The Sun is a variable star, meaning that its activity varies over time. The changing degree of activity is due to two factors: its rotation and its convection processes, which transport hot gas from the solar interior to the surface. The interaction of these two motions-rotation and convection-generates powerful magnetic fields and influences the cyclic activity level demonstrated by the ebb and flow of sunspots and solar flares.
Ulysses became the first spacecraft to explore the Sun from polar orbit.
Additionally, activity in the Sun's corona-the white "halo" of gas seen during total eclipses-causes ejection at very high velocities of a hot electrified gas called the solar wind, which courses throughout the solar system transporting energy to Earth and all the other planets. Its interaction with Earth's magnetic fields causes a whole range of effects, such as the aurora magnetic storms, disruption of radio communications, and power surges in transmission lines.
An important facet of NASA's space science program is solar-terrestrial research, which embraces the study of the Sun as a variable star, the origin and transmission of the solar wind, its interactions with Earth's magnetosphere, and how all these phenomena connect the Sun to the Earth and the heliosphere, the vaguely-bounded region of space where the Sun's magnetic field and the solar wind extend.
Over the years, NASA has employed a number of spacecraft to study the processes that link Earth with the Sun. In the 1990s, this activity has expanded under the International Solar Terrestrial Physics (ISTP) program, a joint effort of NASA, the European Space Agency (ESA) and the Japanese Institute of Space and Astronautics.
Not a component of ISTP but a joint NASA/ESA mission, Ulysses-launched October 6, 1990-is the first effort to explore the heliosphere from solar polar orbit over a full range of solar latitudes. After a roundabout four-year flight from Earth, Ulysses reached an area of the Sun's south pole in June 1994, then flew into the northern hemisphere of the heliosphere in 1995 and conducted a four-month observation of the north polar region. Late in 1995, Ulysses completed its primary mission, having returned volumes of invaluable data on the Sun's corona, wind, solar and non-solar cosmic rays, solar radio bursts and plasma waves, and the heliosphere's magnetic field. Ulysses is managed for NASA by Jet Propulsion Laboratory.
The Solar Heliospheric Observatory (SOHO), launched December 2, 1995, surprised scientists by its initial reports, which cited intense activity on the Sun's surface at a time-the low ebb of the 11-year solar cycle-when it should have been relatively inactive. A part of the ISTP program, SOHO is a joint NASA/ESA mission designed to perform remote measurements of the Sun and in situ measurements of the solar wind to improve knowledge of the corona and the origin of the solar wind.
The two-ton spacecraft was launched into orbit at a point about a million miles sunward from Earth, a point where the gravities of the Sun and Earth cancel each other out and provide a stable position from which to conduct long term continuous observation of the Sun. In the early months of 1996, SOHO returned to Earth motion pictures of the Sun's activity and a mass of data on solar interior dynamics and the composition of the solar wind. In addition to NASA and ESA, participants include Germany, the United Kingdom, France, Finland and Switzerland; Goddard Space Flight Center manages the NASA-provided elements of SOHO.
Another component of the ISTP program, launched into polar orbit on February 24, 1996, is the Polar spacecraft, built by Lockheed Martin Astro Space under the management of Goddard Space Flight Center. Polar and its sister spacecraft Wind (launched November 1, 1994) are a pair of complementary spacecraft, developed under NASA's Global Geoscience Space Program to gain broader understanding of the relationship between solar plasma emitted by the Sun and its interaction with Earth's magnetosphere, ionosphere and magnetic poles.
Polar's job is to measure the energy, energization and transport of plasma into the magnetosphere by the solar wind. Additionally, it is making direct measurements of global energy deposition into Earth's atmosphere. At midyear 1996, NASA was receiving good data from all 11 of Polar's instruments.
Two veteran spacecraft launched almost 30 years ago-Voyagers 1 and 2-are now playing a part in solar-terrestrial research. Having completed their grand tours of the solar system and flybys of the outer planets, they have been assigned new jobs: to observe cosmic rays and the solar wind, and to search for the transition boundary between the solar wind and interstellar space as they head out of the solar system on escape trajectories. Both spacecraft are estimated to have about 20 years of useful life remaining.