On 2 January 2004, NASA\u2019s 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.<\/p>\n
Two years later, a capsule carrying those grains entered Earth\u2019s atmosphere and landed by parachute on the Utah Test and Training Range. It was the first deliberate return of solid material from a comet and the first extraterrestrial sample brought to Earth from beyond the Moon\u2019s orbit.<\/p>\n
The mission joined three difficult operations: meeting a small moving comet, collecting fragile dust without erasing all of its scientific value, and delivering less than a milligram of material safely through atmospheric entry. The fact that the cargo was microscopic made none of those tasks small.<\/p>\n
Stardust entered the coma, not the distant tail<\/p>\n
The headline uses the familiar image of catching a comet\u2019s tail. More precisely, Stardust sampled the coma, the broad cloud of gas and dust released around Wild 2\u2019s solid nucleus. On its closest approach it passed within about 236 kilometres, according to NASA\u2019s current Wild 2 overview<\/a>.<\/p>\n That distinction is physical rather than pedantic. A comet\u2019s tails extend away from the nucleus under the influence of sunlight and the solar wind. The coma is the denser region immediately surrounding the nucleus, where grains recently lifted from the surface can be intercepted.<\/p>\n Wild 2, pronounced \u201cVilt 2\u201d, was an unusually useful target. Before 1974, it followed an orbit lying farther from the Sun, between the paths of Jupiter and Uranus. A close gravitational encounter with Jupiter reshaped that orbit, giving the comet its present period of about 6.4 years. When Stardust arrived, Wild 2 had experienced relatively few close solar passages and less repeated heating than many familiar short-period comets.<\/p>\n The spacecraft also photographed the nucleus. Its 72 images showed a small body about 2.75 kilometres across at its longest dimension, with steep scarps, depressions and active jets. Stardust was not simply collecting anonymous dust. It supplied the geological setting from which the sample had emerged.<\/p>\n The collector was tiled with a solid that was 99.8 per cent air<\/p>\n Capturing dust at 6.1 kilometres per second presented an awkward materials problem. A hard plate would stop the grains over an extremely short distance, producing intense heat, fragmentation and mixing with the collector. Slowing them gently required a medium offering very little initial resistance.<\/p>\n Stardust carried a tennis-racket-shaped grid containing more than 1,000 square centimetres of aerogel. The translucent silica material was 99.8 per cent air and roughly 1,000 times less dense than glass. Its density increased with depth, allowing a grain to meet a very light surface and then experience progressively stronger braking.<\/p>\n As particles entered, they carved narrow tracks that often widened near the impact point and tapered towards the surviving material. Some looked like tiny carrots embedded in blue smoke. Laboratory teams could map each track, cut out a wedge of aerogel surrounding it and work towards the terminal grain.<\/p>\n \u201cSoft catch\u201d is a relative term at this speed. Larger or loosely bound particles fragmented, some components melted, and organic molecules could be altered by capture heating. Scientists have had to distinguish original cometary material from aerogel, spacecraft contamination and products of the impact itself. A NASA curation review<\/a> describes the resulting work as delicate microsurgery rather than simple removal.<\/p>\n The collector also had two sides. One face was exposed to Wild 2 during the 2004 encounter. The reverse faced streams of possible interstellar dust during other collection periods. Whipple shields on the spacecraft absorbed the more dangerous impacts while the navigation camera and dust instruments continued operating.<\/p>\n The 4.6-billion-kilometre figure covers a seven-year route<\/p>\n Stardust launched from Cape Canaveral on 7 February 1999 aboard a Delta II rocket. It completed solar orbits, returned past Earth for a gravity assist in January 2001 and flew by the small asteroid Annefrank in November 2002 before reaching Wild 2.<\/p>\n NASA\u2019s mission history<\/a> gives the journey before sample return as 2.9 billion miles, or 4.63 billion kilometres. That is the total spacecraft route from launch through its loops around the Sun, comet encounter and Earth return. The captured grains did not travel the full distance after entering the aerogel. They joined Stardust for the final two years of the seven-year mission.<\/p>\n After the flyby, the collector folded back into the sample return capsule and remained sealed. Stardust itself was not designed to land. It would release the capsule near Earth, divert back into solar orbit and later be reused for the Stardust-NExT flyby of comet Tempel 1 in 2011.<\/p>\n The capsule arrived faster than any previous artificial object<\/p>\n At 05:57 UTC on 15 January 2006, the 46-kilogram return capsule separated from the main spacecraft. Four hours later it reached the top of Earth\u2019s atmosphere at about 12.8 kilometres per second, or roughly 46,000 kilometres per hour. NASA described it as the fastest atmospheric entry yet made by a human-built object, surpassing the Apollo 10 command module.<\/p>\n The capsule crossed from space to desert without guidance or propulsion. A carbon-based heat shield absorbed and shed the entry energy. A drogue parachute deployed high in the atmosphere to stabilise and slow the capsule; the main parachute opened at about three kilometres altitude.<\/p>\n It touched down at 10:10 UTC within the planned landing region on the U.S. Air Force Utah Test and Training Range. High winds carried it north of the expected ground track, but a radio beacon led recovery teams to the capsule 44 minutes after landing.<\/p>\n The canister was opened in a temporary clean room at the nearby Dugway Proving Ground, packaged under controlled conditions and flown to NASA\u2019s Johnson Space Center in Houston. The collector returned more than 10,000 Wild 2 particles larger than one micrometre. The grains were small enough to disappear in an ordinary laboratory, so documentation and contamination control became part of the science.<\/p>\n The first surprise was how much of the comet had formed hot<\/p>\n Comets assemble in cold outer regions and preserve ices and dust from the Solar System\u2019s early history. Before Stardust, it was reasonable to expect Wild 2\u2019s non-volatile grains to be dominated by material inherited from the cold molecular cloud and outer solar nebula.<\/p>\n The returned sample was more mixed. Researchers found crystalline silicates and refractory minerals that form at temperatures associated with the hot inner Solar System. A calcium-aluminium-rich inclusion named Inti resembled some of the oldest high-temperature objects found in primitive meteorites.<\/p>\n