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Country USA/ESA
Mission Solar Polar Orbiter
Launch Date October 6, 1990
Launch Vehicle Space Shuttle with IUS & PAM-S upper stages
Spacecraft Mass 370 kg
Key Dates Feb 08, 1992 - Jupiter Flyby
Jun 26, 1994 - Sun's South Pole
Jul 31, 1995 - Sun's North Pole
End of Mission 2006?
Comments First Solar Polar Orbiter
First Detection of Interstellar Dust Particles

uls1_s.jpg The Ulysses spacecraft, an international mission to study the poles of the Sun and interstellar space above and below the poles, was the third spacecraft to be lofted into space by a space shuttle. Space shuttle Discovery carried the spacecraft into low Earth orbit on October 6, 1990, atop a two-stage Inertial Upper Stage (IUS) rocket and a smaller engine, called the Payload Assist Module (PAM-S). Once it had been released from the cargo bay, the spacecraft's booster rockets fired to send Ulysses on its way to Jupiter, where it used the giant planet's gravity to gain more momentum to swing out of the ecliptic plane and onward to the polar regions of the Sun.

Jupiter Flyby

uls_s.jpg Sixteen months and 925 million kilometers (575 million miles) after launch, Ulysses reached the huge Jovian magnetosphere. Jupiter's magnetic field forms a windsock around the planet -- the magnetosphere -- that is blown by the solar wind. The magnetosphere is known to vary in size and configuration over time, depending on the amount of force exerted on it by the solar wind. Ulysses was the first spacecraft since the Voyagers to return to Jupiter, and it encountered many changes in this magnetic sheath in 13 years time. By February 1992, the solar wind had died down and was not nearly as powerful as it had been during the Voyager encounters. As Ulysses journeyed through Jupiter's magnetic bubble, data coming back from the spacecraft began to suggest that Jupiter's magnetic field had ballooned over time. Scientists described the bubble as "more stretched out" and "flattened" by comparison to the magnetosphere that the Voyagers had encountered. Other surprises presented themselves during Ulysses' 11-day tour of Jupiter. Among the most important, Ulysses found less volcanic activity occurring on Io than had been the case 13 years before.

On February 8, 1992, at 12:01 Universal Time (4:01 a.m. Pacific Standard Time), Ulysses sailed past Jupiter at closest approach of 446,552 kilometers (277,474) miles from the planet's center and boomeranged out of the ecliptic plane. Ulysses detected dust as it neared Jupiter, recording periodice bursts of sub-micrometer dust particles at approxiamtely monthly intervals. The source of the of the dust particles could be Jupiter or the volcanoes from Io.

Ulysses also detected larger dust particles moving in a direction opposite to the motion of the planets. This dust is believed to travel throughout the solar system and moves against the direction of the planets and asteroids. This is the first detection of dust particles of interstellar origin.

The Ulysses Comet Watch Program

The Ulysses Comet Watch Network has as its main goal to gather cometary data during times of polar passage by the Ulysses spacecraft. If successful, the comets studied could be used as indicators of high-latitude solar winds. The Network was modeled after the very successful International Halley Watch observation networks, which operated during 1985 and 1986 gathering thousands of Halley images. For the Ulysses Comet Watch, two components were proposed: a bright comet network of both amateurs and professionals, doing work similar to the Halley watch effort; and a faint comet group consisting of professional observatories and technically advanced amateurs.

The Network was established in late 1992, and a request for participation was published in Sky & Telescope magazine. To date, more than 250 observers from around the world have volunteered for the network. Currently, they are kept informed of upcoming comets through two vehicles: the Ulysses Comet Watch Newsletter (of which ten have been sent to date), and via an E-mail distribution list. This Website is the latest addition to the Ulysses Comet Watch outreach efforts. The Network's approach has been to gather data on appropriate comets, with an eye toward eventual data reduction and correlation with relevant Ulysses data, and finally publication.

Because of the unique orbit Ulysses is following, several important criteria have been established for network observations. First, bright comets (total magnitude greater than 6) are most desirable. Second, the major observation periods are limited to times when a comet is with 2 AU of the sun and 20 degrees latitude of the spacecraft. The following comets have been observed by the network: Borrelly, D'Arrest, Encke, Honda-Mrkos-Padjuakova, Mueller, Pons-Winnecke, Tempel 2, Tuttle, Tuttle-Giacobini-Kresak, Hyakutake, and de Vico. The comet under observation now is Hale-Bopp, although we are asking observers to try for the difficult-to-observe Machholz 1. Observers have sent in more than 200 observations since the network was formed.

Ulysses Sun Polar Passes

Ulysses began its pass over the southern pole of the Sun on June 26, 1994, and became the first spacecraft ever to explore that region of space. The north and south poles of the Sun are theoretically defined as the regions above or below 70 degrees of the Sun's equator. Ulysses spent four months in this high-latitude region, gathering data on the complex forces at work over the Sun's pole. On September 13, 1994, the spacecraft reached its highest latitude of 80.2 degrees south of the Sun's equator. The southern pass was completed less than two months later, on November 5, 1994, as Ulysses dropped back into lower latitudes and began to head for the Sun's equator. The second phase of the mission, traversing the Sun's northern pole, begins in June 1995 and will continue through early November of that year.

Scientists are elated to be able, finally, to carry out observations in the Sun's polar regions. So far the mission has been a complete success. The probe and its instruments are performing well and transmitting 60 million bits of data daily. The information has yielded major discoveries concerning the Sun's environment at high latitude. In particular, scientists are reporting that the Sun is not the clearly defined sphere that appears in conventional optical images. Beyond its bright gaseous surface is an atmosphere that is 200 times hotter, thousands of millions of times less dense and millions of times less bright. This atmosphere, known as the corona, is unstable and too hot to be confined by the Sun's gravity. As it flows outward, it evaporates, producing a continuous outflow of magnetic gas, which spreads across the entire planetary system and becomes the solar wind. As the solar wind reaches Earth, its density reaches 5 atoms per centimeter cubed and fluctuates in speed from between 1.5 million to 3 million kilometers per hour.

Ulysses is managed jointly by the European Space Agency (ESA) and NASA. The spacecraft, weighing in at 370 kilograms (814 pounds), was designed to study three major topics in solar physics: the Sun, the solar wind and interstellar space. The spacecraft carries nine scientific instruments and a radio system that is also used for scientific experiments. Both U.S. and European science teams provided the instruments. The spacecraft was built by Dornier Systems of Germany for ESA, which is responsible for on-orbit operations. NASA provided the space shuttle Discovery, the Inertial Upper Stage (IUS) booster and Payload Assist Module (PAM-S). The U.S. Department of Energy supplied the radioisotope thermoelectric generator. The Jet Propulsion Laboratory manages the U.S. portion of the mission for NASA.

Ron Baalke, STARDUST Webmaster,
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