Stardust: From Comet Flyby to Sample Return

On 7 February 1999, Stardust lifted off from Launch Pad 17A at Cape Canaveral, Florida, and began a journey around the Sun. Its objective was to fly past comet 81P/Wild 2, collect dust from its coma, and return the samples to Earth. Along the way, the opposite face of its collector also captured candidate interstellar dust particles passing through the Solar System. The mission did not require a landing on the comet's nucleus.

NASA organized the mission through its Discovery Program, with the Jet Propulsion Laboratory managing it. Donald Brownlee of the University of Washington was principal investigator, and Peter Tsou was deputy principal investigator. Lockheed Martin Space Systems in Denver designed, built and operated the spacecraft. The research team had to integrate spacecraft navigation, particle capture and terrestrial sample analysis into one continuous mission sequence.

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The spacecraft had a launch mass of about 385 kilograms, including a roughly 46-kilogram return capsule and 85 kilograms of propellant, and was launched on a Delta II rocket. Its design combined new low-density aerogel capture technology with existing hardware: the navigation camera incorporated spare Voyager mission components, a sensor design from Galileo and updated electronics.

The two faces of the tennis-racket-shaped collector were used for cometary and interstellar dust, respectively. Silica aerogel increased in density from the surface inward, slowing high-speed particles along slender tracks that researchers could later use to locate them. Shields protected the main spacecraft. A dust-flux monitor recorded the number and size of impacts, while another analyzer used ions generated by impacts for time-of-flight mass spectrometry. The latter was developed by a German team, with the Finnish Meteorological Institute participating in software development.

References: [1]

Stardust conducted two interstellar dust-collection campaigns, in 2000 and 2002, and used Earth's gravity to alter its trajectory on 15 January 2001. On 2 November 2002, it flew past the asteroid Annefrank, rehearsing ground operations and onboard procedures for the later comet encounter.

On 2 January 2004, the spacecraft flew past Wild 2 at a relative speed of about 6.1 kilometers per second, passing close through the coma and taking 72 images of the nucleus. The images showed steep cliffs, depressions and multiple active jets. Dust impacts did not rise and fall smoothly; instead, dense groups of particles alternated with gaps. About six hours after closest approach, the collector folded back into the return capsule, sealing the samples to await the Earth encounter two years later.

References: [1]

At 05:57 Universal Time on 15 January 2006, the return capsule separated from the main spacecraft. About four hours later, it entered Earth's atmosphere, slowed with its heat shield and parachutes, and landed at the Utah Test and Training Range at 10:10. The main spacecraft performed a separate avoidance maneuver and continued orbiting the Sun. The samples' landing therefore did not mean that the entire spacecraft had returned to the ground.

The returned cometary samples contained more than 10,000 micrometer-sized particles. Ground investigations examined not only fragments preserved at the ends of tracks in the aerogel but also impact marks on the aluminum frame. Capture itself could fragment or heat particles, so these effects had to be distinguished when determining their original composition. About 200 researchers participated in the preliminary analysis, allowing the same set of samples to be studied with different instruments and methods.

References: [2] [3]

In their preliminary study published in 2006, Brownlee and colleagues reported that the samples contained olivine, pyroxene and other high-temperature minerals, as well as organic matter. They were not a collection of compositionally uniform, wholly unprocessed fine interstellar dust. The team argued that some material formed in hot regions of the inner solar nebula had subsequently been transported to the outer regions where comets formed.

This shifted the question from whether comets preserved early material to how material from different sources became mixed. The samples could be compared with meteorites and remotely measured spectra and could continue to be studied with more advanced microscopes and mass spectrometers. The preliminary findings supported large-scale material transport, but a single set of particles did not establish one unique transport mechanism.

References: [2]

The main spacecraft subsequently undertook the Stardust New Exploration of Tempel 1 mission, or Stardust-NExT. On 15 February 2011 Universal Time, it flew past comet Tempel 1 at a distance of about 181 kilometers and returned 72 images. These were used to compare the comet with observations from the Deep Impact mission and identify the crater that mission had produced.

On 24 March 2011, the team scheduled a final engine burn to exhaust the propellant; it lasted 146 seconds. Engineers used the records to check fuel-consumption models. Final communications ended at 16:33 Pacific Daylight Time that day. From launch in 1999, sampling in 2004 and capsule landing in 2006 to the end of operations in 2011, Stardust linked flyby observations, physical sample analysis and a repeat visit in a twelve-year mission.

References: [3] [4]