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NASA’s mission ...to Catch Imagine . . . a lone spacecraft hurtling through space, millions of kilometres from Earth. As its approaches a comet, its electronic systems awake from slumber and fire off an impactor module. Jam-packed with guidance and imaging systems, this projectile locks on to its target, hurtling towards imminent collision. In the final moments before its destruction, the spacecraft beams crucial photometry back to Earth and then rips a massive crater into the comet 8  Silicon Chip www.siliconchip.com.au n: By SAMMY ISREB h a Comet N o, this isn’t the plot of a Hollywood blockbuster. It’s the goal in a series of three NASA missions to study comets within our Solar System. For thousands of years man as been fascinated by the phenomenon of comets. Up until a few centuries ago, the sight of the bright halo-like streaks across the night skies brought with it a sense of awe, for an apparently heavenly body. As science evolved, an understanding developed that comets were merely rocky projectiles, be it beautiful ones, hurtling through the vastness of space, propelled and guided by gravitational forces. As astronomical techniques advanced, scientists were able to view comets in detail and they have developed the hypothesis that comets have a composition of ice and dust, probably around a rocky core. This hypothesis should be proved (or not) by a series of spacecrafts being launched by NASA. In January 1999, NASA launched the first of this series of spacecraft, Stardust, and this has just been followed on July 3rd, 2002, by CONTOUR (Comet Nucleus Tour). See www.contour2002.org The final craft, Deep Impact, is set for launch in January 2004. somewhat closer to the sun following a near collision with Jupiter in 1974. As Wild 2 is distinctly smaller than comet Halley and orbits further from the sun, it lacks the signature tail of Halley, instead exhibiting a dull glow when observed from Earth. When a comet passes by the sun and is heated, sublimation effectively boils off material to produce the coma, the gaseous halo around the core. With each periodic fly-by of the sun, more and more of this volatile material is boiled off, eventually leading to an inactive comet, devoid of a coma. As Wild 2 has only recently commenced flying by the sun since its orbit alteration several decades ago, it is an ideal choice for study due to its high level of volatility. With each fly-by of the sun, Wild 2 will throw off fresh core material. It is this fresh material which is of special interest to the Stardust mission, which aims to catch a small quantity of these particles before returning them to Earth for analysis. This will be a world first, giving scientists detailed information on the composition of comets. Along with the actual ‘capture’ of particles, scheduled to take place during January 2004, the Stardust craft is configured for a rendezvous period commencing 100 days prior to and concluding up to 150 days after this The Stardust Mission The Stardust craft, launched in January 1999, has already clocked up an astounding 2.263 billion kilometres towards its January 2004 rendezvous with the comet Wild 2. This periodic comet had its orbit deflected www.siliconchip.com.au In the Payload Hazardous Servicing Facility, a worker looks over the solar panels of the Stardust spacecraft before it undergoes lighting tests. September 2002  9 the SRC, will then alter its trajectory to avoid crashing into the Earth. As Stardust slingshots back into space, the ejected SRC module will hurtle towards Earth, 125km above the surface and travelling at 12.8km/s (ie, 46000 kilometres/hour!). As the SRC descends through the atmosphere, a protective thermal shell will absorb 99% of the capsule’s kinetic energy and protect the sensitive internals from the immense heat. At 3km above the landing site in Utah, the SRC will deploy its internal parachute, guiding the comet dust payload back to Earth. Aerogel Capture Medium Artists rendition of the Stardust trailing the Wild 2 comet. During the January 2004 encounter, the craft will as shown, extend the aerogel collector array in order to capture comet debris. date. During this extended window additional data in the form of photometry across many spectral bands will be acquired and transmitted to Earth. Around six hours before Stardust reaches the closest approach to Wild 2 (100km from the sunlight side of the comet), the dust collector containing Aerogel material will be deployed. The craft will then manoeuvre to align both its dust shield and the collector array, perpendicular to the dust stream. By then, Stardust will be moving at a velocity of 6.1km/s, relative to Wild 2. At this speed the Aerogel material will best capture the dust ejected from the comet, ranging in size from one to 100 microns. Two years later, in January 2006, Stardust will be in the final stages of the Earth Approach Subphase. At this time the Sample Return Capsule (SRC), containing the retracted Aerogel collector array, will detach from the main craft. The Stardust craft, minus With the primary mission of Stardust being to capture dust particles from Wild 2, the Aerogel capture material is one of the most important parts of the craft. Travelling at up to 12.8km/s, the particles possess huge kinetic energy. The aerogel material must strip this energy from the particles without allowing them to alter their composition by being heated or pulverized. Aerogel is a silicon-based porous, sponge-like structure, with 99.8% of its