A former groundskeeper and log home-builder in a small Ontario town has helped discover what is likely the first speck of space dust from outside our solar system ever collected and analyzed. The discovery reveals new insights about interstellar space and the dust that permeates it.
While on disability after a stroke, 51-year-old Bruce Hudson of Midland, Ont., spent thousands of hours in front of his computer, poring through images of samples brought back by a NASA spacecraft as part of the Stardust citizen science project.
In the process, he managed to find a precious dust particle captured by the spacecraft — one of only two from the spacecraft's sample collectors that could be analyzed. A third left tracks, but went so fast that it vaporized in the process.
The new analysis led by Andrew Westphal, a physicist at the University of California, Berkeley's Space Sciences Laboratory, has revealed that those dust particles were tiny fluffy clumps, each about two thousandths of a millimetre wide. They contained geometric crystals on the inside and a shapeless, glassy material on the outside that was likely generated by heating and melting followed by sudden cooling. The two dust specks each contained different minerals.
"These two particles probably formed around other stars," Westphal said in an interview with CBC News. They have been in the "interstellar medium" between stars for millions of years. "And each had different histories and underwent all kinds of adventures."
The analysis of the two particles, along with much tinier dust particles trapped in the aluminum foil of the spacecraft, revealed that interstellar dust is far more diverse and complex than scientists would have guessed from their previous observations by telescope. From the overview of the dust provided by the telescopes, scientists had created very simple models of what the dust might be like.
"It's a bit of a surprise and a delightful surprise to find these are much richer on a particle-by-particle basis than these simple models," said Westphal.
He and his team published the results in the journal Science on Thursday.
NASA's Stardust spacecraft launched in 1999 to sample both the dust from a comet and stardust from the interstellar medium — a stream of dust from deep space that our solar system is moving through.
Driving through a cloud of snowflakes
Westphal said the way our solar system moves through the interstellar dust is like a car driving through a cloud of snowflakes — the relative movements make it appear that all the snow or dust is coming from a particular direction, toward the front windshield. The Stardust spacecraft was designed to capture the dust as gently as possible, to maximize the chance that they would survive rather than being smushed in the process like bugs on a windshield.
"So what you're going to do is you drive your car in the direction of the wind," he explained, "and you'll collect snowflakes gently on your back windscreen."
Stardust collected the particles in 132 panels, each two centimetres by four centimetres, made of a transparent, spongy, foam-like material called aerogel that is so fine and lacy that it's made mostly of air.
"They're very light — somewhere around a tenth of a gram per cubic centimetre," said Rhonda Stroud, a physicist at the Naval Research Laboratory in Washington, D.C. who co-authored the paper."
The idea was that the microscopic dust particles would burrow through the aerogel and get stuck there, leaving behind a small track. Their direction of travel would give away whether they were part of the interstellar dust.
After its journey through space, the Stardust spacecraft dropped the aerogel samples by parachute back down to Earth in 2006 so they could be analyzed.
That's where volunteers like Hudson came in. Looking for the tracks in the aerogels was like hunting a needle in a haystack. To make matters worse, computers weren't very good at it identifying them.
"It turns out that human eyes are just much better at recognizing these particular features," Stroud said.
Westphal and his team scanned thousands of microscope images of the aerogels and turned them into movies that looked like a microscope focusing at different depths. They recruited volunteers through a citizen science project called Stardust@home to help find the tracks.
When Stardust@home launched in 2006, Hudson had just spent a few years in and out of the hospital recovering from a stroke — learning to walk and talk again while dealing with seizures and the side effects of many different drugs that left him unable to work.
"And I thought, 'Well, now what am I supposed to do?'" he recalled.
Like playing slots
Hudson, who had long been interested in astronomy and science shows on TV, heard about Stardust@home and decided to give it a try.
He went on the website where he was told how the tracks would look — "like a carrot growing in the ground — you got the top part and you got the root, right down to the tip." Particles tend to break up as they pass through the aerogel — smaller pieces end up in the "tip," while bigger pieces remain in the upper part.
Hudson estimates he spent thousands of hours on the project.
"Some days, I felt OK. I'd go, you know, 14 hours plus," he said. He likened it to playing slots at the casino. "It's almost like you put in that $20 bill, spin it and you don't know what going to be next. The next one might be the winner."
Sure enough, Hudson eventually found a winner. When he received an email from Westphal with the news, his first reaction was to ask if it was a joke. Westphal responded that no, it wasn't.
"I said, 'Oh my god, really?" said Hudson, recalling that he felt "a little tingling thing going on."
"I was like, 'Cool.' I never thought I would ever get one. There's so many people doing it."
Altogether, Stardust@home has had more than 30,000 volunteers, who have so far searched 77 of the 132 aerogels.
The track and dust particle discovered by Hudson, which he named Orion after a constellation in the night sky, contains the minerals olivine, magnesium spinel, and some iron with calcium, chromium, manganese and nickel thrown in. The other, found by a Naomi Wordsworth of South Buckinghamshire, England, and named Hylabrook, after a Robert Frost poem, contains olivine along with magnesium, iron and silicon. The particles were analyzed with X-rays from two different particle accelerators.
Four space dust particles 1,000 times smaller than the specks in the aerogel were found in the aluminum foil around the aerogels as part of a search led by Stroud. Some of those specks contained sulphur — a bit of a surprise, since many scientists had thought solid sulphur wouldn't exist in interstellar space.
The researchers say the composition of the seven dust particles and the direction of travel of the larger particles through the aerogel strongly suggest they are from outside the solar system.
But in order to really confirm that, they need to fingerprint the oxygen in the particles. The oxygen in our solar system has a very characteristic fingerprint in terms of the ratio of different isotopes — forms of oxygen with different masses.
"That will be the smoking gun," Westphal said.
The challenge will be figuring out a way to conduct the analysis without damaging or losing the seven precious dust specks, given that there are no current plans to collect more interstellar dust.
"At least for the next decade, this is the entire inventory that we have to work with," he added. " So we have to be very careful."