OrbitalHub

In the coming days, a spacecraft launched from Cape Canaveral in October 2023 will pass close enough to Mars to feel the planet's gravity bend its trajectory. NASA's Psyche spacecraft is executing a gravity assist maneuver, using Mars as a gravitational redirector to adjust its speed and direction toward a distant asteroid in the outer main belt. The flyby is scheduled for approximately May 15, 2026, when the spacecraft will pass within 1,500 kilometers of the Martian surface, close enough for ground-based telescopes to detect it and for the spacecraft's instruments to record data about the planet's environment.

The gravity assist is not an accident or an afterthought. It is an intentional engine of the mission design, reducing the propellant the spacecraft must carry for its journey to 16 Psyche, a metal-rich asteroid that orbits the Sun at approximately 2.9 times the Earth-Sun distance. Without the Mars flyby, reaching 16 Psyche would require more acceleration from the spacecraft's solar-electric propulsion system and a longer travel time. The maneuver leverages orbital mechanics to do in minutes what would otherwise require months of thrusting.

16 Psyche is one of the most massive objects in the asteroid belt, with a diameter of approximately 280 kilometers. What distinguishes it from most asteroids is its composition. Spectroscopic observations from Earth suggest that the asteroid may be composed primarily of iron and nickel, similar to the metallic cores of rocky planets like Earth. The leading hypothesis is that 16 Psyche is the exposed core of a protoplanet that was disrupted by collisions early in the solar system's history, stripping away its rocky mantle and leaving the metallic interior as a separate body. If this interpretation is correct, 16 Psyche offers a direct view of planetary core material without the need to drill through hundreds of kilometers of overlying rock.

The Psyche mission will test this hypothesis through a year-long scientific investigation beginning in August 2029. The spacecraft carries a magnetometer to search for evidence of a remnant magnetic field, which would support the core remnant hypothesis. A gamma ray and neutron spectrometer will characterize the elemental composition of the surface, distinguishing iron-rich regions from silicates. A multispectral imager will map the surface geology and topography. A technology demonstration called the Deep Space Optical Communication system will test high-bandwidth laser communication at interplanetary distances.

The solar-electric propulsion system aboard Psyche uses xenon as its propellant. The xenon atoms are ionized by electrons emitted from hollow cathodes, accelerated through a series of electrostatic grids, and expelled at velocities exceeding 19 kilometers per second. The resulting thrust is modest, on the order of a few pounds, but it operates continuously over months, producing a cumulative velocity change that equals or exceeds what a chemical rocket could achieve with far more propellant mass. The system is the highest-power electric propulsion ever flown on a planetary mission, drawing up to 45 kilowatts from the spacecraft's large solar arrays.

The Mars flyby serves multiple engineering purposes simultaneously. The primary objective is trajectory modification, changing the spacecraft's heliocentric orbit to align with 16 Psyche's orbital plane and reduce the arrival velocity. The secondary objective is calibration of the science instruments, which will observe Mars during the approach and departure phases, providing an opportunity to compare the spacecraft's measurements against known values for a well-characterized planetary body. The magnetometer will pass through Mars's bow shock and magnetotail, providing data on the planet's interaction with the solar wind.

Lindy Elkins-Tanton, the mission's principal investigator, noted on April 29 that NASA's Eyes on the Solar System simulation tool had been updated to show the upcoming flyby, allowing the public to visualize the encounter in real time. Raw images from the spacecraft are available through NASA's public image archive, and the team is expected to release imagery from the Mars approach beginning in early May. The spacecraft is operating nominally, according to periodic updates from the Jet Propulsion Laboratory, with no reported anomalies in the weeks leading up to the encounter.

The timing of the flyby reflects the orbital geometry of the mission. Psyche launched in October 2023, placing it in a trajectory that intersects Mars's orbit at the appropriate point in May 2026. The 2029 arrival date is fixed by the orbital mechanics of the transfer trajectory from Earth to the asteroid belt. The launch window for 16 Psyche occurs only once every 4.7 years, when the relative positions of Earth, Mars, and the asteroid align. If Psyche had missed the 2026 Mars flyby opportunity, reaching 16 Psyche would have required waiting for the next window in 2031, at which point the mission would arrive in 2036.

A gravity assist works by exploiting the orbital motion of a planet. When a spacecraft approaches a moving planet, the planet's gravitational field redirects the spacecraft's path. More precisely, the spacecraft is falling toward the planet, but because the planet itself is moving, the spacecraft's velocity relative to the Sun changes as it swings around the planet's trailing side. In the planet's reference frame, the spacecraft approaches and departs at the same speed but in different directions. In the Sun's reference frame, the spacecraft has gained or lost velocity depending on whether it passed behind or ahead of the planet's motion.

The Psyche spacecraft is passing behind Mars as seen from the Sun, which means it will gain velocity relative to the Sun, raising its orbital energy and moving it outward toward the asteroid belt. The magnitude of the velocity change, approximately 2.5 kilometers per second, is modest compared to the total velocity budget of the mission but occurs at precisely the right location and direction to maximize its effect on the trajectory.

Navigation of the flyby requires precise knowledge of the spacecraft's position and velocity relative to Mars at the time of closest approach. The navigation team at JPL uses ground-based tracking data, including measurements from the Deep Space Network, to estimate the trajectory and command correction maneuvers when necessary. The margin for error is small: arriving at Mars with a velocity error of even a few meters per second would change the trajectory after the flyby by enough to require additional correction burns that consume propellant and alter the arrival time at 16 Psyche.

The instruments aboard Psyche that will observe Mars during the flyby include the magnetometer and the multispectral imager. The magnetometer will detect perturbations in the interplanetary magnetic field caused by Mars's bow shock, the boundary where the solar wind encounters the planet's magnetic environment. The imager will acquire context images of the Martian surface from a distance, providing an opportunity to test the camera's performance on a real planetary target before the encounter with 16 Psyche.

The Deep Space Optical Communication technology demonstration, which uses a laser transmitter to send data at rates far exceeding what conventional radio systems can achieve, will be tested during the flyby. The Mars proximity provides a useful target for the optical communications link, with ground stations on Earth pointing toward the spacecraft as it passes near the planet. The test will demonstrate whether optical communication can be used for high-bandwidth science data transmission during future deep space missions, potentially revolutionizing the data return capabilities of interplanetary spacecraft.

Video credit: NASA Jet Propulsion Laboratory