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  Mysteries of the Solar Nebula

A few billion years ago, after generations of more ancient suns had been born and died, a swirling cloud of dust and gas collapsed upon itself to give birth to an infant star.

As the ball-shaped cloud fell in on itself, it began to flatten and rotate, eventually resembling a spinning pancake. Mostly the stuff of the cloud was simple atoms of hydrogen and helium, but it was peppered here and there by more complicated elements forged in the internal furnaces and death explosions of older stars. Even as a new sun took shape at the center of the cloud, disturbances formed farther out. In a remarkably short time by astronomical standards — "just" tens of millions of years, or less — these disturbances of matter turned into planets.

Today that star system is home to an amazing diversity of environments — from immense mountains and enormous, jagged canyons on rocky inner planets to sulfur volcanoes and ice geysers on moons circling huge gas planets farther out from the star, their orbits crisscrossed by legions of comets and asteroids.

That is the story, astronomers tell us, of how the Sun, our Earth and the solar system that both of them occupy came to be. There is plenty of evidence from observations over many decades to establish the broad outlines of that story. But exactly how the placental cloud of dust and gas, called the "solar nebula," turned into the solar system that we see around us today still poses many mysteries for scientists. One of the main ways that scientists approach the question of how the solar system formed is by comparing the elements and isotopes that made up the original cloud of dust and gas to the compositions of the planets, moons, asteroids and comets in the solar system today. (An isotope is a variation of an element that is heavier or lighter than the standard form of the element because each atom has more or fewer neutrons in its nucleus.) But what were the ingredients in the original solar nebula?

Fortunately, nature provides a fossil record of the solar nebula. Like other stars its size, the Sun has an outer atmosphere that is slowly but steadily flowing off into space. This material, consisting mostly of electrically charged atoms called ions, flows outward past the planets in a constant stream called the "solar wind." This wind is a snapshot of the materials in the surface layers of the Sun, which in turn reflects the makeup of the original solar nebula. This is the rationale of the Genesis mission. By flying out beyond the interfering influences of Earth's magnetic fields, the spacecraft can collect samples of the solar wind revealing the makeup of the cloud that formed the solar system nearly 5 billion years ago.

Solar Studies Past and Present
Past Missions to Collect Solar Wind
Apollo 11, 12, 14, 15 and 16 (NASA)
: The solar wind composition experiment on these missions that took astronauts to the Moon between 1969 and 1972 was a 1.4- by 0.3- meter (55- by 11-inch) aluminum foil sheet on a pole. This sheet was exposed to the Sun for periods ranging from 77 minutes on Apollo 11 to a period of 45 hours on Apollo 16. On Apollo 16, a platinum sheet was also used. Solar wind particles embedded themselves in the foil, which was returned to Earth for laboratory analysis. The chemical composition of the embedded solar wind included isotopes of the light noble gases helium-3, helium-4, neon-20, neon-21, neon-22 and argon-36. The Apollo foils showed that the ratio of neon-20 to neon-22 in the solar wind was almost 40 percent higher than what is found in Earth's atmosphere. Such a large difference was totally unanticipated. Many scientists believe that this difference was caused by an early major loss of Earth's atmosphere. Comparison of Genesis data with the terrestrial atmosphere for nitrogen and the other noble gases (argon, krypton and xenon) may well provide a definitive test of that theory.

Currently Operating Space-Based Missions
Ulysses (European Space Agency and NASA): Launched October 6, 1990, Ulysses explores the Sun from a perspective above and below the ecliptic, the plane in which most of the planets orbit the Sun, to study the environment around the Sun's north and south poles. Scientists have used Ulysses data to define different types of solar wind, of which Genesis will return separate samples. In addition, they have measured the strength of magnetic fields that surround the Sun and related them to the solar wind.

Wind (NASA): Launched November 1, 1994, the Wind spacecraft is currently in orbit around the same point in space targeted by Genesis — the Lagrange 1 point, or "L1," where the gravities of Earth and the Sun cancel each other out. It is studying various facets of the interaction of Earth's magnetic environment and the solar wind.

Solar and Heliospheric Observatory (European Space Agency and NASA): Soho also orbits the Lagrange 1 or "L1" point. Launched on Dec. 2, 1995, Soho uses 12 instruments to study the physical processes in the Sun's corona and changes in the Sun's interior by making observations in visible, ultraviolet and extreme ultraviolet light.

Advanced Composition Explorer (NASA): This spacecraft launched Aug. 25, 1997, carries nine instruments to study the formation and evolution of the solar system and the astrophysical processes involved. It does this by sampling low-energy particles from the Sun and high-energy particles from elsewhere in the galaxy. Like Genesis, it orbits the L1 point to get a prime view of the Sun and the galactic regions beyond. The spacecraft measures particles of a wide range of energies and nuclear mass, under all solar wind flow conditions and during both large and small particle events including solar flares. It provides a one-hour warning when solar events will cause a geomagnetic storm that can interfere with the operations of satellites and telecommunications systems on Earth.

Transition Region and Coronal Explorer (NASA): This spacecraft, called by its acronym Trace, images the solar corona and the transitional region between the Sun and surrounding space. Launched April 2, 1998, Trace enables solar physicists to study the connections between the Sun's magnetic fields and associated plasma structures on the Sun. It does this by taking sequences of images of different areas — the photosphere, or the Sun's visible surface; the corona, the outermost region of the Sun's atmosphere, consisting of hot ionized gases; and the transitional region between the Sun and surrounding space.

Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics mission (NASA): This mission, known by its acronym Timed, is studying the influences of the Sun and humans on the least explored and understood region of Earth's atmosphere — the mesosphere and lower thermosphere/ionosphere. This region is a gateway between Earth's environment and space, where the Sun's energy is first deposited into Earth's environment. This mission focuses on a portion of this region at an altitude of about 60 to 180 kilometers (40 to 110 miles). Launched Dec. 7, 2001, the mission is helping scientists understand how conditions vary in this region, allowing predictions of effects on communications, satellite tracking, spacecraft lifetimes, degradation of spacecraft materials and on the reentry of vehicles piloted by human crews. The mission's study of space weather will help scientists gain a better understanding of the dynamics of this gateway region.

Reuven Ramaty High Energy Solar Spectroscopic Imager (NASA): This mission is exploring the basic physics of how particles are accelerated and energy is released in solar flares. It approaches this task by making high-resolution images of solar flares studying the spectrums of released energy across wavelengths from X-rays to gamma rays. It is expected to observe tens of thousands of microflares, more than a thousand X-ray flares and more than one hundred gamma ray flares. The spacecraft was launched Feb. 5, 2002.

More information about Sun-exploring missions.

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Curator: Aimee Meyer
Updated: November 2009

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