Stardust
NASA's Comet Sample Return Mission
Having just finished receiving, researching, and posting the Orgueil meteorite page, I realized I needed to look closer into Pre-Solar Grains (PSGs).
So, in my beginning research, I've came across Stardust, which brought back a few PSGS, and the famous Comet Wilt 2 photo.
So, let's start here, at a great robotic mission.
Figure 1.
Some History
Comet Wild 2 - pronounced "Vilt 2" after the name of its Swiss discoverer - has a very long and very complicated life.
Figure 2.
It started in the earliest stages of the Solar System 4.56 billion years ago. At that time, the rocky, metallic and icy components that ultimately formed our system's planets started to condense.
The ices of water, ammonia, carbon dioxide, and carbon monoxide condensed in the cold outer parts of the Solar System. Most of these ices ended up as part of the gas giant planets, but some remained in 1-10 km sized solid bodies called planetesimals.
As the gas giant planets formed, their massive gravity caused these early comets to disperse.
Some were dispersed to surround the furthest reaches of the Solar System into what is called the Oort cloud of comets.
Figure 3.
However, such comets have very long orbital periods (e.g. hundreds or thousands of years) so their passages are difficult to predict and were of no use for the Stardust mission.
Other comets with short periods (e.g. under 200 years) lie within Neptune’s orbit. These are part of the Kuiper Belt.
Here's the orbits of three comets that show a wide range of orbit periods.
Figure 4.
- Tempel 1 completes an orbit every 5.5 years.
- Halley's comet is visible from Earth every 75 to 76 years.
- Hyakutake orbital period was about 17,000 years, but the gravitational influence of the giant planets increased this period to 100,000 years.
Here's an illustration to give you a better sense of the relationship between the Oort Cloud and the Kuiper Belt.
Figure 5.
Comet Wild 2 was in this region of the Solar System until 1974 when it passed close to Jupiter, causing it to be deflected into the inner Solar System.
It now has an orbital period of 6 years with perihelion (nearest point to Sun) just beyond the distance of Mars and an aphelion (farthest point to Sun) near Jupiter’s orbit.
In 1978 it was discovered by astronomer Paul Wild, and this famous comet takes his name.
Because of its short period, Comet Wild 2 became the focus of the Stardust program; it was an ideal comet to study.
Mission Overview
Stardust was the first U.S. space mission dedicated solely to the exploration of a comet, and the first robotic mission designed to return extraterrestrial material from outside the orbit of the Moon.
The Stardust spacecraft was launched on February 7, 1999, from Cape Canaveral Air Station, Florida, aboard a Delta II rocket.
Figure 6.
The primary goal of Stardust was to collect dust and carbon-based samples during its closest encounter with Comet Wild 2. The rendezvous was scheduled to take place in January 2004, after nearly four years of space travel.
Comet Wild 2 – Up close
Here's the famous Comet Wild 2 composite image:
Figure 7.
This composite was taken by the navigation camera during the close approach phase. Several large depressed regions can be seen.
Comet Wild 2 is about 3.1 miles in diameter. To create the composite image, a short exposure image showing tremendous surface detail was overlain on a long exposure image taken just 10 seconds later showing the jets of gas.
Together, the images show the intensely active surface, jetting dust and gas streams into space and leaving a trail millions of kilometers long.
Comet Storm
The Stardust Project Manager Tom Duxbury, NASA's Jet Propulsion Laboratory, said the craft flew through sheets of cometary particles that jostled the spacecraft and that on at least 10 occasions the first layer of its shielding was breached.
Stardust entered the comet's coma - the vast cloud of dust and gas that surrounds a comet's nucleus - on December 31, 2003.
Figure 8.
Halley's Comet becomes visible to the unaided eye about every 76 years as it nears the sun.
Credit: Lick Observatory
From that point on, it kept its defensive shielding between it and what scientists hoped would be the caustic stream of particles it would fly through. And fly through cometary particles Stardust did, but not in the fashion its design team had thought about.
The designers thought they would see a uniform increase in the number of particles the closer the craft came to the comet's nucleus and then a reduction in activity.
However, Dr. Don Brownlee, Stardust Principal Investigator, said according to spacecraft data, it flew through a veritable swarm of particles, then almost nothing, only to fly through another swarm.
Stardust scooped up the swarming particles, analyzed them with on-board instruments and stored other particles for later, in-depth analysis when back on Earth.
Along with dust collecting, the spacecraft also took some remarkable images of the comet.
The following images are a good representation of the closest images of comet Wild 2.
Figure 9.
From the upper left and continues to the right on the first three rows.
The overexposed and out-of-sequence images at the bottom are long exposures taken for autonomous tracking and yield the best jet images.
Only minutes after its closest approach with the comet, Stardust pointed its high gain antenna at Earth and began transmitting a data stream that took over 30 hours to send. The information relayed will keep cometary scientists busy for years to come.
About six hours later, the collector grid was retracted into the spacecraft's sample return capsule to be opened in a clean room at the Johnson Space Center after returning to Earth in January 2006.
Here's a picture of the sample return capsule (the white-topped, blunt-nosed cone seen on the top of the spacecraft). It was jettisoned as Stardust swung by Earth.
Figure 10.
Having weathered a sandblasting by cometary particles at six times the speed of a rifle bullet, the spacecraft was on its two-year, 708 million mile, trek back to Earth.
Two years later, Stardust Capsule Return was seen from NASA's DC-8 Airborne Laboratory. The mission of the DC-8 lab was to explore the conditions during reentry. Lab technicians surveyed the light emitted by the fireball when the capsule streaked through the sky.
The aircraft was located near the end of the trajectory.