volume being air space. Although made from silica, aerogel is less than 1/1000th the density of glass, making it one of the world’s lightest solids. The large amounts of air in the aerogel are used to provide a cushioning effect, so when a particle hits the surface it buries itself in the aerogel, creating a long track up to 200 times the length of the particle. This allows the particle to slow down and prevents it from altering in physical or chemical composition. Back on Earth, scientists will remove the dust particles from the aerogel for extensive analysis. Stardust technical specs Weighing in at 380kg including (Left): Though with a ghostly appearance like an hologram, aerogel is very solid. It feels like hard styrofoam to the touch. (Right): A close-up of the collector array, fitted with the aerogel collection media. This array will eventually return the collected sample to earth. 10  Silicon Chip www.siliconchip.com.au propellant, the 1.7m long craft was developed by NASA and Lockheed Martin Astronautics. The craft’s payload consists of various scientific instruments. Along with the Aerogel Sample Collectors, other instruments are onboard are: * Comet and Interstellar Dust Analyser (CIDA): A real-time mass spectrometer to determine the composition of individual dust grains as they collide with a silver plate during flight. * Navigational Camera: In addition to acquiring high resolution images of Wild 2, to be transmitted back to Earth, this camera is used to navigate towards the Wild 2 nucleus during the dust capture portion of the mission. * Dust Flux Monitor (DFM): Mounted in front of the protective shield, the DFM unit gathers data on the density, distribution and direction of particles passing the craft. The Aerogel Sample Collectors are integrated into the Sample Return Capsule (SRC), an advanced blunt body re-entry capsule, with parachute and mortar assembly. The propulsion system of Stardust consists of a small amount of hydrazine (N2H4) propellant. Most of its ultimate velocity will be derived by planetary fly-bys. Electrical power is generated by 6.6 The aerogel material is super strong! A 2.5kg brick is supported on top of a piece of aerogel weighing only 2 grams. www.siliconchip.com.au Scheduled for return in January 2006, the Sample Return Capsule (SRC) will bring the collected comet debris back to earth for analysis. square metres of solar panels, along with a 16 amp-hour nickel hydrogen backup battery. Data and communication systems use a 32-bit embedded CPU with 128Mb of memory. Acquired data is temporarily stored in this memory, before being transmitted back to Earth via the Deep Space Network X-band up/down link. All of the Stardust subsystems are built around an aluminum honeycomb core frame, surrounded by panels of graphite fibres encapsulated in polycyanate material. The front of the craft uses a “Whipple Shield” made up of several advanced materials, including ceramic blankets. Comet Enke is one of the most easily observable comets from Earth, having orbited the sun thousands of times over its life. Due to its ‘old’ nature, Enke gives off little dust and gas, which have boiled off long ago. This will give CONTOUR excellent visibility on the approach to its nucleus, with little risk of being bombarded by a high density of particles, which would be present in a more active comet. Discovered only 70 years ago, comet Schwassmann-Wachmann 3, has since split into several pieces. CONTOUR will fly within 100km of these pieces. CONTOUR Mission Contour technical specs CONTOUR (Comet Nucleus Tour), launched on July 3rd, 2002, is the second mission of the series. The 4-year mission includes a meeting with comet Enke on the 12th November 2003, followed by a fly-by of comet Schwassmann-Wachmann 3, on the 19th June 2006. The eight-sided CONTOUR craft measures 1.8m in height and 2.1m in width, and weighs 970kg. 503kg are the rocket motor, with another 80kg of hydrazine fuel. Electrical power comes from nine Gallium Arsenide solar panels, feeding nickel cadmium backup batteries. It has dual 5-Gigabit September 2002  11 designed for use at a range greater than 2000km from the nucleus of the comet under investigation. CFI will be first used to locate the target comet, from a great distance, against a backdrop of stars. CFI will then take colour images of the nucleus of the comet and its distinguishing features such as gas and dust jets. Lastly, CFI will use narrow bandwidth filters, tuned to the unique emissive frequencies of dissociated water, carbon, and dust, to allow identification of the active nucleus elements. CONTOUR Neutral Gas and Ion Mass Spectrometer (NGIMS): The NGIMS instrument is a highly sensitive mass spectrometer, designed specifically to determine the composition of incident gas from within the coma. The 13.5kg apparatus will measure the relative abundance of water, methane, carbon dioxide, ammonia and hydrogen sulphide. Comet Impact Dust Analyser (CIDA): Identical to the CIDA unit which has been launched on the Stardust mission, the 10.5kg CIDA instrument is used to determine the size and composition of inbound particles. In order to do this, as the dust particles fall upon a charged grid. Depending on the size of the particles, a certain number of charged ions may be extracted by the charged grid. These then move through the instrument, past a reflector, and are measured by a special detector. As there is a relationship between the size of the dust particle, and the time it takes for the proportionally sized ions it releases to travel