Comets and Kuiper Belt objects are, in a real sense, the bits of the early Solar System that never got used. They formed when the Sun did, stayed cold enough that their ices never evaporated or their organic chemistry never cooked, and have been sitting in deep freeze for 4.5 billion years. Study them and you are studying the unprocessed starting material of every planet and moon. A handful of missions have done this in detail. A new one launches soon.
What a Comet Actually Is
A comet nucleus is a few-kilometre body of water ice, carbon dioxide ice, other frozen volatiles, rock, and dust — the famous "dirty snowball" picture, which is roughly right. When a comet's orbit takes it close to the Sun, sunlight sublimates surface ices directly into gas, and the pressure of that outflow drags dust particles with it. The gas and dust expand around the nucleus to form the coma, a diffuse fuzzy envelope that can reach tens of thousands of kilometres across. Solar wind and radiation pressure push some of that material into the two visible tails: a straight ion tail pointing directly away from the Sun, and a curved dust tail following the comet's orbit.
Most comets come from one of two reservoirs. Short-period comets (orbital periods under about 200 years) come from the Kuiper Belt and the scattered disk, out beyond Neptune. Long-period comets (periods up to millions of years, and in some cases essentially unbounded) come from the Oort Cloud, a roughly spherical halo of icy bodies surrounding the entire Solar System out to possibly 100,000 AU. Nothing from the Oort Cloud has ever been visited directly.
What Rosetta Did
The most detailed study ever made of a comet is ESA's Rosetta mission to comet 67P/Churyumov–Gerasimenko. Rosetta arrived at 67P in August 2014, after a ten-year cruise, and spent over two years orbiting the comet — the first spacecraft ever to do that. In November 2014, it dropped the Philae lander, which made the first soft landing on a comet's surface, though Philae bounced and ended up partially shadowed, cutting short its science operations.
Rosetta's findings reshaped the field. The comet turned out to have two lobes joined by a neck, probably two separate bodies that gently merged early in the Solar System's history. Its water ice had a deuterium-to-hydrogen ratio different from Earth's, weakening the case that comets delivered Earth's oceans (though 67P is one comet; the population varies). Organic molecules, including the amino acid glycine, were detected in material around the comet. And the nucleus outgassed in distinct jets tied to specific surface features, giving the first detailed look at how comet activity actually works.
What New Horizons Did
NASA's New Horizons mission flew past Pluto in July 2015, returning the first detailed images of Pluto's surface and confirming it as a more geologically complex world than anyone expected: nitrogen ice plains, cryovolcanoes, and a thin atmosphere undergoing seasonal change.
Four years later, in January 2019, New Horizons flew past a small Kuiper Belt object formally designated 486958 Arrokoth. Arrokoth is the most distant object ever visited by a spacecraft — about 6.6 billion km from Earth at the encounter. Like 67P, it has a contact-binary shape (two lobes touching), but on a much smaller, older, and colder body. Arrokoth is essentially a frozen sample of what the outer Solar System looked like before planet formation really got going, and it has stayed that way for longer than anything else we have close-up data on.
New Horizons is still in flight, past 60 AU, and could potentially encounter a third target later in the decade if a suitable KBO is found in its trajectory.
Comet Interceptor
Every comet flown past so far has been a short-period comet — a body that has been through the inner Solar System many times before and has had its surface layers processed by solar heating. The scientifically valuable target would be a dynamically new long-period comet, one falling in from the Oort Cloud for the first time, with surface chemistry essentially unchanged since the Solar System formed. The problem is that such comets are usually discovered only a few months before perihelion, which is not enough time to build and launch a dedicated mission.
ESA's Comet Interceptor solves this by launching first and picking a target later. The spacecraft will be launched around 2029 and parked at Lagrange point L2, waiting. When a suitable long-period comet is discovered on an approach trajectory that Comet Interceptor can reach with its onboard propellant, it will leave L2 and intercept. Three separate spacecraft — a main module and two smaller probes — will fly through the coma at different distances, capturing simultaneous multi-point measurements that no single-spacecraft flyby has ever managed.
Comet Interceptor is also designed to be able to intercept an interstellar object if one comes through the Solar System on a reachable trajectory. The two interstellar visitors observed so far — 'Oumuamua in 2017 and 2I/Borisov in 2019 — were both discovered too late for any mission to chase them. A future interstellar visitor might not be.
Why This Matters
Planets process their starting material beyond recognition. Every rock on Earth has been melted, metamorphosed, eroded, or chemically reworked. A Kuiper Belt object or a long-period comet has not. If you want to know what the Solar System was made of before any of that processing happened, you have to go to the bodies that never got warm. Rosetta showed how much can be learned by going there. New Horizons added the Pluto system and the first up-close KBO. Comet Interceptor adds the missing piece: an unprocessed visitor from the far reaches.