Stardust was a robotic spacecraft launched by NASA on February 7, 1999. Its main goal was to gather dust samples from the coma of Comet Wild 2 and cosmic dust, then return them to Earth for study. It was the first mission of its type to return samples from a comet. During its journey to Comet Wild 2, it also passed by and studied the asteroid 5535 Annefrank. The mission was completed successfully on January 15, 2006, when the sample return capsule landed on Earth.

A mission extension, named NExT, ended in February 2011 when Stardust flew by Comet Tempel 1, a small object in the solar system previously visited by the Deep Impact mission in 2005. Stardust stopped operating in March 2011.

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

Mission background

Beginning in the 1980s, scientists started planning a special mission to study a comet. In the early 1990s, several missions to study Comet Halley were the first to successfully return close-up data. However, the U.S. mission called Comet Rendezvous Asteroid Flyby was canceled because of budget problems. In the mid-1990s, support was given to a less expensive mission called Stardust, which would study Comet Wild 2 in 2004.

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

Comet Wild 2 was chosen as the mission’s target because it offered a rare chance to study a long-period comet that had come close to the Sun. In 1974, Jupiter’s gravity changed Wild 2’s orbit, bringing it closer to the Sun. Scientists expected the comet still had material from when it formed long ago.

The mission’s main goals included collecting samples from the comet and studying its composition. 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 feet 7 inches) long and 0.66 meters (2 feet 2 inches) wide. It used materials like graphite fiber panels and aluminum honeycomb for strength. To save costs, it reused designs from past missions. The spacecraft had five scientific tools, including a sample collection tray that would return to Earth.

The spacecraft used eight hydrazine thrusters to control its movement and orientation. It had 80 kilograms of fuel for maneuvers. A star camera, inertial measurement unit, and Sun sensors helped track its position. The spacecraft communicated with Earth using an X-band antenna and a 15-watt transponder.

Power came from two solar arrays that provided 330 watts of energy. The arrays had protective shields to guard against comet dust. A nickel–hydrogen battery stored energy when sunlight was weak. The computer used a radiation-hardened processor and had 128 megabytes of memory.

Comet and interstellar dust were collected in aerogel, a very light material with a sponge-like structure. The collector tray had 90 blocks of aerogel, covering over 1,000 square centimeters. When a particle hit the aerogel, it created a long path up to 200 times the particle’s length. The aerogel was stored in a capsule released near Earth in 2006.

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

Stardust carried two sets of silicon wafers with names of over one million people who joined a public outreach program. One set was on the spacecraft, and the other was on 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 the Cape Canaveral Air Force Station in Florida, using a Delta II 7426 launch vehicle. The rocket’s engines burned for 27 minutes, placing the spacecraft into an orbit around the Sun. This path would take the spacecraft past Earth in 2001 for a gravity assist maneuver, allowing it to reach asteroid 5535 Annefrank in 2002 and Comet Wild 2 in 2004. The spacecraft passed Comet Wild 2 at a speed of 6.1 km/s. In 2004, a course correction was made so the spacecraft could return to Earth in 2006 to release the Sample Return Capsule for landing in Utah’s Bonneville Salt Flats.

On 15 January 2006, the Sample Return Capsule was released during the second Earth encounter. Immediately after, Stardust performed a maneuver to avoid entering Earth’s atmosphere. The spacecraft had less than 20 kilograms of fuel remaining. On 29 January 2006, Stardust entered hibernation mode, with only its solar panels and receiver active, in an orbit around the Sun that would return it near Earth on 14 January 2009.

A mission extension was approved on 3 July 2007 to operate Stardust again for a flyby of Comet Tempel 1 in 2011. This extension allowed the spacecraft to revisit a small object in the Solar System and used the remaining fuel, marking the end of its operational life.

On 2 November 2002 at 04:50:20 UTC, Stardust passed 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 2003 Comet Wild 2 flyby.

