Stardust was a 385-kilogram robotic space probe launched by NASA on February 7, 1999. Its main mission was to collect dust samples from the coma of comet Wild 2 and samples of cosmic dust, then return them to Earth for study. It was the first mission of its kind to bring samples back from space. During its journey to Comet Wild 2, it passed by and studied the asteroid 5535 Annefrank. The main mission was completed on January 15, 2006, when the sample return capsule returned to Earth.

A mission extension called NExT ended in February 2011, when Stardust met Comet Tempel 1, a small object in the Solar System that was visited by Deep Impact in 2005. Stardust stopped working in March 2011.

On August 14, 2014, scientists found possible interstellar dust particles in the Stardust capsule that returned to Earth in 2006.

Mission background

In the 1980s, scientists started looking for a mission to study a comet. During the early 1990s, several missions to study Comet Halley were the first to return close-up data. However, the U.S. mission called Comet Rendezvous Asteroid Flyby was canceled because of budget issues. In the mid-1990s, support was given to a less expensive mission, called a Discovery-class mission, to study Comet Wild 2 in 2004.

Stardust was chosen in the fall of 1995 as a low-cost NASA Discovery Program mission with clear science goals. Construction of Stardust began in 1996, and the spacecraft followed strict rules to prevent contamination, called level 5 planetary protection. Scientists believed the risk of spreading alien life was low because particles moving at over 450 meters per second (1,000 mph) would likely destroy any known microorganisms, even if they hit aerogel.

Comet Wild 2 was selected as the mission’s target because it offered a rare chance to study a long-period comet that had come close to the Sun. After an event in 1974, when Jupiter’s gravity changed Wild 2’s orbit, the comet now has a shorter orbit that brings it closer to the Sun. Scientists expected the comet to still contain material from when it formed.

The mission’s main goals were to collect and return samples of comet and interstellar dust for analysis. The spacecraft was built by Lockheed Martin Astronautics in Denver, Colorado, as a Discovery-class mission. NASA’s Jet Propulsion Laboratory (JPL) managed the mission. Dr. Donald Brownlee from the University of Washington led the science team.

The spacecraft was 1.7 meters (5 ft 7 in) long and 0.66 meters (2 ft 2 in) wide. Its design was adapted from a previous spacecraft called SpaceProbe. The body used graphite fiber panels with an aluminum honeycomb structure and was covered with protective materials like polycyanate and Kapton sheeting. To save costs, the spacecraft used parts and technologies from past missions and the Small Spacecraft Technologies Initiative (SSTI). Five scientific instruments were included, one of which was the Stardust Sample Collection tray, which would return to Earth for analysis.

The spacecraft used eight 4.41 N hydrazine thrusters and eight 1 N thrusters to control its movement and orientation. It carried 80 kilograms of fuel. Positioning information was gathered using a star camera, an inertial measurement unit, and two Sun sensors. The star camera software was provided by Intelligent Decisions, Inc.

To communicate with Earth, the spacecraft used an X-band antenna with a 0.6-meter (2 ft 0 in) parabolic dish, along with medium- and low-gain antennas. The antenna system was originally designed for the Cassini spacecraft.

The spacecraft was powered by two solar panels that generated an average of 330 watts of energy. The panels included Whipple shields to protect against dust in the comet’s coma. The design followed guidelines from the Small Spacecraft Technology Initiative (SSTI). The panels could switch between series and parallel connections depending on the spacecraft’s distance from the Sun. A nickel–hydrogen battery was also included to provide power when sunlight was limited.

The spacecraft’s computer used a radiation-hardened RAD6000 32-bit processor card. It had 128 megabytes of memory, 20% of which was used by the flight system software, which ran on VxWorks, an embedded operating system.

Comet and interstellar dust were collected in aerogel, a material with a sponge-like structure that is 1/1000 the density of glass. The collector tray, shaped like a tennis racket, had 90 blocks of aerogel covering over 1,000 square centimeters. When a particle hit the aerogel, it created a long track inside the material. The aerogel was stored in an aluminum grid inside a Sample Return Capsule (SRC), which was released as the spacecraft passed Earth in 2006.

