New Year Invites Wild Bash

Stardust is the first U.S. mission dedicated solely to a comet and will be the first to return extraterrestrial material from outside the orbit of the Moon. In the four years since its launch, the spacecraft has set the distance record for a solar-powered probe, making a loop around the Sun beyond Mars and the asteroid belt to where the solar flux is ten percent of what we see on Earth. Along its journey, it achieved a close flyby of the asteroid, Annefrank. But on January 2, 2004, on its final solar orbit, Stardust will perform it main task: to fly past comet Wild 2, collect samples and return them to Earth in 2006. In January 2006, the samples will be delivered by parachute inside the Stardust Sample Return Capsule to the Utah Testing and Training Range. The total trip entails two billion miles out to meet the comet, and a billion miles home.

first view of annefrank
During the comet flyby, giant 20-story tall antennas on Earth, called the Deep Space Network, will receive transmissions of pictures and other scientific data.
Credit: NASA/JPL

"Scientists have long sought a sample directly from a known comet because of the unique chemical and physical information these bodies contain about the earliest history of the solar system," said Dr. Edward Weiler, NASA’s associate administrator for space science. "Locked within comet molecules and atoms could be the record of the formation of the planets and the materials from which they were made."

Over the course of seven years, it will collect interstellar dust on two different orbits, and will fly through the coma, or cloud of dust and debris, surrounding Comet Wild-2 (pronounced "Vilt" 2). Finally the spacecraft will approach Earth, ejecting an entry capsule that will descend to the Utah desert carrying cometary and interstellar dust samples. The encounter of Wild-2 will occur on January 2, when the comet is 1.86 astronomical units from the Sun (almost twice Earth’s distance from the Sun) and 97 days after the comet has rounded the Sun. "This is an exciting time," said University of Washington Professor Don Brownlee, principal investigator for Stardust.

At its closest approach, Stardust will be traveling at a speed of 6.1 kilometers per second (about 13,650 miles per hour) relative to the comet. The velocity has been chosen to optimize the capture of particles by the aerogel collector; at this speed, it can "soft-catch" the comet samples without changing them greatly. The passage through the most intensive rain of debris within the coma will last about eight minutes. Wild-2 will be far from its peak period of activity and be relatively safe for a close flyby. The spacecraft will approach Wild-2 from above the comet’s orbital plane, then dip slightly below it. In effect, the comet will "run over" the spacecraft.

Approaching from the sunlit side and northern portion of the comet, the spacecraft’s flight path through the coma will take Stardust within about 150 kilometers (100 miles) of the comet’s nucleus. This "miss distance" was selected to balance between the need to protect the spacecraft and the objective of sampling the freshest possible material off the comets nucleus. Stardust’s navigation camera, which is fixed to the spacecraft body, will take images of the comet nucleus. The camera will be protected from direct hits because it faces away from the direction of the particle onslaught. It will record images of the comet through the reflection in a movable mirror. The mirror will provide image-motion compensation that is, it will move to keep the reflection of the comet in the camera’s field-of-view and minimize image smearing during the flyby.

Wild-2 is an ideal target in part because it has only recently been deflected from a distant orbit into its current orbit which brings it into the inner solar sytem. Its drastic orbit change resulted from a very close approach that the comet made to Jupiter in September 1974. Before that, the comet was in a much longer orbit and had made fewer passages of the Sun, so it is more pristine than most short-period comets. Since Comet Wild-2′s orbit change in 1974, it has looped in toward the Sun three times. Comet scientists anticipate that 1 to 5 percent of Wild-2′s nucleus surface could be active with gas and dust jets erupting from the surface. With a relatively slow flyby, the existence and activity of jets should be well observed.

Wild Encounter

For planning purposes, the mission team has defined an encounter period spanning from 100 days before to 150 days after the comet flyby. During the segment beginning 100 days before the comet encounter, the navigation camera will image Wild-2 weekly to assist in targeting the spacecraft. Stardust will repeatedly image the comet through the cameras eight different filters, acquiring as many images of the comet as can be practically returned until 12 hours before the closest approach to the nucleus. One closeup will be sent back to Earth near the point of closest approach. Subsequent imaging data will be recorded onboard the spacecraft for transmission to Earth after the fly-through of the coma has been completed. The mission’s core period for science data collection runs from about five hours before to five hours after its closest approach to the comet nucleus. Five hours and about 100,000 kilometers (60,000 miles) away from the nucleus, the spacecraft will begin to enter the coma.

aerogel
Aerogel panel used to capture comet particles.
Credit: JPL/Tsou

The comet’s nucleus should begin to emerge in the navigation cameras field-of-view as an extended dark body. All comet science experiments will be taking data, and the spacecraft will be tracked continuously throughout this period. Just before the spacecraft’s closest approach to the nucleus, Stardust will deploy its aerogel collector, with the "A side" facing the direction of incoming comet particles.

