Scientists caught a tiny piece of space dust outside the ISS. What they found was unusual

It was only a few dozen micrometres long, narrower than a human hair. Yet the tiny particle contained a combination of minerals never before identified in chondrule-like material. Scientists recovered it from an experiment mounted on the outside of the International Space Station (ISS).

Named Hoshi, the Japanese word for “star”, the particle was the subject of a study published in Earth and Planetary Science Lettersin 2011. Unlike most extraterrestrial material, which reaches scientists after surviving a fiery passage through Earth’s atmosphere, Hoshi was captured directly in space, making it a rare scientific find.

A collection panel that spent years outside the station

It began in 2001, when the Japan Aerospace Exploration Agency (JAXA) mounted an experiment called MPAC & SEED on the Russian Zvezda service module of the ISS. The full name is Micro-Particles Capturer and Space-Environment Exposure Device.


Its purpose was twofold. It was meant to collect tiny particles moving through space, and to test how materials cope with the space environment. For the collecting, it used silica aerogel, a light and porous material that can trap a particle without wrecking its structure.

JAXA’s account says several panels were brought back between 2002 and 2005. The one that came down in 2005 held the particle that later got its name.

That was only the start. The sample still had to be taken out, studied and compared with other extraterrestrial material.

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What scientists found inside the speck

The findings, published in Earth and Planetary Science Letters, describe Hoshi as having a texture similar to that of a porphyritic olivine chondrule. Chondrules are small, rounded mineral grains found in many meteorites. Scientists study them to better understand how the Solar System formed.

Inside Hoshi were olivine, pyroxene and an iron sulphide rich in nickel. No one had earlier identified chondrule-like material with that same combination, and that is what made it stand out.

The researchers also measured oxygen-isotope ratios in the particle’s. Isotopes are versions of the same element that have different numbers of neutrons. The proportions in which they occur let scientists compare materials and consider where they may have originated.

Their reading was that Hoshi is a fragment of a primitive parent body that had been heated to some extent. Which asteroid or other body that was, they could not say.

So the finding has a clear limit. It didn’t trace the particle’s whole journey. What it gave scientists was a set of mineral and chemical clues to hold up against other known samples.

Why collect dust in space instead of on Earth?

By the time a particle reaches the ground, the atmosphere has already heated it. Weathering and contamination can follow. Delicate material is harder to study once that has happened.

A collector positioned outside Earth’s atmosphere avoids much of the damage caused by atmospheric entry. Silica aerogel slows incoming particles and traps them, while the tracks they leave behind help researchers locate and recover the samples for analysis.

Hoshi was significant because it provided evidence of a chondrule-like object collected directly in space. In its published findings, JAXA said the discovery suggested that other primitive extraterrestrial materials with unfamiliar mineral characteristics might also exist.

The discovery did not establish that a new planet had been found, nor did it pinpoint Hoshi’s exact origin. Its significance lay in the unusual mineral combination, which did not fit neatly into categories established through the study of previously collected material.

A small sample, with limits to its story

Particles smaller like this are easy to overlook, but Hoshi is an exception. For a speck that is barely visible, it can hold information about its minerals, structure and its heating history, provided it is recovered and examined carefully.

It also shows where the evidence stops. The team could describe what the particle was made of and draw conclusions about the conditions it may have passed through. Naming the parent body was beyond them.

More than a decade after the paper, Hoshi remains a good example of what a carefully designed experiment can bring back from space. The panel didn’t return a map of where the particle had been, or a label with its birthplace. It returned a fragment that scientists could examine, measure and compare.

For researchers trying to understand the early Solar System, that small fragment was enough to add a new piece of evidence, even if it could not tell the whole story.