Six decades have passed since a University of Chicago physicist pondered why the tail of a comet, no matter the direction it traveled, always pointed away from the Sun. From this and other observations made by astronomers, Eugene Parker theorized that there must be some kind of stream of particles flowing away from the Sun. He called this the "solar wind."

It turns out that such a stream of charged particles does exist, a moving plasma of electrons, protons, and other particles that varies in density and speed. Over time, as observations mounted, astronomers moved from a position of skepticism about Parker's solar-wind idea to one of acceptance. Then, nearly 30 years ago, scientists launched their first mission, Ulysses, to make direct measurements of the phenomenon.

Astronomers have since come to realize the profound importance of this solar wind for our planet and the rest of the Solar System. For example, we now know that a few hundred million years after its formation, the early solar wind accelerated ions in the upper Martian atmosphere to an escape velocity, stripping the young planet of much of its atmosphere over time.

Scientists are keen to better understand the solar wind (and larger bursts called coronal mass ejections) in part because of how this all affects space weather above the Earth, which can disrupt the function of satellites and other spacecraft. A better understanding of the wind's features would allow for the hardening or protection of space-based assets.

But to truly grok the solar wind, we need to get closer to the Sun. Scientists have been thinking about such a mission since the late 1970s, and formal discussions began with NASA in 1993. The space agency eventually selected the Solar Probe mission, but budget cuts under NASA Administrator Sean O'Keefe canceled the project in 2003.

Seeking the origin

Since then, the mission was revised so that it won't fly so close to the Sun—the spacecraft will now get to within 8.86 Solar radii, or about 6.2 million kilometers, rather than about 4.5 Solar radii—and this allows for slightly less shielding and a more ambitious scientific payload. Even so, the solar probe will still come seven times closer to the Sun than any other human scientific spacecraft.

"This is a mission to the origins of the solar wind," said David Alexander, a solar physicist who directs the Rice Space Institute in Houston.

Astronomers think the solar wind is accelerated in the region of 4 to 8 solar radii. Although they have some ideas about what is happening in the Sun's corona with regard to ions and mass ratios, they really want to pin down the physics of this acceleration region. The velocity of the solar wind varies widely, from less than 300km/s to more than 800km/s. "If we can really understand why, there are a lot of things we can do practically for humans," Alexander said.

Scientists would also like to better predict coronal mass ejections and to understand what kinds of particles are feeding these ejections. Armed with this knowledge, they would be better able to protect not only satellites, but also humans during spacewalks or astronauts on the Moon or en route to Mars.

More than two decades of fits and starts with the solar probe mission—now named the Parker Solar Probe after the discoverer of the solar wind—have brought the 685kg spacecraft to a launchpad at Cape Canaveral, Florida. The next step involves a launch aboard a Delta IV Heavy rocket, the second most powerful rocket in the world, to fling the probe toward Venus. Liftoff is scheduled for 3:33am ET (07:33 UTC) on Saturday.

Once near Venus, the probe will make seven flybys as it aligns itself into an elliptical orbit that will bring it close to the Sun 24 times. At its closest approach, the probe will be traveling 700,000kph, a faster velocity relative to the Sun than any other human-made object. The spacecraft should complete the last of its two-dozen elliptical orbits of the Sun in 2025.