To collect particles without damaging them, Stardust will use an extraordinary substance called aerogel a silicon-based solid with a porous, sponge-like structure in which 99 percent of the volume is empty space. Originally invented in 1933 by a researcher at the College of the Pacific in Northern California, aerogel is made from fine silica mixed with a solvent. The mixture is set in molds of the desired shape and thickness, and then pressure-cooked at high temperature. Over the past several years, aerogel has been made and flight-qualified at the Jet Propulsion Laboratory for space missions. A cube of aerogel looks like solid, pale-blue smoke. It is the lightest-weight, lowestmass solid known, and has been found to be ideal for capturing tiny particles in space.

When a particle hits the aerogel, it will bury itself, creating a carrot-shaped track in the aerogel up to 200 times its own length as it slows down and comes to a stop. The aerogel made for the Stardust mission has extraordinary, water-like clarity that will allow scientists to locate a particle at the end of each track etched in the substance. The Stardust science team expects that the samples returned will be profoundly complex, and each particle will be probed for years in research labs.

halley_comet
Icy-rock core of Halley’s Comet.
Credit: ESA/Giotto spacecraft flyby

The main Whipple shield, equipped with its dust flux monitor, will be counting particle hits. The comet and interstellar dust analyzer will be measuring comet particle composition during the flythrough. From -5 hours through -4 minutes, and again from +4 minutes to +5 hours, the spacecraft will transmit a continuous stream of imaging and other observations. At -4 minutes, when the comet nucleus occupies 60 by 60 picture elements (pixels) in the cameras field-of-view, a final black-and-white picture of the nucleus will be transmitted directly to Earth. Any images taken from -4 minutes to +4 minutes will be stored onboard for later transmission. From -3 minutes to +4 minutes, one major goal will be to keep the comet nucleus within the cameras field-of-view.

To accomplish this, the scanning mirror can adjust its position from viewing forward to backward, and the spacecraft itself can tilt to add a second axis to the mirrors position. This is expected to result in significant spacecraft motion, so mission planners expect to temporarily lose radio contact with Stardust. Should this loss of signal occur, Stardusts medium-gain antenna will take over communications functions until the dish-shaped high-gain antenna can be pointed again at Earth. Once Stardust has completed its voyage through the coma, Stardust’s aerogel collector will be stowed for the final time, sealing its comet and interstellar samples inside the sample return capsule. Imaging and other science data recorded onboard during the coma fly-through will be transmitted to Earth, and Stardust will begin the final leg of its journey back toward Earth.

Sample Return to Earth

Stardust is scheduled to fire its thrusters three times as it approaches Earth to fine-tune its flight path. The first trajectory maneuver is scheduled 13 days before Earth entry, the second three days before entry and the final maneuver 3 hours before entry. Earth entry will take place on January 15, 2006.

first view of annefrank
Artist rendering of pickup scene, Utah Test Range
Credit: NASA/JPL, U. Wash

Soon after the final trajectory maneuver at an altitude of 110,728 kilometers (68,805 miles), Stardust will release the sample return capsule. A spring mechanism will impart a spin to the capsule as it is pushed away from the spacecraft in order to stabilize it. After the capsule has been released, the main spacecraft will perform a maneuver to divert itself to avoid entering Earths atmosphere. The spacecraft will remain in orbit around the Sun. The capsule will enter Earths atmosphere at a velocity of approximately 12.8 kilometers per second (28,600 miles per hour), faster than the Apollo Mission capsules and 70% faster than the reentry velocity of the Shuttle . The capsules aerodynamic shape and center of gravity are designed like a badminton shuttlecock so that the capsule will automatically orient itself with its nose down as it enters the atmosphere.

