After prospecting for 850 days in a gravity hole nearly a million miles from Earth, the Genesis spacecraft is scheduled to return home today and drop off a unique cargo that may contain new clues about the creation of the sun and planets.
The payload, enclosed in a capsule to be snatched in mid-air by helicopter over Utah's salt flats, will weigh only one hundred thousandth of an ounce. But it will contain a billion billion atoms that originated from the surface of the sun and represent the original building blocks that formed the solar system some 4.5 billion years ago.
Scientists have a good idea of how the solar system came into being: A giant cloud of gas and dust collapsed under its own gravity into a spinning pancake-shaped disc. As gravity tightened, 99 percent of the cloud's matter was pulled into the sun, leaving behind tiny ripples that produced the planets.
But therein lies the puzzle. All the planets are different - those closest to the sun, like Mars, Mercury and Earth, are rocks; those farthest away, like Saturn, Jupiter and Uranus, are balls of gas.
The composition of the rocky planets varies, and so does the composition of the gas balls. Even the moons orbiting some of the planets are different - ranging from solid ice to molten furnaces.
"Even where you expect everything to be the same, they're not," said Genesis principal investigator Don Burnett, a California Institute of Technology geochemist.
The question facing scientists, then, involves the composition of the original cloud of gas and dust from which everything was created. Was it homogenous, the different elements of the periodic table scattered equally everywhere? Or was it heterogeneous, with clumps of different elements occupying specific neighborhoods of the cloud?
Scientists will examine the nuggets brought back by Genesis to see if they can find the answers.
The atoms collected by the spacecraft come from the sun's outer atmosphere, which is still made up of the gas and dust that went into forming the sun. It has not undergone any chemical changes since then because it is too far away from the intense heat in the sun's core to be affected.
As energy radiates out from the center of the sun, it constantly blows away some of the atmosphere, which spreads through space as the solar wind.
The only other time bits of that solar wind were captured and brought back to Earth was when astronauts set up screens on the airless surface of the moon to catch the isotopes that make up the solar wind. Isotopes are different forms of the same atoms and can be heavier or lighter than the stable atoms seen on Earth.
Those samples only added to the puzzle of creation. The ratio of neon-20 to neon-22 isotopes in the solar wind, for example, was almost 40 percent higher than what is found in Earth's atmosphere. That could mean a totally different birth process for Earth - or that much of the Earth's early atmosphere was somehow lost.
The $264 million Genesis spacecraft was launched Aug. 8, 2001, and arrived at the gravity hole known as Lagrange 1 point four months later. Lagrange 1 is a point in space where the gravity from the sun and Earth cancel each other, allowing Genesis to linger there without expending much rocket fuel.
With its uninterrupted view of the sun, Genesis deployed an array of solar wind collectors made of silicon, gold, sapphire, diamond and other materials to which the isotopes of more than 60 different elements in the solar wind stick.
"Sample returns are a great way to do planetary science," Burnett said. "When you bring something back to Earth, you can turn all the analytic firepower of our laboratories on it, instruments that you can't fly in space."