Traveling to the Sun might seem like a straightforward journey, but the reality is far more intricate. Reaching our star involves a complex dance with orbital mechanics and immense energy considerations. A spacecraft must overcome the very motion that keeps Earth safely in its orbit.
More Than Just a Straight Shot
Earth orbits the Sun at an average speed of approximately 67,000 miles per hour (107,000 kilometers per hour). Any spacecraft launched from Earth shares this inherent orbital velocity, moving sideways around the Sun. To “fall” into the Sun, a spacecraft must shed this significant orbital velocity. It is energetically more challenging to slow down enough to fall toward the Sun than to accelerate and escape the solar system entirely.
This initial velocity poses a substantial hurdle. Imagine trying to throw something into the center of a merry-go-round while spinning rapidly on its outer edge; the object needs to slow its tangential speed to move inward. Without significant deceleration, a probe would simply continue orbiting the Sun, never reaching the star itself.
The Gravitational Challenge
To overcome Earth’s orbital velocity and enable a spacecraft to fall closer to the Sun, engineers employ gravity assist maneuvers. This method uses the gravitational pull of planets to change a spacecraft’s speed and direction without expending large amounts of propellant. When a spacecraft approaches a planet, it exchanges energy with the planet’s orbital motion, gaining or losing momentum.
For Sun-bound missions, gravity assists decelerate the spacecraft relative to the Sun. This is achieved by approaching a planet, like Venus, in a trajectory opposite to the planet’s orbital direction. The spacecraft “slingshots” around the planet, transferring some orbital momentum, causing it to slow down and fall closer to the Sun. This reduction in orbital velocity allows the probe to overcome the Sun’s strong gravitational pull and achieve an inward spiraling trajectory.
Pioneering Missions to the Sun
Humanity has sent several pioneering missions to study the Sun, demonstrating the complex orbital mechanics required. The Helios 1 and Helios 2 probes, launched in December 1974 and January 1976, were among the first. They reached their closest approaches within three months. Helios 1 passed within 29 million miles (47 million kilometers) of the Sun, while Helios 2 achieved an even closer distance of 27 million miles (43.4 million kilometers) and set a speed record of about 150,000 miles per hour (241,350 kilometers per hour).
More recently, NASA’s Parker Solar Probe (PSP), launched on August 12, 2018, represents the most ambitious mission. This probe uses seven Venus gravity assists to gradually reduce its orbital energy and spiral inward. Its first close approach occurred around November 5, 2018. The PSP’s trajectory allows increasingly closer passes, with its final planned gravity assist on November 6, 2024, setting up its closest approach on December 24, 2024. At this point, the probe will come within 3.8 million miles (6.1 million kilometers) of the Sun’s surface, traveling at up to 430,000 miles per hour (690,000 kilometers per hour), making it the fastest human-made object.
Survival in the Sun’s Extreme Environment
Reaching the Sun is only part of the challenge; surviving its extreme environment demands extraordinary engineering. Near the Sun, spacecraft face intense heat, powerful solar radiation, and high-speed solar particles. The Parker Solar Probe is equipped with a specialized Thermal Protection System (TPS), an eight-foot diameter, 4.5-inch thick heat shield. This shield, constructed from carbon-carbon composite with a lightweight carbon foam core, features a white reflective coating to deflect energy.
While the TPS outer surface can reach 2,500 degrees Fahrenheit (1,370 degrees Celsius), most instruments are maintained at 85 degrees Fahrenheit (30 degrees Celsius) in its shadow. A sophisticated water-cooled system manages heat for the solar arrays, circulating 3.7 liters of deionized water through channels and radiators. Due to significant communication delays, the Parker Solar Probe operates with high autonomy, enabling quick reactions to the Sun’s dynamic conditions.