Just as Comet Hale-Bopp continues its journey across the nighttime sky, another currently visible comet, Wild 2, is capturing the attention of NASA's Stardust Project for a vitally important reason.
Stardust, a spacecraft with a planned 1999 launch, will capture samples of comet dust from Wild 2 in 2004 for return to Earth in 2006. This current appearance by Wild 2 (pronounced "Vilt 2"), offers the Stardust team a prime opportunity to prepare for the spacecraft's historic journey by gathering data on the comet's brightness and the size and quantity of its gas and dust particles.
The spacecraft is protected from oncoming cometary particles with a front "Whipple Bumper," a shield named for renowned astronomer Fred Whipple, with a composite structure that includes metals and several curtains of the same material as bullet-proof vests. However, the bumper does not offer unlimited protection against a barrage of numerous, large particles.
"We want to study the dust envelope of Wild 2 so we'll know how close we can fly without jeopardizing the spacecraft," said Stardust Project Manager Dr. Kenneth Atkins. "These current observations will help us significantly reduce the risk."
By observing Wild 2 in both visible and infrared light, the Stardust team will be able to fine-tune models of the comet environment and mission logistics. Final trajectory adjustments may be made up to a few hours before encounter, using observations made by Stardust en route to Wild 2. That encounter will take place on January 2, 2004, about 98 days after the comet's perihelion, or closest pass by the Sun. During Wild 2's current visit, scientists will study the comet's activity at a comparable post-perihelion point, gathering data crucial to the success of the Stardust mission.
Wild-2 studies are being conducted at numerous observatories, including Lowell Observatory, Flagstaff, AZ; the Lunar and Planetary Laboratory, University of Arizona, Tucson; the W.M. Keck Observatory and other major telescopes atop Mauna Kea, HI; and the 200-inch Hale telescope at Palomar Observatory near San Diego, CA. Amateur astronomers can see Comet Wild 2 with a relatively small telescope through August, with the best viewing before the end of May. With an apparent magnitude of 9.6, Wild 2 is currently visible almost directly overhead about one hour after sunset and is located in the constellation of Cancer close to the Praesepe open star cluster.
Comet Wild 2, a short-period comet with a six-year orbit, was discovered in 1978 by the Swiss astronomer Paul Wild, after its close encounter with Jupiter in 1974. This may have been the comet's first journey to the inner solar system in recent centuries, which means it is in a more pristine state than other comets which have been "around the block" more often. For this and several other reasons, Wild 2 was chosen as the destination comet for Stardust.
As Atkins explained, "Wild 2's orbit presents attractive features for doing a sample return. The comet will be in the right place at the right time so that when Stardust encounters it, Wild 2 will have a relatively low flyby speed of 6 kilometers per second (3.7 miles). This makes the task of catching the comet dust as it whizzes by much easier. In addition, the orbital geometry of Wild 2 enables us to save money by launching Stardust on a Delta rocket and designing an efficient trajectory."
To capture the comet dust without harming it, Stardust will use aerogel, a spongy, silica-based solid with 99 percent empty space. The tiny cometary particles will bury themselves in the transparent aerogel, awaiting retrieval by scientists on Earth. On its way to Wild 2, Stardust will loop twice around the Sun and collect interstellar dust particles. By returning these space and cometary materials to Earth, Stardust will mark the first space sample return mission since the Apollo missions collected moon rocks in the 1960s and 1970s.
Stardust co-investigator Ray Newburn said comets are apparent leftovers from the formation of the solar system and may unlock many cosmic secrets. As he put it, "Comets are a different sort of beast. They've been in a cosmic deep-freeze for most of the solar system's 5-billion-year history. Many scientists believe comets added complex organic molecules to the primordial soup of oceans that helped form life. Stardust should give us some hard facts about Wild 2 and other comets, including chemical composition and age."
Stardust is one of NASA's Discovery missions, which team the agency with industry and universities to launch low-cost spacecraft in a short time frame with highly-focused science goals. Stardust's principal investigator is Dr. Don Brownlee of the University of Washington, Seattle, WA.
The Stardust spacecraft and sample return capsule are being built by Lockheed Martin Astronautics in Denver, CO. The mission is managed by NASA's Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, DC. JPL is also developing the aerogel and the spacecraft's navigational camera, also to be used for scientific imaging. Stardust's cometary and interstellar dust analyzer instrument is provided by Jochen Kissel through the Max Planck Institute in Germany; the University of Chicago, IL, is building a Whipple Shield dust impact counter.
Additional information is available on the Stardust home page on the World Wide Web at: http://pdc.jpl.nasa.gov/stardust/home.html.
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