Ulysses (spacecraft) - astronomy.

Ulysses (spacecraft) - astronomy. I INTRODUCTION Ulysses (spacecraft), name of an interplanetary spacecraft and its mission to measure the solar wind and magnetic field over the Sun's poles during periods of both high and low solar activity. The Ulysses mission is a joint project of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). Launched in October 1990 from the space shuttle Discovery, Ulysses was first sent to Jupiter, which was used as a gravitational slingshot to bend the trajectory of the spacecraft into a path perpendicular to the plane in which all of the planets orbit the Sun. Ulysses is the first spacecraft ever to follow such a path. During early planning stages, the Ulysses mission was named the International Solar Polar Mission, but in 1984 its name was changed to Ulysses in honor of the mythical Greek hero, who quested for unknown regions beyond the Sun. The Ulysses craft passed by Jupiter in 1992 and by the Sun in 1994-1995. It passed Jupiter's orbit again in 1998 and by the Sun a second time in 2001. II THE ULYSSES SPACECRAFT The body of the Ulysses spacecraft is 2.14 m (7.06 ft) high, 3.33 m (11 ft) wide, and 3.24 m (10.7 ft) long. Within the body are control systems, scientific instruments, and a fuel tank (33.5 kg/68.7 lb) full of hydrazine, a fuel used by small thruster rockets to aim and correct Ulysses's course. Mounted on top of the body, a 1.65-m (5.5-ft) dish antenna is used to communicate with headquarters on Earth. On one side of Ulysses, a small nuclear reactor generates 280 watts of electrical power for instruments and controls. Instruments sensitive to radiation are mounted on a 5.5-m (18-ft) radial boom extending from the side of the body opposite the reactor. Instruments account for 55 kg (120 lb) of Ulysses's 366 kg (807 lb) mass. Four instruments are mounted on the radial boom opposite the reactor. Two magnetometers are used to measure the magnetic fields of the Sun and Jupiter during Ulysses's flyby. The Solar X-ray and Cosmic Gamma-ray Burst Instrument (GRB) collects data on solar flare X rays and cosmic gamma rays, and the Unified Radio and Plasma Wave Instrument (URAP) detects long radio waves from sources at great distances and short plasma-wave frequencies produced by solar eruptions near the Sun. The URAP instrument also searches for the origins of Jupiter's radio waves. A 72-m (238-ft) wire antenna and a 7.5-m (25-ft) axial antenna for this instrument were deployed in flight. Five instruments are located on the spacecraft body. The Solar Wind Plasma Experiment (SWOOPS) measures the solar wind, which is the number and energy of particles emitted by the Sun. The Solar Wind Ion Composition Spectrometer (SWICS) measures the temperature and ionization, or electrical charge, of heavy atoms such as oxygen, silicon, and iron in the solar wind. The Energetic Particle Composition instrument (EPAC) detects medium-energy ions, or electrically charged particles, in the solar wind. EPAC also detects helium atoms that originate beyond the solar system. The Heliospheric Instrument for Spectral Composition and Anisotropy at Low Energies (HI-SCALE) measures low energy interplanetary ions and high-energy electrons released by solar flares. The Cosmic Ray and Solar Particle Instrument (COSPIN) is designed to detect and characterize cosmic rays, a wide range of particles ejected by the Sun, and particles emitted by Jupiter. III THE ULYSSES MISSION Astronomers have known for many years that the Sun's magnetic field is like a giant bar magnet with its polar axis perpendicular to the plane of the ecliptic--the plane of Earth's and most of the others planets' orbits (see Solar System: Movements of the Planets). However, previous spacecraft explorations were confined to the ecliptic, so there were no direct measurements of the magnetic fields over the Sun's poles or the solar wind emitted from the polar regions. The Ulysses mission was designed to send a spacecraft into polar orbit around the Sun and directly measure the polar solar wind and magnetic field. Previous spacecraft that had explored the solar system, such as the United States spacecraft Galileo, Pioneer, Voyager, and Mariner, attained speeds high enough to orbit the Sun by launching in the same direction that Earth orbits the Sun. These missions essentially used Earth as a booster platform and only added a relatively small extra push to the craft so that they could