through the apparatus, the CIDA can accurately infer the size of the incident dust particles. (Above): The Comet Nucleus Tour (CONTOUR) spacecraft on display in the Spacecraft Assembly and Encapsulation Facility right before being assembled onto the launch vehicle. (Below): The partially assembled Delta II rocket, containg the CONTOUR craft, was eventually launched into space on the 3rd of July 2002. solid state recorders for data storage. When CONTOUR has passed the comet and has a clear radio path to Earth, the data will be transmitted to the Deep Space network on Earth. CONTOUR uses four state-of-the-art instruments in order to obtain mission data, as well as providing navigational inputs to assist in steering the craft towards the comet targets: CONTOUR Remote Imaging Spectrograph (CRISP): Supplied by the Applied Physics Laboratory at John Hopkins University, the CRISP unit is a high resolution camera, operating in both the visible and infrared spectral ranges. It weighs At 26.7kg. With the approach to the comet Enke reaching a velocity of 28.2km/s, the CRISP unit relies on advanced optoelectronics to produce high resolution images at these speeds. Light wavelengths shorter than 800nm are separated via a beamsplitter towards a high resolution CCD camera. The imager contains a 10-position selectable filter wheel, with one clear and nine coloured filters. These coloured filters have central bandpass wavelengths ranging from 450nm to 770nm and are used for determining the geological composition of the surface being imaged. Light wavelenghts longer than 800nm (infrared) are directed to the spectrometer portion of CRISP, where separation into 256 different infrared wavelengths from 800nm to 2500nm occurs. The result is measured by a mercury cadmium telluride detector, cooled to minus 183°C, to obtain a two-dimensional spectral map. CONTOUR Forward Imager (CFI): This tiny 9.7kg instrument is a high sensitivity ultraviolet imaging apparatus, 12  Silicon Chip www.siliconchip.com.au This is an artist’s rendition of the flyby spacecraft releasing the impactor, 24 hours before the impact event. Pictured from left to right are comet Tempel 1, the impactor and the flyby spacecraft. The impactor is a 370-kilogram mass with an onboard guidance system. Deep Impact Mission The Deep Impact Mission is arguably one of the most amazing missions in NASA’s history. Reading like the plot of a science fiction movie, Deep Impact will be launched in January 2004 on board a Delta II rocket to make a rendezvous with Comet Tempel 1 in July 2005. Around 24 hours before the encounter, Deep Impact will release a 370kg projectile equipped with electronic guidance and imaging equipment. It will send high resolution images right up to the moment when it crashes into the comet. The impact is planned to (hopefully?) release core fragments, which will float towards the Deep Impact craft which will be trailing the comet. Also, after the impact fragments have been released, the fresh surface of core material in the crater will be visible to the Deep Impact craft. Along with the impactor module, Deep Impact will carry three scientific instruments: High Resolution Instrument (HRI): HRI is a high resolution telescope, with inbuilt infrared spectrometer. The resolving power of this instrument is so high, that from 700km away the HRI is able to image the comet with better than 2 metres per www.siliconchip.com.au pixel resolution. Following the impactor module’s collision with the comet, the HRI will commence acquiring high resolution visual images, in addition to providing spectral analysis of the composition of the comet’s nucleus. Around 300 megabytes of this data will be produced in the minutes following the collision. Medium Resolution Instrument (MRI): The MRI serves as a backup for the HRI device, delivering a lower resolution of 10m at a distance of 700km in the visible spectrum. As the MRI has a wider field of view than the telescopic HRI, it is better suited to viewing the stars and navigating towards the comet in the days leading up to the approach. Impactor Module The impactor module is designed to separate from the fly-by spacecraft around 24 hours before it impacts into the comet Tempel 1. Weighing a mere 370kg, the impactor is intended to deliver 18 Gigajoules (roughly equivalent to 4.5 tonnes of TNT explosive) and is expected to blast a massive crater into the comet. In order to achieve this high energy collision, the impactor module will be travelling at 10.2 km/second (36720km/h) just before impact. Given that the module will be released more than 800,000km from the comet which is only 6km in diameter, it is a complex task to ensure the impactor is on course. To do this, a specially designed instrument, known as the Impactor Target Sensor (ITS), feeds data to auto-navigation algorithms developed by the Jet Propulsion Laboratory, to make trajectory corrections via the small onboard hydrazine propulsion system. After impact the fly-by craft will take visual images of the newly formed crater, as well as performing infrared spectroscopy analysis of the ejected material in order to determine the composition of the comet’s nucleus. The impactor module is made of 49% copper and 24% aluminum. These materials, not believed to be found within the comet, are used so that the analysis of the ejected material is not affected by the remains of the SC impactor module. Acknowledgement: Our thanks to NASA/JPL for their assistance with the details and photographs/illustrations for this article. September 2002  13