“I’m always amused when someone says, ‘Shoot X or so-and-so into the sun,’” says Rand Simberg, a space consultant and an engineer. “Because they have no idea how hard that is to do.”

The reason has to do with orbital mechanics, the study of how natural forces influence the motions of rockets, satellites, and other space-bound technology. Falling into the sun might seem effortless since the star’s gravity is always tugging at everything in the solar system, including Earth. But Earth—along with all the other planets and their moons—is also orbiting the sun at great speed, which prevents it from succumbing to the sun’s pull.

This arrangement is great if you’d like to avoid falling into the sun yourself, but it’s rather inconvenient if you want to launch something there.

“To get to Mars, you only need to increase slightly your orbital speed. If you need to get to the sun, you basically have to completely slow down your current momentum,” says Yanping Guo, the mission-design and navigation manager for the Parker Solar Probe. Based at the Johns Hopkins Applied Physics Laboratory, Guo has been working on the probe for about 17 years.

Probes bound for deep-space destinations like Mars can piggyback off Earth’s momentum to fly faster. For a spacecraft to launch toward the sun, on the other hand, it must accelerate to nearly match the Earth’s velocity—in the opposite direction. With the planet’s motion essentially canceled out, the spacecraft can surrender to the sun’s gravity and begin to fall toward it. But this is almost impossible with current rocket technology, so spacecraft have to get some help, in the form of slingshot maneuvers off other planets, called gravity assists.

Spacecraft usually use gravity assists to travel deeper into the solar system; in 2007, the Pluto-bound New Horizons spacecraft approached Jupiter, dipped into the massive planet’s immense gravity, and then bolted away, moving faster than it approached.

This time, the Parker Solar Probe will experience seven gravity assists from Venus in order to draw closer to the center of the solar system. With each pass, the spacecraft will shed some of Earth’s motion.

The team behind the solar probe initially imagined the spacecraft would get this gravitational boost from Jupiter, the king of gravity assists thanks to its massive size and corresponding gravity. Parker would have needed to swing past Jupiter only twice. The sun “is the most challenging destination to reach in the entire solar system without a gravity assist,” Guo says. “Any available launch vehicle—even near-future, the most powerful—it won’t be able to shoot a spacecraft to get to the sun. You must use gravity, and not just a general gravity assist—you have to use the most powerful gravity assist.”

Such a long detour would have required the Parker Solar Probe to run on nuclear power. But about a decade ago, NASA told the Parker team it couldn’t build its spacecraft to operate that way. Engineers had to completely rethink their planned trajectory: Solar-powered spacecraft, they believed at the time, would be too far from the sun to function near Jupiter. (Things eventually changed, but not soon enough for Parker; the solar-powered Juno reached Jupiter in 2016 and has been orbiting happily ever since.)