Figure 11.
NASA's Stardust sample return capsule landed at the U.S. Air Force Utah Test and Training Range at 2:10 a.m. Pacific time.
Figure 12.
The whole mission - February 1999 to January 2006 - seven years, there and back, has provided a range of scientific discoveries.
Here's the Mission Wrap Up – from NASA
The international team of 200 scientists found that the particles brought back were indeed ancient building blocks of the solar system - but the nature and origin of the particles was quite unexpected.
Before the mission, there were very good reasons to believe that we knew what comets would be made of and there was a general expectation was that the particles collected from comet Wild 2 would be mainly be dust that formed around other stars, dust that was older than the Sun.
Such particles are called stardust or pre-solar grains and this was the main reason why the mission was named Stardust.
But Stardust found more than what they expected:
- Most of the comet's rocky matter formed inside our solar system - at extremely high temperature.
In great contrast to its ice, Wild 2's rocky material had formed under white-hot conditions.
- Comets are a mix of materials made by conditions of both "fire and ice".
Comet ice formed in cold regions beyond the planet Neptune but the rocks, probably the bulk of any comet's mass, formed much closer to the Sun in regions hot enough to evaporate bricks.
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The material collected contained pre-solar "stardust" grains - but those grains are very, very rare (few).
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Wild 2 contained components of primitive meteorites - rocks from asteroids that formed between Mars and Jupiter.
- Rounded particles called chondrules.
- White irregular particles known as Calcium Aluminum Inclusions (CAIs).
Chondrules are the dominant material in many primitive meteorites, and they are rounded droplets of rocks that melted and then quickly cooled as they orbited the Sun.
CAIs are much rarer than chondrules. They are also the oldest solar system materials and are composed of exotic minerals that form at the very high temperature.
- Wild 2 showed that matter abundantly formed in the inner solar system was somehow transported to the edge of the young solar system where comets formed.
There are some theories that suggest that CAI's formed just a few radii from the surface of the Sun, 4.567 billion years ago. The finding that inner solar system materials, formed at very high temperature, were transported all the way to the edge of the Solar System to the region where Pluto is one of the major scientific findings of Stardust.
In other words, instead of being dominated by particles formed around other stars, our comet's rocks were predominantly formed close to the Sun. Thus, these comet sample studies have provided a direct look at the nature and origin of the building blocks of planets, materials that were sprayed all over the young solar system and must have been incorporated into all planets and moons.
- The most unexpected was discovery of the amino acid glycine by a team of scientists from the Goddard Space Flight center.
While perhaps not totally unexpected that a comet would contain amino acids, it was unexpected that this molecule could be detected in the tiny particles that were collected at such high speed (six times the speed of a rifle bullet!).
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Team developed methods that made the detection of amino acids possible and proved that the glycine was an Earth contaminant.
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The significance of this discovery is that comets must have delivered at least one amino acid to our planet before it had life. Because most stars have comets, it suggests that all Earth-like planets obtain important pre-biotic molecules from space.
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Another surprise came when Wild 2 flew through the dust escaping the comet - The rate of impact rate changed in spurts, probably caused by entering and exiting "jets" of dust flowing off the nucleus and also the breakup of "cometary dirt clods" as they drifted away from the nucleus and lost ice which had served as glue to hold them together.
- The flyby photos changed how scientists thought the comet would look like – it wasn't a bland object looking somewhat like a black potato.
It was quite dramatic: kilometer-sized deep holes bounded by vertical and even overhanging cliffs; flat topped hills surrounded by cliffs; spiky pinnacles hundreds of meters tall, pointed skyward: in addition to the numerous jets of dust and gas escaping into space.
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Dust jets also came from the comet's night side - a region expected to be inactive because if its lack of heating by sunlight.
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Wild 2 did not contain impact craters, such those found on the Moon, Mars and practically every other surface exposed to space. The lack of impact craters indicates the surface is new, the old cratered surface is gone.
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The surface of Wild 2 was very different from the surfaces of any other asteroid and comet that have been imaged by spacecraft. It was rougher and much more dramatic.
Some of the Stardust Team
Figures from stardust.jpl.nasa.gov
Stardust Mission Operations
Stardust Navigators
Deep Space Operation Center Members
Ground Data Systems Engineers
Group Photo – Science Team
Links
This animation depicts Stardust's turbulent ride through three of more than two dozen jets that blast outward from comet Wild 2. The jets pummeled the spacecraft with about a million particles per second. Twelve particles, some larger than a bullet, penetrated the top layer of the spacecraft's protective shield.
stardust.jpl.nasa.gov
This movie strings together a series of still images of comet Wild 2. The hig h-resolution pictures were taken by NASA's Stardust spacecraft during its historic flyby of the comet on Jan 2, 2004.
stardust.jpl.nasa.gov
Figures & Acknowledgments
Figures
Figure 1. From stardust.jpl.nasa.gov
Figure 2. From sdnext.org
Figure 3. From www.astrobio.net
Figure 4. From deepimpact.umd.edu
Figure 5. From http://en.wikipedia.org/wiki/File:Kuiper_oort.jpg
Figure 6. Photo at stardust.jpl.nasa.gov
Figure 7. From stardust.jpl.nasa.gov
Figure 8. From www.astrobio.net
Figure 9. From stardust.jpl.nasa.gov
Figure 10. From stardust.jpl.nasa.gov
Figure 11. From stardust.jpl.nasa.gov
Figure 12. Photo from stardust.jpl.nasa.gov
Acknowledgments
With respect and thanks to the Stardust Mission Team, NASA, and all of the great websites that cover the study and publication of comet information and science.
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