Cosmic dust collected by space missions
To be sure exactly where a grain of dust came from, you have to fetch it yourself. Spacecraft have caught comet dust in flight, counted impacts from dust far beyond Neptune and brought back grains from asteroids.
Micrometeorites found on Earth could come from any of thousands of comets and asteroids. Space missions solve that problem by collecting from one known body. NASA's Stardust caught dust from comet Wild 2 in a featherweight foam called aerogel. Rosetta studied dust around comet Churyumov–Gerasimenko. Hayabusa2 and OSIRIS-REx brought back pieces of the asteroids Ryugu and Bennu, which turn out to look a lot like some micrometeorites.
Key facts
- Stardust
- Flew through comet Wild 2's dust in 2004; capsule landed in Utah in January 2006
- Rosetta
- Orbited comet 67P/Churyumov–Gerasimenko from 2014 to 2016, analysing its dust
- Hayabusa2
- Returned about 5.4 g from asteroid Ryugu in December 2020
- OSIRIS-REx
- Returned about 122 g from asteroid Bennu in September 2023
Counting dust in orbit
The first space-age measurements of dust simply counted hits. Spacecraft carried sensors that recorded each impact, and returned hardware showed microscopic craters. The Long Duration Exposure Facility, retrieved in 1990 after almost six years in orbit, gave one of the most quoted estimates of how much dust meets the Earth (see how much falls). Space-station panels, satellite surfaces and the windows of space shuttles have also been studied for tiny pits made by dust and by human-made debris.
Stardust: comet dust in a cloud of glass
NASA's Stardust spacecraft was launched in 1999 to collect dust from comet 81P/Wild 2. Catching grains moving at about 6 km/s without destroying them needed a remarkable material: aerogel, a silica foam so light that it is mostly empty space. Grains ploughed into it like bullets into snow, leaving carrot-shaped tracks with the particle at the end.
Stardust flew past the comet in January 2004, and its capsule parachuted into the Utah desert in January 2006. The biggest surprise was that the comet dust contained minerals that formed at very high temperatures, including fragments like chondrules and calcium–aluminium-rich inclusions. They must have formed close to the young Sun and then been carried out to the cold region where comets were made, showing that material was mixed across the whole early Solar System.
Stardust also carried a second collector facing the stream of dust that blows in from between the stars. Volunteers searching scanned images of the aerogel online helped find tracks, and in 2014 researchers reported a handful of candidate interstellar grains.
Comets up close
The European spacecraft Giotto flew through the dust around Halley's comet in 1986 and measured the make-up of individual grains, finding many rich in carbon, hydrogen, oxygen and nitrogen. Three decades later, the European Rosetta mission spent two years alongside comet 67P/Churyumov–Gerasimenko. Its instruments collected dust grains drifting from the comet and examined them with a microscope and a mass spectrometer. They found fluffy aggregates of tiny grains, with a large share of carbon-rich organic matter, very like the chondritic porous interplanetary dust particles caught in the stratosphere.
Dust far out in the Solar System
NASA's New Horizons spacecraft carries a dust counter designed and built by students, the first science instrument of its kind on a planetary mission. On its way past Pluto and into the Kuiper belt, it has kept recording dust impacts, and has found more dust far beyond Neptune than models predicted. Some of that distant dust is expected to drift inwards and eventually reach the Earth.
Asteroid samples and micrometeorites
Japan's Hayabusa mission returned tiny grains from the stony asteroid Itokawa in 2010, confirming that ordinary chondrites come from such bodies. Its successor, Hayabusa2, brought back about 5.4 g from the dark asteroid Ryugu in 2020, and NASA's OSIRIS-REx delivered about 122 g from the asteroid Bennu in 2023. Both samples are rich in clay minerals, carbonates and organic matter, similar to the rarest carbonaceous chondrites, CI chondrites.
These samples have never been exposed to Earth's air or water, so they are a perfect benchmark. Researchers have compared them with fine-grained micrometeorites and found close similarities in mineralogy and texture, supporting the idea that dark, water-rich asteroids and comets supply much of the dust that reaches the Earth. They also show that fragile material like this is badly under-represented among meteorites, which is one reason micrometeorites matter.
Did you know?
Aerogel holds world records as one of the lightest solid materials ever made; a block the size of a brick weighs only a few grams.
Some grains caught by Stardust had formed at temperatures above 1,000 °C, inside a comet that formed in one of the coldest parts of the Solar System.
The student-built dust counter on New Horizons was named after Venetia Burney, the English schoolgirl who suggested the name Pluto in 1930.