The vast expanse between stars is not quite empty—it’s home to a diffuse spread of dust, aptly called the interstellar medium (ISM). Studies of the ISM have largely relied on inferences since direct observations of the dim, diffuse material are difficult. But now it seems we have a chance for an up-close look. According to a recent study, seven dust particles caught by the Stardust spacecraft likely came from the ISM. If true, the particles may be the first material captured from outside the Solar System, allowing scientists to study the ISM more directly.

Assuming that they did originate outside the Solar System, the particles can be used to address unanswered questions about the ISM. For one thing, scientists are still unsure whether most of the gas in the ISM is atomic, molecular, or ionic, or whether any of these phases is dominant at all. Scientists would also hope to address whether most particles there are crystalline or amorphous in structure and how much iron is present.

(There are materials called GEMS—glass with embedded metal and sulfides—that may also originate from outside the Solar System, but this is a contentious subject within the scientific community.)

Stardust

The Stardust spacecraft, which was retired in 2011, was equipped with a 0.1 square meter plate designed to capture interplanetary dust. Most of the plate’s volume (85 percent) was an ultralow density silica aerogel.

Aerogel, nicknamed “frozen smoke,” replaces the liquid component of a conventional gel with gas, leading to an incredibly lightweight and low density material that feels like styrofoam to the touch. It’s useful for capturing dust moving at high velocities because it gradually decelerates the particles, rather than catching them via hard impacts. This makes it more likely that the particles will be captured intact.

The remaining 15 percent of the plate was covered with aluminum foil, which is complimentary to the aerogel. When a dust particle impacts with the foil, it leaves residue there, and the composition of that residue can be analyzed. This keeps the dust material from getting mixed in with the gel, which could make it harder to differentiate the two materials, a problem that the aluminum foil doesn’t share.

The plate was angled roughly in the direction of the constellation Ophiuchus to collect dust. This collection took place over 195 days, spread out over the years 2000 to 2002. By the end of the collection period, the plate had 71 tracks in the aerogel, made by captured particles.

Finding Tracks

The tricky part was not the capture of the particles—it was their identification and analysis. Over 700,000 micrograph images needed to be looked over, a prohibitively time-consuming endeavor.

To increase the project’s manpower, scientists opened it up to the general public as Stardust@Home, a citizen science project. Over 30,000 volunteers pored over the aerogel images, identifying the tracks therein.

The first data from Stardust@Home was finally released in 2006, at which point it could be analyzed.

Of the 71 tracks discovered, 46 of them were found to originate not from interstellar space but from the solar panel that was powering Stardust. The remaining 25 tracks had a chance of coming from interstellar space, but these could have also originated from micrometeoroid impacts striking the plate’s containment capsule.

Of the 25, the scientists arbitrarily chose 13 to analyze. Some of those were found to have originated from Earth, while others were more ambiguous. But the scientists were able to select three that were likely to have originated from outside the Solar System.

The three particles were even given names: Orion (a mixture of olivine, magnesium spinel, and iron-bearing phases with small amounts of calcium, chromium, manganese, and nickel), Hylabrook (a magnesium-, iron-, and silicon-rich particle with an olivine core), and Sorok

(about which there is some ambiguity, because the track made by the particle contains silicon and carbon, but it is not clear whether the carbon is residue from the particle or from the aerogel).

Interstellar Origin

To determine whether these particles actually came from outside the Solar System, the team had a set of criteria to compare them to. First, the particle must have been moving at high velocity. Orion and Hylabrook are calculated to have been captured at around 10 kilometers per second, while Sorok was captured at over 15 km/s. All three particles meet the first criterion.

Next, a particle must have a composition consistent with having formed in space, which is inconsistent with any of the materials on the spacecraft. All three of the particles meet this requirement as well.

The most definitive evidence that a particle comes from outside the Solar System would be a composition that includes an oxygen isotope not normally found here. However, if a particle’s oxygen isotopes are consistent with the Solar System, it doesn’t mean the particle originated here—it would simply mean that there isn’t definitive confirmation of an extra-solar origin.

Unfortunately, all of the particles examined here either have oxygen isotopes matching the Solar System, or there’s not enough data to say anything definitive. But because of their compositions, impact angles and speeds, and comparison with models (which predict likely angles of approach and speeds for particles originating in the Solar System), the scientists concluded that these particles most likely originate in the ISM.

They calculate that the likelihood of the particles originating within the Solar System is less than 0.03 percent.

Besides Orion, Hylabrook, and Sorok, the team also analyzed four particles that impacted on the aluminum foil. These were also likely to have come from the ISM. These match more closely with the predictions of ISM particle sizes (based on astronomical observations) than the three from the aerogel.

More Work to be Done

But unanswered questions remain. For one thing, the densities of the particles are still unknown.

If the particles are very compact and have high densities, then the new study is at odds with theoretical predictions but in agreement with astronomical observations of the ISM. If, however, the particles have low densities, the study may roughly agree with both. The latter possibility is more likely, the scientists note in the paper, based on the information gathered about the seven particles studied (three in the aerogel and four on the aluminum foil).

If that’s the case, the paper’s authors conclude, it’s good news for future missions hoping to bring back more interstellar dust.

As for the seven particles collected by Stardust, there is more work to be done to conclusively determine whether they originated outside the Solar System. The particles must next be carefully transferred from the aerogel into instruments for further analysis of the isotopes in the grains. Developing the techniques necessary to do this “will be fundamentally boring but necessary," Stardust team member Andrew Westphal told Science.

Science, 2014. DOI: 10.1126/science.1252496 (About DOIs).