What Two Comets Are Leaving Behind
This week, Earth is walking through the dust of a comet that visited 40 years ago and won't return until 2061. Meanwhile, scientists released the first close-up data on an interstellar comet older than our Sun. Two comets, two timescales, the same week.
This week, Earth is walking through the dust of a comet that visited 40 years ago and won't be back until 2061. While we're inside that trail, scientists are still piecing together what they learned from a different comet, one that came from another star system, probably predates our Sun, and will never come anywhere near us again.
Two comets, two utterly different timescales, the same week to think about both.
Halley's Comet, twice removed
Halley's Comet last passed through the inner Solar System in 1986. It will return in 2061. That part everyone knows. The bit that catches me is what Halley left behind in between.
A comet is essentially a dirty snowball, and every time it gets close to the Sun, it sheds material. After thousands of passes, Halley has scattered a wide, sparse trail of dust around its orbit. Earth crosses through that trail every May. Most of those particles are smaller than a grain of rice. They hit our atmosphere at about 40 miles per second. They burn out before they reach the ground, and what we see from Earth are streaks of light that appear to radiate out from the constellation Aquarius.
Those streaks are the Eta Aquarid meteor shower. The peak is May 5th and 6th this year. From the UK and northern latitudes you'll see fewer than from the southern hemisphere, where the radiant climbs higher in the pre-dawn sky, but you can still catch some in the last hour or two before sunrise if the moon isn't washing the sky out. This year, a bright moon will erase the fainter ones. You'll still see the bright ones.
The thing about meteor showers is they're easy to mistake for novelty. They aren't. The Eta Aquarids have been streaking out of Aquarius every May for as long as anyone has kept records. What changes year to year is just whether the moonlight cooperates and whether we choose to look.
A child watching this week's meteor shower at age eight will be forty-three when Halley itself returns. Most adults looking up this week will never see the comet again. But we are inside its dust right now.
A visitor that arrived once
While Earth was crossing Halley's old trail, a totally different comet was being studied by a spacecraft that was never supposed to be looking at it.
The European Space Agency's JUICE mission is on its way to Jupiter. Its real job, when it arrives in 2031, is to study Jupiter's icy moons, the ones with buried oceans where something might be alive. JUICE is built for that. It carries instruments designed to detect water ice, water vapour, and the chemistry of frozen worlds.
In November 2025, JUICE was nowhere near Jupiter. It was on its long looping route through the inner Solar System, picking up gravity assists. While it was out there, the interstellar comet 3I/ATLAS swung close to the Sun. ESA decided to point JUICE at it.
The data took three months to make it back to Earth. ESA released the first analysis last month. Five instruments on the spacecraft turned out to be a near-perfect match for an interstellar visitor, because instruments built to study the icy moons of Jupiter are also instruments built to study a comet made of ice. That happy accident is honestly the part of this story I keep thinking about.
Among the findings: four days after its closest approach to the Sun, 3I/ATLAS was shedding water at a rate of 2,000 kilograms per second. That is roughly seventy Olympic swimming pools of water vapour per day, blasted out of a comet smaller than most cities, in a stream pointing back toward the Sun. The tail it left stretched more than five million kilometres behind it.
And the comet itself is probably more than ten billion years old. Our Sun is four and a half billion years old. The Earth is younger than that. The comet that JUICE caught a glimpse of formed around a different star, in a different part of the galaxy, before the rock you're standing on existed.
It is gone now. It is not coming back. The data we got from JUICE is essentially all we will ever have.
Talking about this with your kids
Two things to try this week.
Set an early alarm for May 5th or 6th and go outside before dawn. Face roughly east. You don't need a telescope, because meteors are best seen with bare eyes. You want as much sky as possible. Give your eyes ten minutes to adjust before you decide there's nothing there. A free stargazing app will help you find Aquarius if you want the radiant point exactly. Tell whoever you bring outside that the streaks of light are tiny pieces of a comet that visited in 1986 and will be back in 2061. If your kid is seven, ask them how old they'll be when Halley returns.
Then, separately, try the harder one. Two comets passed through this same Solar System in the last few years. One of them is older than our Sun. That comet is gone, and we won't see it again, but for a few weeks last autumn a spacecraft on its way to Jupiter and its icy moons happened to be in the right place to take a closer look.
We tend to think of Solar System news as things happening out there. The Eta Aquarids are a useful reminder that the Solar System is also passing through us, in dust and light, every May.
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