NEOWISE

In the summer of 2020, a small, dark lump of primordial ice swept in from the outer edge of the Solar System and briefly became the brightest comet the Northern Hemisphere had seen in more than two decades. Catalogued as C/2020 F3 and known to the public simply as NEOWISE, it hung in the twilight below the Big Dipper for weeks, trailing a golden dust tail and a fainter blue-white streak of glowing gas. For many people who watched it, it was the first great comet of their lifetimes — and the last they will ever see, because NEOWISE will not return for roughly 6,800 years.

The comet takes its name from the mission that found it: NEOWISE, the reactivated Near-Earth Object hunting phase of NASA's Wide-field Infrared Survey Explorer (WISE) space telescope. On 27 March 2020, the observatory's infrared detectors picked out the comet's warm coma against the cold background of space, and a modest icy body no bigger than a mountain was on its way to becoming the Great Comet of 2020.

A visitor from the deep freeze

NEOWISE is a long-period comet on an almost parabolic, retrograde orbit that carries it far beyond the planets — its path is inclined about 129 degrees to the plane of the Solar System, meaning it loops around the Sun 'backwards' relative to Earth and the other worlds. Its orbit is enormous: the outbound semi-major axis stretches to roughly 358 astronomical units, hundreds of times Earth's distance from the Sun, out into the frigid realm where comets sit for aeons largely unchanged since the Solar System formed.

That vast orbit sets the timetable for its visits. Before its 2020 passage the comet's period was around 4,500 years; the gravitational tug of the giant planets nudged it onto a slightly higher-energy path, lengthening the outbound period to roughly 6,800 years — about 2.48 million days. The last time NEOWISE rounded the Sun, near the dawn of recorded history, the earliest cities and monuments of the ancient world were only just taking shape. The next time, no one alive today, nor any of their descendants for hundreds of generations, will be here to see it.

A nucleus the size of a mountain

The solid heart of the comet — its nucleus — is a dirty snowball of ice, dust and rock only about 5 kilometres across, small enough to fit inside a large city. Infrared measurements from the NEOWISE spacecraft itself pinned down that size and showed the nucleus is quite dark, reflecting only a few per cent of the sunlight that hits it. It spins roughly once every 7.6 hours.

Small as it is, the nucleus is the engine for everything spectacular about the comet. As sunlight warmed it on approach, frozen gases sublimated straight from solid to vapour, dragging dust off the surface and inflating a glowing coma far larger than the nucleus. Crucially, NEOWISE survived that heating intact — many bright comets crumble near the Sun, but this one held together and put on its show.

Grazing the Sun, then swinging past Earth

NEOWISE reached perihelion, its closest approach to the Sun, on 3 July 2020 at a distance of about 0.29 AU — roughly 43 million kilometres, inside the orbit of Mercury. That searing pass is exactly what made the comet so bright: the intense heat drove furious outgassing and dust release, and NASA's Sun-watching spacecraft, including the Parker Solar Probe and the SOHO observatory, tracked it as it rounded the Sun.

Having survived perihelion, the comet then swept toward Earth, making its closest approach to our planet on 23 July 2020 at about 0.69 AU (some 103 million kilometres). It never came dangerously close — that is a comfortable two-thirds of the Earth–Sun distance — but the geometry was ideal for observers in the north, lifting the comet high enough into a dark sky to be admired with the naked eye.

Three tails and a splash of colour

Photographs of NEOWISE captured two prominent tails fanning out from the coma. The broad, curving, golden-yellow tail is made of dust grains pushed gently off the nucleus and lit by reflected sunlight. Above it ran a straighter, fainter blue tail of ionised gas — including carbon monoxide ions — sculpted into a thin streak by the solar wind and pointing almost directly away from the Sun.

NEOWISE also showed a rarer, third feature: a faint sodium tail, reported in mid-July 2020, produced by sodium atoms escaping the coma and glowing in a distinctive yellow-orange line. Sodium tails have been documented on only a handful of very bright comets, most famously Hale-Bopp, which makes NEOWISE a valued case study for astronomers probing what these icy bodies are made of.

The brightest comet in a generation

At its best, NEOWISE shone at around magnitude 0.5 to 1 — comparable to a bright star — making it the most impressive comet visible from the Northern Hemisphere since Hale-Bopp in 1997. It has since been informally recognised as the Great Comet of 2020. For a few weeks it was a genuine public event, drawing crowds to dark hillsides and filling social media with long-exposure photographs of its twin tails arcing over the horizon.

The show did not last. Comets fade quickly as they retreat from the Sun and their outgassing slows; through late July 2020 NEOWISE dimmed steadily toward the edge of naked-eye visibility, and it soon required binoculars and then telescopes to follow at all as it headed back out into the deep freeze.

Watching it, if you had the chance

For the few who still have images and memories of that July, NEOWISE was easiest to find in the northwest after sunset, low beneath the bowl of the Big Dipper in the constellation Ursa Major, best viewed from a dark site away from city lights. Binoculars revealed the golden dust tail beautifully, while cameras on tripods, using exposures of several seconds, brought out the colour and the delicate ion tail invisible to the eye.

That window is now closed. NEOWISE is far beyond the planets again, dimming toward the outer Solar System, and it will not brighten our skies again for millennia. On Temposia you can instead trace its steep, retrograde orbit and see how a body from the outer dark briefly crossed the inner Solar System before vanishing for another 68 centuries.