escape the gravitational pull of the planet. The situation for engineers planning the flight of Ulysses was not so simple, however, for the desired path--nearly perpendicular to the plane of the ecliptic--meant that Earth's orbital velocity could not be used to attain the required speed. In fact, Earth's orbital speed would have to be canceled in order to attain the proper direction. To achieve such objectives directly would require rockets many times more powerful than any that exist today. Therefore, flight engineers for Ulysses devised a path that would take the spacecraft first to Jupiter. They planned to use Jupiter's gravitational field as a slingshot to bend the trajectory of the craft into the proper angle and at the speed required to maintain a polar orbit about the Sun (see Gravitation). Ulysses was launched with the space shuttle Discovery on October 6, 1990. From a high earth orbit, three upper-stage rockets boosted the Ulysses spacecraft to the speed required to leave Earth orbit and reach Jupiter--11.3 km/s (24,400 mph). The initial trajectory given to the spacecraft was just slightly to the north of the ecliptic. After 16 months, in February 1992, Ulysses passed over Jupiter's north pole. Jupiter's gravity bent Ulysses's path into a southward arc that took the spacecraft around Jupiter and over Jupiter's south pole, then on a path nearly perpendicular to the plane of the ecliptic and headed toward the Sun. In November 1994 Ulysses passed almost directly over the Sun's south pole at a distance of about one and one-half times the radius of Earth's orbit. The Sun's gravity bent the spacecraft's path into a trajectory northward around the Sun. The spacecraft passed over the Sun's north pole in July 1995. After passing over the Sun's north pole, Ulysses started back toward Jupiter. The entire duration of Ulysses's first pass by the Sun occurred during a period of low solar activity. The spacecraft passed Jupiter's orbit again in May 1998 and started back toward the Sun. Jupiter was far away on the other side of the Sun when Ulysses crossed the planet's orbit, so the spacecraft's orbit was not affected by Jupiter. Ulysses passed a second time over the Sun's polar regions in 2000 and 2001, this time during a period of high solar activity. After its second pass by the Sun, Ulysses was coordinated with ESA's Solar and Heliospheric Observatory (SOHO) spacecraft to jointly observe the Sun's corona and solar wind. IV RESULTS Prior to the Ulysses mission, scientists believed that the intensity of the solar wind was greater over the Sun's poles than at the ecliptic. However, Ulysses measured the intensity of the solar wind as roughly constant at all latitudes. Scientists also believed that the Sun's magnetic field lines at its poles would be dragged into space by the solar wind, increasing the magnetic field strength over the poles. Measurements showed, however, that pressure forces created by the ionized gas of the solar wind tend to even out the Sun's magnetic field strength. Data gathered by Ulysses also revealed large-amplitude magnetic waves over the Sun's poles that strongly scatter cosmic rays. During close approaches to Jupiter, Ulysses measured the magnetosphere and radiation belts surrounding Jupiter and ionized particles emanating from Jupiter's moon Io. The SWICS instrument detected a lower concentration of helium 3 in interplanetary space than astronomers had expected to find. This result may indicate that the universe contains more dark matter, or matter that cannot be detected by observing electromagnetic radiation, than had been thought. Ulysses was also used to study comet Hale-Bopp, which was first seen in 1995. Hale-Bopp travels an unusual path at a large angle to the ecliptic, and the trajectory of Ulysses was well suited to capture images of Hale-Bopp as it made its way around the Sun. Astronomers paid particular attention to Hale-Bopp's plasma tail--a plume of ionized, or electrically charged, gas within the main tail that streams away from the comet when it approaches within about 1.5 AU from the Sun. They found that the plasma tail disconnects from the comet when the comet passes through a region where the Sun's magnetic field reverses. By observing Hale-Bopp's reaction to the solar wind, astronomers have gained valuable information about both the comet and the Sun's magnetic field. Contributed By: Colin A. Fries Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.