It’s Easier to Leave the Solar System Than to Reach the Sun
The center of the solar system is a tricky destination, but NASA is going.
In a very short time, we human beings have seeded our corner of the universe with all kinds of signs of our existence. We have flung hundreds of satellites into the sky, cloaking the Earth in technology. We sent spacecraft to swing by planets and moons, to orbit them, to roam their surfaces. A few years ago, we reached the invisible line between the end of our solar system and the beginning of everything else, and then pierced it, hurtling into the darkness beyond.
This last achievement, humanity’s escape from the solar system, was certainly astonishing, a testament to human ingenuity and engineering. But it was much easier than what we’re trying to do next.
The Parker Solar Probe, a NASA mission, will blast off from the Florida coast in the early-morning hours of Saturday. Next month, the spacecraft will reach Venus, its sidekick on a long journey. Parker will swing past the planet seven times, slowing down with each pass. Eventually the probe will end its rendezvous with Venus and move into a closer orbit around the sun, coming within 3.9 million miles of the sun’s surface to graze its edge. It will be more than seven times closer than any probe has flown before.
As strange as it may sound, it’s much more difficult to reach the sun than it is to leave the solar system altogether.
“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.)
The Parker team studied its options and settled on Venus. In some ways, the new trajectory works out well. With Jupiter, the probe would have come closer to the sun, but it would have made only two passes. With Venus, the Parker Solar Probe will make 24 passes over its lifetime. The probe will spend more time sampling new territory around the sun and, scientists hope, provide answers to some outstanding questions about our star.
NASA has previously launched several satellite missions toward the sun, but the field of heliophysics—the study of the sun’s effects on the solar system—remains quite new. The Parker Solar Probe will fly through one of the most mysterious regions of the sun: the corona, the outer layer of hot plasma stretching millions of miles from the surface, or photosphere. Unlike the photosphere, the corona is visible to the naked eye only during a solar eclipse, like the kind that swept across the United States last year.
Scientists don’t know why the corona is so hot; temperatures there can exceed 1.8 million degrees Fahrenheit (1 million degrees Celsius), while the photosphere remains at a comparatively cool 10,000 degrees Fahrenheit (6,000 degrees Celsius). Nor do they know how exactly it generates constant streams of charged particles that unfurl across the entire solar system like tentacles, a phenomenon known as the solar wind.
Scientists have spent 60 years thinking about the solar wind. Specifically, Eugene Parker, the American astrophysicist for whom the NASA mission is named, first described the dynamics of solar wind in 1958. “It was something most people couldn’t seem to swallow. They expressed stern disbelief,” Parker told Rebecca Boyle in Air & Space magazine this summer.
The disbelief was muffled four years later, when instruments on NASA’s Mariner 2 spacecraft observed the presence of solar wind on the way to Venus. Evidence steadily piled up in the decades that followed, as more spacecraft and satellites launched into the sky and felt the breeze. In 2013, as Voyager 1 departed the solar system, the spacecraft’s instruments detected hints of our sun’s wind as it crashed into the cooler particles of interstellar space.
The Parker Solar Probe is better dressed for its fiery occasion than previous spacefarers. The probe wears a carbon-composite shield that is 4.5 inches thick and capable of withstanding external temperatures of nearly 2,500 degrees Fahrenheit (1,377 degrees Celsius). Water will circulate through tubes in the solar arrays and into large radiators, cooling the probe down. An autonomous computer system will gauge the heat outside and tuck some of the spacecraft’s solar panels away, or expose them, based on the surroundings.
Safe from the sun’s glare, the probe’s scientific instruments will operate at a balmy 78 degrees Fahrenheit (26 degrees Celsius).
The Parker mission is scheduled to end in 2025. After it pings home a final time, the spacecraft will, over the course of many years, succumb to the sun’s gravity. There, beyond the pull of anything else, Parker will dip closer and closer to our star. Under the scorching conditions, it will crumble first into pieces, and then into dust. Years after learning to leave the solar system, humanity will have finally managed to plunge right into the heart of it.