Stardust (spacecraft)

• 1. Introduction

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  Stardust was a 385-kilogram robotic space probe that launched on the 7th of February 1999 on a mission to do something no spacecraft had ever done: fly through the coma of a comet, scoop up dust grains traveling at over 1,000 miles per hour, and bring them home to Earth alive. The probe was the first of its kind, a sample-return mission targeting Comet Wild 2. When its sample capsule touched down in Utah on the 15th of January 2006, it became the fastest human-made object ever to re-enter Earth's atmosphere, hitting the upper air at 46,440 kilometers per hour. Inside that capsule, buried in a ghostly silica foam called aerogel, were grains that had floated through the solar system for billions of years. What those grains contained would challenge assumptions about the early solar system, the outer reaches beyond Neptune, and the very origins of life.

• 2. A Mission Born of Canceled Dreams

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  Scientists began pushing for a dedicated comet mission in the 1980s, and early missions to Comet Halley in the 1990s gave them their first close-up data from a comet. A more ambitious American project, Comet Rendezvous Asteroid Flyby, was on the drawing board but was canceled for budgetary reasons. What emerged from that disappointment was a leaner ambition: a Discovery-class mission with focused science goals and a tight budget. Stardust was competitively selected in the fall of 1995. Construction began in 1996 under the most stringent contamination restrictions, level 5 planetary protection. Scientists had to ensure nothing from Earth hitched a ride and corrupted the samples, though the risk of spreading alien microbes was judged low. Particles hitting the aerogel collector at over 1,000 miles per hour were believed to be lethal to any known organism. The principal investigator was Dr. Donald Brownlee from the University of Washington, and the spacecraft was designed, built, and operated by Lockheed Martin Astronautics in Denver, Colorado. The target, Comet Wild 2, was chosen for a specific reason: it was a long-period comet from the distant outer solar system that had only recently been nudged into a tighter orbit. In 1974, Jupiter's gravity had pulled Wild 2 inward, making it newly accessible. Scientists believed its original primordial material would still be largely intact.

• 3. A Spacecraft Built on Borrowed Ideas

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  The Stardust spacecraft bus measured 1.7 meters long and 0.66 meters wide, adapted from the SpaceProbe deep space bus that Lockheed Martin had developed earlier. Its structure was built with graphite fiber panels over an aluminum honeycomb, then wrapped in polycyanate and Kapton sheeting for additional protection. To keep costs low, engineers drew heavily on technology from other missions, including the Small Spacecraft Technologies Initiative. The navigation camera used the optical assembly from the Voyager Wide Angle Camera, the only remaining Voyager spare camera assembly was even repurposed as a testing tool. The 15-watt transponder originally designed for the Cassini spacecraft was installed for communications. The computer ran on a radiation-hardened RAD6000 32-bit processor card with 128 megabytes of storage and an operating system built on VxWorks, an embedded software developed by Wind River Systems. Power came from two solar arrays producing an average of 330 watts, with a single nickel-hydrogen battery for backup. Those arrays were fitted with Whipple shields to protect them from Wild 2's coma. The probe launched aboard a Delta II 7426 rocket from Space Launch Complex 17A at Cape Canaveral Air Force Station in Florida at 21:04:15 UTC on the 7th of February 1999.

• 4. The Aerogel Trap

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  Catching comet dust at flyby speeds required an extraordinary material. Aerogel is a silicon-based solid where 99.8 percent of the volume is empty space, a porous, sponge-like structure dense enough to stop a high-speed particle but light enough to avoid destroying it. When a grain hits the aerogel, it buries itself and carves a track up to 200 times the length of the grain itself. The Stardust sample collector was roughly the size of a tennis racket, containing ninety blocks of aerogel providing more than 1,000 square centimeters of capture surface. The collector was packed in an aluminum grid inside the Sample Return Capsule. Imaging every grain captured would require one million photographs, a task too large for any single research team. NASA distributed the images to home computer users through a citizen science program called Stardust@home, mirroring the model of other distributed computing projects. The program allowed volunteers to make real discoveries: in 2010, Dr. Andrew Westphal announced that volunteer Bruce Hudson had found a track labeled "I1043,1,30" that may contain an interstellar dust grain. Hudson named his find "Orion." In April 2014, NASA reported that seven particles of actual interstellar dust had been recovered from the aerogel, the first physical samples ever retrieved from outside the solar system.

