orbited comet 67P/Churyumov–Gerasimenko from 2014 to 2016, watching, month by month, exactly how sunlight drives jets, builds the coma, and feeds the tails. NEOWISE, glowing over the July horizon, was simply the same physics written large enough for the naked eye.

The two tails of a comet are made of different stuff and obey different forces — which is why they point in slightly different directions.

Property	Dust tail (Type II)	Ion tail (Type I)
Made of	Micron-sized silicate/organic grains	Ionised gas (CO⁺, H₂O⁺, CO₂⁺)
Pushed by	Solar radiation pressure (photon momentum)	Solar wind + draped magnetic field
Colour	Yellow-white (reflected sunlight)	Blue (fluorescence of CO⁺)
Shape	Broad, curved, lagging the orbit	Narrow, straight, nearly antisolar
Speed away from Sun	Slow — grains barely outrun gravity	Fast — up to ~100+ km/s
Points	Roughly away from Sun, curved back	Almost exactly away from Sun