Two Missions, Two Very Different Worlds
Launched in 2003, the European Space Agency's Mars Express has been a stalwart of planetary science, orbiting the Red Planet for over two decades. Its goal has been a comprehensive investigation of Mars: its surface, atmosphere, and the history of its water.
Now, consider NASA's Psyche spacecraft, launched in 2023. It isn’t going to a planet at all. Its destination is (16) Psyche, a mysterious, metal-rich asteroid orbiting the Sun between Mars and Jupiter. Scientists believe this asteroid could be the exposed iron core of an ancient, shattered protoplanet. Psyche’s goal is not to study a whole world, but to explore a new type of world—one made of metal, not rock or ice. This fundamental difference in targets is the key to understanding why their scientific instruments are both similar and wildly different.
The Eyes of the Mission: Seeing in Different Light
Both spacecraft are equipped with powerful cameras, but they are designed to 'see' in ways that serve their unique missions. Mars Express carries the High Resolution Stereo Camera (HRSC). Its purpose is to create high-resolution 3D maps of the Martian surface, helping scientists understand the planet's geology and topography. It captures stunning, detailed landscapes. NASA's Psyche, on the other hand, has a pair of Multispectral Imagers. While they will also take pictures to map the asteroid's surface, their primary job is to see in different wavelengths of light, from visible to near-infrared. This allows scientists to infer the asteroid's composition. Different minerals and metals reflect light differently, so these imagers will help create a surface map of what Psyche is made of—is it all metal, or is there rock mixed in? It's the difference between taking a detailed photograph and performing a chemical analysis with a camera.
Sensing the Unseen: From Subsurface Radar to Gamma Rays
Much of space science involves detecting what the human eye cannot. Mars Express famously carries MARSIS, a subsurface sounding radar designed to look for liquid water or ice buried kilometres beneath the Martian crust. This was revolutionary for a mission searching for signs of past or present water on Mars. Psyche, travelling to a barren metallic world, has no need for such an instrument. Instead, its most specialized tool is the Gamma-Ray and Neutron Spectrometer (GRNS). This device will be bombarded by cosmic rays that hit the asteroid's surface and cause the atoms there to emit gamma rays and neutrons. The GRNS can read the energy signatures of this radiation to directly identify elements like iron, nickel, and silicon on the surface. It's a way of directly tasting the asteroid's elemental makeup, crucial for answering whether it is truly a planetary core.
A Cosmic Compass: Mapping Invisible Fields
One of the most direct points of comparison is that both spacecraft carry a magnetometer. A magnetometer is essentially a highly sensitive cosmic compass used to measure the strength and direction of magnetic fields. But even with the same type of instrument, the scientific questions are different. Mars Express uses its magnetometer to study how Mars's thin atmosphere interacts with the solar wind, and to investigate the remnants of the planet's ancient global magnetic field, which has long since vanished. Psyche’s magnetometer has a more focused job: to determine if the metal asteroid has a magnetic field of its own. If it does, it would be powerful evidence that the asteroid is indeed the core of a long-dead protoplanet, which once generated a magnetic field just as Earth’s core does today. It’s the same tool used to find a definitive clue to Psyche’s origin.
A Flyby for Calibration
The connection between the two missions became very real in May 2026, when Psyche performed a gravity-assist flyby of Mars. As it zipped past the Red Planet to gain speed for its journey, mission planners coordinated with the Mars Express team. Both spacecraft pointed their imagers at the same regions of the Martian surface at roughly the same time. This wasn't just for a photo opportunity. By comparing the images taken by Psyche's new Multispectral Imager with the well-understood data from the veteran HRSC on Mars Express, scientists could calibrate Psyche's instruments against a known target. This dry run ensures the equipment is perfectly tuned for its arrival at the main-belt asteroid in 2029.