• 5. Through the Coma of Wild 2

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  At 19:21:28 UTC on the 2nd of January 2004, Stardust encountered Comet Wild 2 on its sunward side, passing at a relative velocity of 6.1 kilometers per second. The original plan had called for a closest approach of 150 kilometers, but a safety review board pushed the distance out to 237 kilometers to reduce the chance of catastrophic dust collisions. The comet actually overtook the spacecraft from behind as both traveled around the Sun, an unusual geometry that required precise maneuvering. The AutoNav software managed the flyby autonomously, deploying the sample collection plate and operating the navigation camera through a sequence of pre-loaded encounter commands. During the approach through the asteroid belt, Stardust had already rehearsed its systems with a flyby of asteroid 5535 Annefrank on the 2nd of November 2002, passing within 3,079 kilometers. That encounter, used mainly as an engineering test, was also the first time the spacecraft's instruments were exercised against a real solar system body. The encounter with Wild 2 set a new record: Stardust became the furthest solar-powered object ever flown, reaching 2.72 AU from the Sun during its extended journey.

• 6. The Fastest Return to Earth

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  On the 15th of January 2006, at 05:57 UTC, the Sample Return Capsule separated from Stardust. It hit the upper atmosphere at 09:57 UTC traveling at 46,440 kilometers per hour, Mach 36, the fastest reentry speed ever recorded for a human-made object. The capsule decelerated from Mach 36 to subsonic speed in 110 seconds. Peak deceleration reached 34 g, encountered 40 seconds into the reentry at an altitude of 55 kilometers above Spring Creek, Nevada. The heat shield, a phenolic-impregnated carbon ablator produced by Fiber Materials Inc., reached temperatures above 2,900 degrees Celsius. After the parachute deployed, the capsule touched down at 10:12 UTC at the Utah Test and Training Range near the U.S. Army Dugway Proving Ground. Military aircraft carried it to Ellington Air Force Base in Houston, then an unannounced convoy delivered it by road to the Planetary Materials Curatorial facility at Johnson Space Center. The clean room there operated at a cleanliness level 100 times that of a hospital operating room. Early estimates suggested at least one million microscopic dust specks were embedded in the aerogel. Ten particles were found measuring at least 100 micrometers, and the largest reached approximately 1,000 micrometers. The combined mass of the entire harvested sample was roughly one milligram.

• 7. What the Dust Revealed

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  In December 2006, seven papers appeared in the journal Science with the first detailed findings from the Wild 2 samples. The results unsettled assumptions about where cometary material forms. Among the compounds found were two organic molecules containing biologically usable nitrogen, along with aliphatic hydrocarbons with longer chain lengths than those seen in the diffuse interstellar medium. The samples contained both amorphous and crystalline silicates, including olivine and pyroxene. The presence of crystalline silicates was significant: these are high-temperature materials that form close to the Sun, not in the cold outer solar system where Wild 2 originated. Their presence in a comet nucleus supported the idea that material formed in the hot inner solar system had been transported outward to the Kuiper belt region. Hydrous silicates and carbonate minerals were notably absent, suggesting Wild 2's dust had never been processed by liquid water. Then, in 2009, NASA announced that scientists had identified glycine, a fundamental amino acid and building block of proteins, in material captured from Wild 2 during the 2004 flyby. Carl Pilcher, who led NASA's Astrobiology Institute, commented that the discovery strengthened the argument that life in the universe may be common. In April 2011, scientists from the University of Arizona found evidence in the samples of iron and copper sulfide minerals that could only have formed in the presence of liquid water, overturning the assumption that comets never get warm enough to melt their icy interiors.

• 8. Stardust-NExT and the End

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  After releasing the sample capsule in January 2006, Stardust was put into hibernation for three years in a heliocentric orbit. On the 3rd of July 2007, NASA approved a mission extension called New Exploration of Tempel 1, or NExT, the first mission ever to revisit a small solar system body. Comet Tempel 1 had been struck by an impactor from the Deep Impact mission in July 2005, but the resulting dust cloud had obscured the crater from Deep Impact's own cameras. Stardust-NExT would try to photograph that crater. At 04:39:10 UTC on the 15th of February 2011, the spacecraft passed Tempel 1 at a distance of 181 kilometers, capturing an estimated 72 images. The images showed terrain changes and revealed parts of the comet surface that Deep Impact had never seen. The impact crater was visible but barely, material that had settled back into the depression had mostly filled it. NExT also made Tempel 1 the most thoroughly mapped comet nucleus to date. On the 24th of March 2011 at approximately 23:00 UTC, with almost no propellant remaining, Stardust conducted a burn to consume its last fuel. Scientists collected data on how the fuel depleted, hoping to improve methods for estimating fuel levels on future spacecraft. The transmitter was then switched off. The spacecraft sent its final acknowledgement from approximately 312 million kilometers away.