To study the aerogel, scientists needed one million images to capture all the dust particles. These images were shared with home computer users through a program called Stardust@home. In April 2014, NASA reported finding seven particles of interstellar dust in the aerogel.

Stardust was launched with two sets of identical silicon wafers, each engraved with over one million names from people who participated in a public outreach program. One set was placed on the spacecraft, and the other was attached to the sample return capsule.

Mission profile

Stardust was launched at 21:04:15 UTC on 7 February 1999 by the National Aeronautics and Space Administration from Space Launch Complex 17A at Cape Canaveral Air Force Station in Florida, using a Delta II 7426 launch vehicle. The burn sequence lasted 27 minutes, placing the spacecraft into an orbit around the Sun. This orbit allowed the spacecraft to return to Earth in 2001 for a gravity assist maneuver, then travel to asteroid 5535 Annefrank in 2002 and Comet Wild 2 in 2004, passing each at a speed of 6.1 km/s. In 2004, a course correction enabled the spacecraft to return to Earth in 2006 to release the Sample Return Capsule for landing in Utah’s Bonneville Salt Flats.

During the 2006 Earth encounter, the Sample Return Capsule was released on 15 January 2006. Immediately after, Stardust performed a "divert maneuver" to avoid entering Earth’s atmosphere. Less than 20 kilograms of propellant remained after this maneuver. On 29 January 2006, the spacecraft entered hibernation mode, with only its solar panels and receiver active, in a 3-year orbit around the Sun that would return it near Earth on 14 January 2009.

A mission extension was approved on 3 July 2007 to reactivate the spacecraft for a flyby of Comet Tempel 1 in 2011. This extension marked the first mission to revisit a small Solar System body and used the remaining propellant, ending the spacecraft’s operational life.

On 2 November 2002 at 04:50:20 UTC, Stardust encountered asteroid 5535 Annefrank at a distance of 3,079 km (1,913 mi). The solar phase angle during observations ranged from 130 degrees to 47 degrees. This encounter tested spacecraft and ground operations in preparation for the 2004 Comet Wild 2 encounter.

On 2 January 2004 at 19:21:28 UTC, Stardust approached Comet Wild 2 on the sunward side at a relative speed of 6.1 km/s, at a distance of 237 km (147 mi). The original planned distance was 150 km (93 mi), but it was increased after a safety review to reduce the risk of dust collisions.

The spacecraft’s speed allowed the comet to overtake it from behind as they orbited the Sun. During the encounter, Stardust was on the sunlit side of the comet’s nucleus, approaching at a solar phase angle of 70 degrees, reaching a minimum angle of 3 degrees near closest approach, and departing at 110 degrees. The AutoNav software guided the spacecraft during the flyby.

During the flyby, Stardust deployed a Sample Collection plate to gather dust grain samples from the comet’s coma and captured detailed images of the icy nucleus.

New Exploration of Tempel 1 (NExT)

On 19 March 2006, Stardust scientists said they were thinking about changing the spacecraft’s path to take pictures of Comet Tempel 1. This comet had been studied before by the Deep Impact mission in 2005, which sent a probe to hit its surface. Taking images of the area where the probe hit might be important because Deep Impact could not see the crater clearly because dust from the impact blocked the view.

On 3 July 2007, the mission was approved and renamed New Exploration of Tempel 1 (NExT). This mission would show how a comet’s surface changes after it passes close to the Sun. NExT would also create the most detailed map of Tempel 1’s surface ever made, helping scientists learn more about how comet surfaces are formed. The flyby mission would use most of the spacecraft’s remaining fuel, meaning the mission would end after this event. A computer program called AutoNav would control the spacecraft for 30 minutes before it reached the comet.

The mission goals included:

At 04:39:10 UTC on 15 February 2011, Stardust-NExT flew past Tempel 1 at a distance of 181 km (112 mi). About 72 images were taken during this event. These images showed changes in the comet’s surface and revealed parts of the comet that Deep Impact had not seen before. Scientists also saw the area where Deep Impact had hit, but it was hard to see because material from the impact had settled into the crater.

On 24 March 2011 at around 23:00 UTC, Stardust used its last fuel in a final burn. The spacecraft had very little fuel left, and scientists hoped the data collected would help improve how fuel levels are measured on spacecraft. After the data was sent, the spacecraft could no longer aim its antenna or send signals. It sent a final message from about 312 million km (194 million mi) away in space.