As the capsule descends, its speed will be reduced by friction on its heat shield, a 60-degree half-angle blunt cone made of a graphite-epoxy composite covered with a new, lightweight thermal protection system. Additional ablative material on the back shell that is similar to material used on the Space Shuttles external tank protects the capsule from the effects of recirculation flow of heat around the capsule. The capsule will slow to a speed about 1.4 times the speed of sound at an altitude of about 30 kilometers (100,000 feet), at which time a small pyrotechnic charge will be fired, releasing a drogue parachute. After descending to about 3 kilometers (10,000 feet), a line holding the drogue chute will be cut, allowing the drogue to pull out a larger parachute that will carry the capsule to its soft landing. At touchdown, the capsule will be traveling at approximately 4.5 meters per second (14.8 feet per second), or about 16 kilometers per hour (10 miles per hour). In all, about 10 minutes will elapse between the beginning of the entry into Earth’s atmosphere until the parachute is deployed.

The landing site at the Utah Test and Training Range near Salt Lake City was chosen because the area is a vast, desolate and unoccupied salt flat controlled by the U.S. Air Force with the Army. The landing footprint for the sample return capsule will be about 30 by 84 kilometers (18 by 52 miles), an ample space to allow for aerodynamic uncertainties and winds that might affect the direction the capsule travels in the atmosphere. To land within the footprint, the capsule’s trajectory must achieve an entry accuracy of 0.08 degree. The sample return capsule will approach the landing zone on a heading of approximately 122 degrees on a northwest to southeast trajectory. Landing time will take place at 3 a.m. Mountain Standard Time on January 15, 2006.

A UHF radio beacon on the capsule will transmit a signal as the capsule descends to Earth, while the parachute and capsule will be tracked by radar. A helicopter will be used to fly the retrieval crew to the landing site. Given the small size and mass of the capsule, mission planners do not expect that its recovery and transportation will require extraordinary handling measures or hardware other than a specialized handling fixture to cradle the capsule during transport. The capsule will be transported to a staging area at the Utah Test and Training Range where the sample canister will be extracted. The sample canister then will be transported to its final destination, the planetary material curatorial facility at NASA’s Johnson Space Center, Houston, TX.

Planetary Protection

The U.S. is a signatory to the United Nations 1966 Treaty of Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies. Known as the "Outer Space Treaty," this document states in part that exploration of the Moon and other celestial bodies shall be conducted "so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter." Comets are believed to be primordial bodies made up of material that is virtually unchanged since their creation when the solar system formed 4.6 billion years ago. This means that any evolutionary processes leading to the emergence of life have not occurred. There is no scientific reason to believe that bacteria or viruses or any other life exist on comets.

origins
Comets may have played a critical role in the origin of life on Earth
Credit: NASA/JPL

One of the objectives of the Stardust mission is to investigate whether the chemical building blocks of life exist on comets. But even if such building blocks do reside there, comets have not provided the hospitable environment required over millions of years to accommodate the complex processes that could result in the emergence of even single-celled organic life. On Stardust, all comet particles that are collected will be heated to extremely high temperature due to their impact speed on the aerogel collector. The temperature caused by the compression interaction between aerogel and any given particle is calculated to be at least 10,000 C (more than 18,000 F). In fact, the collector material literally melts to encapsulate the captured particles. Such high temperatures are naturally sterilizing. As a particle hits the aerogel sample collector, it will come to a dead stop within a microsecond, having traveled about 3 centimeters (1.2 inches) into the aerogel. By that point, the aerogel, which is silica-based, will have melted around the particle, trapping it in glass. It should be noted that particles from space, including material from comets, fall onto Earth’s surface at a rate of approximately 40,000 tons per year, and some of this material is believed to survive atmospheric entry without severe heating.

The Stardust project sponsored a "Send Your Name to a Comet" campaign that invited people from around the world to submit their names via the Internet to fly onboard the Stardust spacecraft. Two microchips bearing the names of more than 1.136 million people are onboard the Stardust spacecraft. The names were electronically etched onto fingernail-size silicon chips at JPL’s Microdevices Lab. Writing on the microchips is so small that about 80 letters would equal the width of a human hair. In addition to holding names from the public-at-large, the second microchip contains all 58,214 names inscribed on the Vietnam Veterans Memorial in Washington, DC, as a tribute to those who died in that war.


Related Web Pages

Stardust
Live Webcam of Stardust Mission
Early Wild Success for Stardust
Telescopes for Stardust
Harpooning a Comet
Two-Way Asteroid Trip Takes Off
Tale of a Comet
We Are All Made of Stars
Winter Boon From Deep Space