Flyby of asteroid Annefrank by NASA’s Stardust mission

The Stardust probe was launched on 7 January, 1999 from Cape Canaveral in Florida, USA on top of a Delta II rocket. The Stardust mission was a part of low-cost missions in the Discovery Programme that was started off with the NEAR Shoemaker mission to asteroid Eros. The 385 kilogram probe was built by Lockheed Martin, and had the primary goal of capturing dust from the coma of Wild 2, and returning to the Earth. Before that daring encounter in 2004 though, mission engineers had slotted in a dating dress rehearsal for the encounter, a flyby of asteroid Annefrank.

The Stardust capsule returning to the Earth. (Image Credit: NASA). 

5535 Annefrank is an asteroid that occupies the main belt between Mars and Jupiter. The ground teams executed the flyby with surgical precision, demonstrating the capabilities of the spacecraft, and sharpening the operational protocols. On board the probe was a sophisticated suite of instruments, including a dust flux monitor, a coma counter, an aerogel collector for shielding the scooped up particles, and a navigation camera, all powered by solar panels that stretched 4.8 metres from tip to tip. The encounter with Annefrank was an opportunity to validate the systems before the Wild 2 encounter, and ensure that the ground teams were adequately prepared.

Annefrank is a 6.5 kilometre wide stony asteroid discovered in 1942 by the German astronomer Karl Reinmuth. The asteroid was picked for the flyby because of the heliocentric orbit of Stardust. Annefrank orbits the Sun every 3.3 years at a distance varying between 2.1 and 2.4 astronomical units, or the distance between the Sun and the Earth. The probe had to tilt itself and run on battery power to spot the faint asteroid. The operations were choreographed by the Deep Space Network operated by NASA’s Jet Propulsion Laboratory, and Lockheed’s Mission Control in Denver.

The encounter was flawless. The navigation camera on Stardust was built using the learnings from the Voyager and Cassini missions, and snapped 72 images of Annefrank, resolving the object down to 185 metres per pixel. The ground teams even briefly opened the aerogel collector to mimic the protocols for the Wild 2 encounter. The spacecraft performed every command perfectly. The high-gain link was lost due to a post-launch glitch, so the ground teams had to rely on a trickle of data from the low-gain antenna on the spacecraft.

Annefrank turned out to be a jagged, triangular prism, possible a fragment of a larger body. There were signs that this parent body was a contact binary system, where a small moonlet grinds the surface of a parent as it orbits it, while maintaining contact. The asteroid seemed to have a stable rotation that was confirmed later by ground photometry. There were no moons or dust clouds surrounding Annefrank, but scattered light blurred some details. The asteroid was five times fainter than expected because its size was not estimated correctly.

A false colour image of the asteroid Annefrank showing variations in surface brightness. (Image Credit: NASA). 

The primary goal of the flyby was operational, to test and validate the systems for the critical comet dust collection, and to provide the ground teams with hands-on experience of operating the spacecraft. Science was not the goal, but the spacecraft delivered. The cratered surface were similar to Gaspra and Ida, investigated by Galileo, and hinted that the Flora family of stony asteroids were the result of collisions in the past. The probe went on to successfully capture 5,000 grains of dust from the coma of Wild 2, returning them to the desert of Utah in 2006.

Stardust was the first sample return mission dispatched beyond the Moon by Earthlings. The silica-based foam used to capture the particles, some of which were formed before the Sun, was 99.8 per cent empty space. The priority of the mission was to return the samples, not reconnaissance of a target. After returning the samples to the Earth, NASA extended the mission, dubbing it Stardust-NExT. The spacecraft then visited Comet Tempel 1, making it the first probe to inspect two comets.

Illustration of the Stardust mission. (Image Credit: NASA). 

The dust grains had an unexpected composition, olivine and pyroxene that were formed in the high heat near the Sun, not in the icy fringes of the Solar System. The discovery indicated that comets help remix Solar System material. The presolar grains were older than 4.6 billion years old, and offered a window into the chemistry of the nebula from which the Sun was born. The observations tied comets to planet formation and asteroids to the origins of meteorites, rewriting human understanding of the Solar System.