On 2 January 2004 at 19:21:28 UTC, Stardust encountered Comet Wild 2 on the side facing the Sun. The spacecraft passed the comet at a distance of 237 km (147 mi) with a relative speed of 6.1 km/s. The planned closest approach was 150 km (93 mi), but this was changed after a safety review to reduce the risk of dust collisions. The comet overtook the spacecraft from behind as they orbited the Sun. During the encounter, the spacecraft was on the sunlit side of the comet’s nucleus, with a solar phase angle of 70 degrees at approach, reaching 3 degrees at closest approach, and 110 degrees at departure. The spacecraft used AutoNav software during the flyby.

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

New Exploration of Tempel 1 (NExT)

On 19 March 2006, Stardust scientists shared that they were thinking about changing the spacecraft’s path for a new mission to take pictures of Comet Tempel 1. This comet was previously studied by the Deep Impact mission in 2005, which sent a probe to hit its surface. The new mission could help scientists see the crater created by Deep Impact, which was not clearly visible 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 coming close to the Sun. NExT would also create the most detailed map of Tempel 1’s surface ever made, helping scientists learn more about the structure of comet nuclei. The flyby mission would use nearly all the spacecraft’s remaining fuel, meaning the spacecraft would no longer be able to operate after this mission. A program called AutoNav (for automatic navigation) would control the spacecraft for 30 minutes before the encounter with the comet.

The mission’s goals included:

At 04:39:10 UTC on 15 February 2011, Stardust-NExT passed by Comet 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 areas that had never been seen by Deep Impact. The Deep Impact crater was also visible, but it was very hard to see because material from the impact had settled into the crater.

On 24 March 2011 at about 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 improve how fuel levels are estimated on spacecraft. After the data was sent, the spacecraft could no longer aim its antenna or send signals. The spacecraft 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 separated from the Stardust spacecraft. The SRC re-entered Earth’s atmosphere at 09:57 UTC, moving at a speed of 12.9 km/s, the fastest reentry speed ever achieved by a human-made object. During reentry, the capsule slowed from Mach 36 to subsonic speed in 110 seconds. The highest deceleration reached 34 g, which occurred 40 seconds into reentry at an altitude of 55 km over Spring Creek, Nevada. The PICA heat shield, made by Fiber Materials Inc., reached temperatures over 2,900 °C during this reentry. The capsule then used a parachute to land 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 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 a million tiny dust particles were embedded in the aerogel collector. Ten particles were at least 100 micrometers (0.1 mm) in size, and the largest was about 1,000 micrometers (1 mm). About 45 interstellar dust impacts were found on the back side of the cometary dust collector. Scientists are observing and analyzing dust grains through the Stardust@Home citizen science project.

The total mass of the collected sample was about 1 mg.

In December 2006, seven scientific papers were published in the journal Science, describing initial findings from the sample analysis. These included a variety 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. Amorphous silicates and crystalline silicates like olivine and pyroxene were present, showing a mix of Solar System and interstellar materials. Hydrous silicates and carbonate minerals were not found, suggesting cometary dust was not processed by water. Limited pure carbon (CHON) was also detected. 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 aerogel images that may contain an interstellar dust grain. Volunteers can name their discoveries. Hudson named his finding "Orion."

In April 2011, scientists from the University of Arizona found evidence of liquid water in Comet Wild 2. They identified iron and copper sulfide minerals that form only in the presence of water. This discovery challenged the belief that comets never warm enough to melt their ice. 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 on display at the National Air and Space Museum in Washington, D.C. It has been exhibited there since 1 October 2008, the 50th anniversary of NASA’s founding. The capsule is shown in sample collection mode, next to a piece of the aerogel used to collect samples.

Results

The comet samples show that the outer areas of the early Solar System were not separated and were not a safe place where materials from space could often stay. The data suggest that material from the hot inner part of the Solar System formed and then moved to the Kuiper belt.

In 2009, NASA announced that scientists found one of the basic chemical parts needed for life in a comet for the first time. Glycine, an amino acid, was found in material released by Comet Wild 2 in 2004 and collected by the Stardust probe. Glycine has been found in meteorites and observed in space gas clouds before, but the Stardust discovery is the first time it was found in cometary material. Studies of isotopes show that the Late Heavy Bombardment included comet impacts after Earth formed but before life developed. Carl Pilcher, who leads NASA's Astrobiology Institute, said, "Finding glycine in a comet helps show that the basic parts of life are common in space and supports the idea that life in the universe may be common rather than rare."