IT之家 On May 17 news, NASA's Psyche spacecraft flew by Mars on May 15 local time, passing closer to the red planet than its two small moons. However, this precisely timed maneuver was not intended to study Mars.

In fact, the Psyche spacecraft's flyby of Mars was to use the planet's gravity for a slingshot acceleration, heading toward its namesake target — the metal-rich asteroid Psyche. The spacecraft came within about 2,800 miles (4,500 kilometers) of Mars at its closest approach, and since early May, Mars had been steadily appearing larger and brighter in the spacecraft's view.

NASA said that on May 15, the Psyche spacecraft flew past Mars at a speed of about 12,333 miles per hour (19,848 kilometers per hour). This flyby not only increased the spacecraft's speed but, more importantly, adjusted its trajectory, steadily propelling it toward its destination — 16 Psyche, an asteroid orbiting between Mars and Jupiter.

Scientists believe that this celestial body, with a diameter of 173 miles (280 kilometers), might be the exposed metallic core of a shattered primordial planet. A violent collision billions of years ago stripped away the outer crust and mantle of this original planet, resulting in its current form. If this hypothesis holds true, the upcoming mission will allow humanity to directly observe core material that is normally hidden deep inside planets like Earth.

Mars Gravity Assist

This Mars flyby is a crucial milestone for the mission, saving precious fuel and putting the probe on the intended trajectory to reach Psyche in 2029.

According to IT之家, this flight maneuver is a widely used gravity assist technique in modern space exploration. By flying precisely close to a moving planet, a spacecraft can gain speed, change its orbit, and save propellant, enabling deep-space missions that would be difficult or impossible with rocket power alone.

This principle may seem counterintuitive, as if the spacecraft acquires energy out of nowhere. As the spacecraft approaches a planet, it accelerates; when it leaves the planet's gravitational influence, it decelerates. Intuitively, the speed changes should cancel each other out.

The key to gravity assist is not simply the planet's gravity, but the planet's own orbital motion around the Sun.

When orbiting the Sun, Mars carries enormous momentum of its own motion. The Psyche spacecraft approaches Mars at a precisely calculated angle and then departs from a specific direction, successfully "borrowing" a minuscule portion of Mars' orbital kinetic energy.

This energy transfer follows Newton's third law, where the action and reaction forces are equal in magnitude and opposite in direction. The spacecraft accelerates by taking an extremely tiny amount of momentum from Mars; this loss is negligible for the planet but is enough to completely alter the spacecraft's flight trajectory.

Shadan Adalan, a scientist who previously worked on NASA's Juno mission to Jupiter, told Space.com in an interview: "This method is efficient, economical, and ingeniously conceived—like a celestial billiard shot that uses a bank shot to gain position."

Gravity assist technology has long helped humanity explore deep space. Its origins trace back to the early space age in 1959, when the Soviet Luna 3 spacecraft used the Moon's gravity to swing around to the far side and capture the first images of the lunar farside.

Many pioneering unmanned deep-space missions have relied on gravity assist. In the late 1970s, the Voyager spacecraft took advantage of a once-in-a-century alignment of the outer planets to travel from Jupiter to Saturn via a gravity slingshot; Voyager 2 went on to fly by Uranus and Neptune, completing a historic tour of the outer solar system.

The Cassini probe used the gravitational assists of Earth, Venus, and Jupiter in sequence to accelerate and gain enough momentum to head toward Saturn; the New Horizons probe flew by Jupiter for a gravity assist, significantly reducing the flight time to Pluto.

Just recently, NASA's highly anticipated Artemis II crewed mission also applied the same principle, using a free-return trajectory, leveraging the Moon's gravity to allow four astronauts to fly around the far side of the Moon and then safely return to Earth without having to start a large number of engines to adjust the course.

A related research paper published last October showed that this Mars gravity assist flyby changed the Psyche probe's velocity relative to the Sun by 2 kilometers per second.

The Psyche probe uses a solar electric propulsion system, relying on solar panels to convert light energy into electrical energy, ionizing and ejecting xenon gas to slowly propel itself forward. This propulsion system is highly efficient, but the long-term thrust output is very weak.

If it relied solely on its own propulsion system to achieve the same amount of speed increase and orbit correction, it would not only be costly but also practically difficult to achieve. This would require a huge amount of propellant, far exceeding the probe's actual carrying capacity, and would also increase overall weight, significantly driving up launch costs.

This Mars flyby allowed the probe to use the planet's gravity to complete most of its orbital and velocity adjustments, saving a significant amount of fuel for the long journey ahead.

The Psyche spacecraft, now like a batted baseball, has left Mars with a new flight momentum and a corrected orbit, heading toward this highly valuable metal-rich asteroid, which may unlock the deep mysteries hidden within planetary interiors. The probe is expected to arrive at asteroid 16 Psyche in July 2029.

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