The fastest object made by humans is just trying to slow down

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Update (6:49am ET, Aug. 11): This story has been updated with the new Aug. 12 launch time. The Aug. 11 launch was scrubbed because of a last-minute helium-pressure problem.

NASA’s Parker Solar Probe is a 1,500-lb. computer a little smaller than a compact car, but in the years ahead, it will reach speeds of 430,000 miles per hour in order pass through the edge of the sun itself. Getting there is no easy task, and neither is understanding the journey.

“I don’t know if I can truly explain to you how the trajectory works—it’s a beautiful trajectory, it’s a complicated trajectory,” Dr. Yanping Guo, a Johns Hopkins University researcher who plotted the spacecraft’s path into the heart of the solar system, told Quartz. “It’s a challenge to explain it in a language other people can understand.”

Let’s try. First, Guo notes, getting to the sun is “the most, most challenging in all space exploration.” (She knows whereof she speaks—Guo also plotted the course for the New Horizons space probe that voyaged to Pluto in 2015.) But that difficulty might not be intuitive. The sun is by far the largest body in the solar system, with a gravitational field so strong that it literally holds all of the planets together. Getting sucked in should be fairly elementary, right?

Remember, though, that the Earth is orbiting the sun, and quickly—19 miles (30 kilometers) per second. Imagine being on a merry-go-round, circling quickly around a central axis. It’s a lot easier to stay in place, or be flung outward, than to climb towards the center. Those same laws of physics, described by Johannes Kepler and Isaac Newton in the 17th century, govern motion in space.

Hang on tight

Let’s stay on our merry-go-round. The sun’s gravity is that central axis, pulling you in, which interacts with your speed around the edge, locking you into an orbit. To get to the interior of the merry-go-round—or the solar system—you must diminish your velocity so the pull from the central axis wins out.

For the Parker Solar Probe, this starts with one of the most powerful rockets in the world, the Delta IV, built by United Launch Alliance. It will hurl the probe from the planet with more than 350 tons of force behind it.

Any object departing from Earth normally shares the planet’s orbit, but Delta IV will launch the probe in the opposite direction. This maneuver will slow the probe, although Earth’s pull is still so powerful that the spacecraft will continue to accompany it for awhile. Guo says the maneuver is akin to “apply[ing] brakes on a fast-moving car, which reduces the spacecraft’s orbital speed but does not change much its moving direction.”

This is where things really get interesting. Even braking out of the gate won’t get the solar probe close enough to the sun in a timely fashion. We need to bring other planets into play.

Assistance in orbit two

Early plans for the Parker Solar mission relied on a maneuver called a “gravity assist,” which would be performed around Jupiter.

The concept is simple: Jupiter is an enormous planet, orbiting at 8 miles (13 kilometers) per second. A space probe can fly into Jupiter’s gravity field and be tugged forward, capturing some of that speed, before slingshotting around the planet and back toward the sun. Imagine a very strong friend hurling you into the center of that merry-go-round.

This plan, however, required equipping the probe with nuclear power, because the solar panels needed to gather energy as far out as Jupiter would be too big. NASA didn’t want to spend the money, or its scarce plutonium supply.

That meant a different trajectory was needed—and that’s where Guo came in. In 2007, most thought that only a Jupiter gravity assist could get a probe to the sun. Guo realized that, with careful planning, the probe could also use Venus to accomplish its goals. Rather than coming in behind Jupiter to capture speed, the probe would pass in front of Venus, slowing as it was tugged backward.

Except Venus is a smaller planet than Jupiter, and doesn’t have as much energy to share. Using it to slow the probe enough to close in on the sun will require seven different passes—a record number.

A diagram of the Parker Solar Probe's orbital trajectory.
A diagram of the Parker Solar Probe’s orbital trajectory.
Image: Yanping Guo

Lining up the probe so that each orbit passes by Venus at the perfect time and velocity is known as a “phasing problem,” a challenging task that calls for a lot of number-crunching. Remarkably, Guo’s trajectory doesn’t require the probe to perform additional maneuvers in deep space. Instead, everything is determined by the gravity assists—a perfect pool shot that banks seven times before hitting the pocket.

“I’m actually surprised that there’s not a deep-space maneuver,” says MIT Professor Richard Binzel, a planetary scientist who works on the New Horizons mission. “It’s a very fine-tuned path—too close, and the planet will pull you all the way in. You’re threading the needle to get just the right tug from the planet.”

In addition to meeting the key requirement of relying only on solar power, Guo’s unique design has other benefits: It will pass closely by the sun 24 times, instead of just twice, and each pass will bring it gradually closer, allowing scientists to calibrate their instruments for more precision.

To touch the sun

So how does this probe, dedicated to slowing down, break speed records? The answer is also in orbits. Each time the probe slows in a passage around Venus, the sun’s gravity pulls it a little closer. As Kepler explained, the closer a spacecraft gets to the body it is orbiting, the faster it goes relative to that body. The spacecraft is crossing the same angular distance around the sun, but as it gets closer, the actual space traversed shrinks.

On its closest orbit, less than 4 million miles from the sun, the probe is forecast to go as fast as 430,000 miles per hour. That will break speed records also set by NASA spacecraft that orbited the sun. Helios 2, the previous record-holder, reached 253,000 miles per hour in an trip around our star in 1976.

The closer it gets to the sun, the faster the Parker Solar Probe will travel.
The closer it gets to the sun, the faster the Parker Solar Probe will travel.
Image: Yanping Guo

While the record for fastest man-made object is impressive, it’s not what motivates the scientists who designed this project, or the engineers who will execute it. The idea of sending a robot to explore the sun has been kicking around NASA since at least 1958, when scientist Eugene Parker theorized (paywall) that the sun sends a massive flow of charged particles out into space, known as “solar wind.” More data about the corona, the super-hot plasma that surrounds the sun and produces this solar weather, could help us understand it—and protect our electrical infrastructure from dangerous magnetic storms.

For many scientists who have worked on Parker Solar, the scheduled 3:31am launch today (Aug. 12) will be the culmination of decades of work. ”I knew a lot of people who worked on this mission for a long time,” says Guo, who has herself worked on the mission since 2007. “I felt very fortunate—I did a study, and I will also be able to participate in the development, and now we are nearly putting the probe up to get it to the sun. How exciting!”

CorrectionAn earlier version of this article misstated Jupiter’s orbital velocity.