Sample return

On 15 January 2006, at 05:57 UTC, the Sample Return Capsule (SRC) successfully separated from the Stardust spacecraft. The SRC re-entered Earth’s atmosphere at 09:57 UTC, moving at a speed of 12.9 kilometers per second, the fastest reentry speed ever recorded for a human-made object. During reentry, the capsule’s speed dropped rapidly from Mach 36 (36 times the speed of sound) to below the speed of sound within 110 seconds. The highest force experienced during reentry was 34 times Earth’s gravity, which occurred 40 seconds after reentry at an altitude of 55 kilometers over Spring Creek, Nevada. The capsule’s heat shield, made of phenolic-impregnated carbon ablator (PICA) by Fiber Materials Inc., reached a temperature of over 2,900 degrees Celsius during this fast reentry. The capsule then used parachutes to descend and landed at 10:12 UTC at the Utah Test and Training Range, near the U.S. Army Dugway Proving Ground. It was transported by military aircraft to Ellington Air Force Base in Houston, Texas, and then moved by road in a secret convoy to the Planetary Materials Curatorial facility at Johnson Space Center in Houston for analysis.

The sample container was placed in a clean room that was 100 times cleaner than a hospital operating room to prevent contamination of interstellar and comet dust. Early estimates showed at least one million tiny dust particles were embedded in the aerogel collector. Ten particles were at least 100 micrometers (0.1 mm) in size, with the largest being about 1,000 micrometers (1 mm). About 45 interstellar dust impacts were found on the back side of the cometary dust collector. Scientists are studying the dust grains through the Stardust@Home citizen science project, where volunteers help identify particles in images of the aerogel.

The total mass of the collected sample was approximately 1 milligram.

In December 2006, seven scientific papers were published in the journal Science, describing initial findings from the sample analysis. These included the discovery of a wide range of organic compounds, including two that contain nitrogen usable by living organisms. Scientists also found aliphatic hydrocarbons with longer chains than those seen in space, as well as amorphous and crystalline silicates like olivine and pyroxene, which support earlier theories about the mixing of materials from the Solar System and interstellar space. Hydrous silicates and carbonate minerals were not found, suggesting that the comet dust was not processed by water. Limited pure carbon (CHON) was also identified in the samples. Methylamine and ethylamine were found in the aerogel but were not linked to specific particles.

In 2010, Dr. Andrew Westphal announced that Stardust@Home volunteer Bruce Hudson discovered a track (labeled "I1043,1,30") in images of the aerogel that may contain an interstellar dust grain. Volunteers who make discoveries can name them, and Hudson named his finding "Orion."

In April 2011, scientists from the University of Arizona found evidence of liquid water in Comet Wild 2. They discovered iron and copper sulfide minerals, which form only in the presence of water. This finding challenged the belief that comets are too cold to melt their icy material. In 2014, scientists announced the recovery of interstellar dust particles from the Stardust mission.

The Stardust samples are now available for public study after completing training on the Berkeley webpage.

The return capsule is currently on display at the National Air and Space Museum in Washington, D.C. It was first shown there on 1 October 2008, the 50th anniversary of NASA’s founding. The capsule is displayed in sample collection mode, next to a piece of the aerogel used to collect samples.

Results

Comet samples indicate that the outer areas of the early Solar System were not separated and were not a safe place where materials from space could often survive. The data show that materials from the hot inner Solar System formed and later moved to the Kuiper belt.

In 2009, NASA announced that scientists found one of the basic chemical building blocks of life in a comet for the first time. Glycine, an amino acid, was found in material from Comet Wild 2, which was collected by the Stardust probe in 2004. Glycine has been found in meteorites and in interstellar gas clouds before, but this discovery is the first time it was found in a comet.

Isotope analysis shows that during the Late Heavy Bombardment, comets hit Earth after Earth formed but before life began to develop. Carl Pilcher, who leads NASA's Astrobiology Institute, said, "Finding glycine in a comet supports the idea that the basic building blocks of life are common in space. This makes it more likely that life in the universe is common, not rare."