How NASA Caught a Comet in a Jar and Rewrote the History of Our Solar System

A Cosmic Errand Like No Other

On January 2, 2004, NASA's Stardust spacecraft flew through the cloud surrounding comet 81P/Wild 2 at 6.1 kilometres per second. A collector filled with silica aerogel faced the incoming dust, slowing microscopic particles that would otherwise have struck like tiny hypervelocity projectiles. The mission's goal was audacious and oddly simple to state: catch a piece of a comet, bring it home, and see what it tells us about the dawn of our solar system.

Two years later, a capsule carrying those grains entered Earth's atmosphere and landed by parachute on the Utah Test and Training Range — the first deliberate return of solid material from a comet and the first extraterrestrial sample brought to Earth from beyond the Moon's orbit. It delivered the first solid samples from space since the 1970s Moon missions. For planetary scientists, it was like finally getting their hands on a time capsule sealed 4.6 billion years ago.

The Genius of Aerogel

Stardust carried a tennis-racket-shaped grid containing more than 1,000 square centimetres of aerogel. This ghostly, almost weightless material — essentially a frozen foam of silica — was the key to the entire endeavour. Particles were brought to a standstill as they penetrated into the aerogel with limited heating or alteration. Thousands of tiny particles, typically leaving carrot-shaped tracks, were trapped, most of them smaller than 10 micrometers in size.

Measurements of dust impacts on the front of the spacecraft suggest that the aerogel particle collector was impacted by approximately 2,800 particles larger than 15 microns in diameter. The spacecraft made its closest encounter at a range of 155 miles (250 kilometres), and the sample collector, deployed on December 24, 2003, was retracted about six hours after closest approach, stowed, and then sealed in the sample vault. During its final plunge Earthward, the capsule became the fastest human-made object to enter our atmosphere, achieving a velocity of about 28,600 mph (12.8 kilometres per second).

What the Comet Revealed

When Stardust launched in 1999, many scientists expected the comet's rocky material would be dominated by the primordial dust that built the solar system. But the actual samples told a different story: Wild 2 contained a potpourri of dust that formed from different events early in the solar system's history. The comet contained an abundance of silicate grains much larger than predictions of interstellar grain models, and many were high-temperature minerals that appear to have formed in the inner regions of the solar nebula — their presence in a comet proving that the formation of the solar system included mixing on the grandest scales.

Scientists also detected the simplest amino acid, glycine, in the particles gathered from Wild 2. Because glycine is one of the building blocks of proteins, its presence in comets strengthens the idea that the molecules necessary to create life may be widespread throughout the universe. Researchers confirmed the finding was genuine: "We discovered that the Stardust-returned glycine has an extraterrestrial carbon isotope signature, indicating that it originated on the comet."

A Legacy That Keeps Giving

Hundreds of scientists and dozens of experimental techniques in facilities around the world contributed to the preliminary examination of the first samples. The Stardust mission samples indicated that some comets may have included materials ejected from the early sun and may have formed very differently than scientists had theorized. Kept in the cold storage of space for nearly its entire lifetime, the comet avoided alteration by heat and water seen in asteroid samples — making it an almost perfect fossil record of the solar system's infancy.

The spacecraft, which was still operational after its primary mission, was later recycled for the Stardust-NExT mission, which flew by comet Tempel 1 on February 14, 2011. The broader lesson from Stardust is that the solar system was far more turbulent and well-mixed in its youth than anyone imagined — and that a handful of microscopic grains, caught in a sheet of aerogel and ferried across 4.6 billion kilometres of space, was all it took to prove it. Future sample-return missions, including those targeting asteroids and eventually other comets, will build on this blueprint, chasing ever older and more pristine pieces of the puzzle that made us.