On 15 May, whilst en route to a metallic asteroid, NASA’s Psyche mission flew just 4500 km above the surface of Mars. It was using the gravity of the Red Planet to boost its speed and adjust its trajectory.

So-called ‘gravity assist flybys’ are common for interplanetary spacecraft. They make journeys to the inner or outer Solar System much more efficient. But they also provide a valuable opportunity to test a spacecraft’s science instruments. Psyche did just that, snapping photos of Mars's dusty surface in the hours before and after its closest approach.

Upon hearing of Psyche's flyby, the teams behind ESA's long-running Mars Express and its High Resolution Stereo Camera (HRSC) wondered if the paths of the two spacecraft would cross, allowing them to point at the same patch of the Red Planet.

Teams working on both missions coordinated their observation plans weeks in advance, exchanging trajectories and observation timelines to photograph the same parts of the martian surface as close in time as possible.

The images here show one of two coordinated observations. On the left is the image captured by Mars Express. On the right is the same landscape captured by Psyche’s camera just 1 hour and 50 minutes later.

The images show part of Noachis Terra, one of the oldest known parts of Mars’s surface. The region is scattered with craters filled with sand dunes that are shown in dark blue.

Though relatively close in time, the images were taken from very different distances and viewing angles. By the time Psyche passed over this region, about 2.5 hours after its closest approach to Mars, its altitude had increased to about 41 000 km. Mars Express, meanwhile, was at an altitude of just 6000 km; it was looking from an inclined angle, whereas Psyche was looking almost straight down towards Mars’s surface.

The two coordinated observations are not only nice visual comparisons, but could be used to help calibrate Psyche's camera. By comparing the brightness and colour recorded in each image pixel, scientists could discover small differences in how the two cameras respond to light. Though more investigation is needed into whether the observations are similar enough to be useful, these comparisons could help with checking and adjusting Psyche's camera, ensuring that it works optimally in future.

Image notes

The Mars Express image has a resolution of 800 metres per pixel. The Psyche image has a resolution of 500 metres per pixel. Though Mars Express's HRSC camera has a resolution of less than 20 metres per pixel in optimum conditions, its observation settings were adjusted to capture an image best comparable to Psyche’s image resolution at the large distance of 41 000 km.

Both images are RGB colour composites generated from the respective red, green and blue filters (Psyche image) or image channels (Mars Express image).

Both images have been stretched for improved comparability.

[Image description: A square image showing a mostly-orange patch of Mars's surface, scattered with sharp craters, many of which are blue inside. A vertical bar lets viewers slide between two views of exactly the same patch of surface. Whilst the landscape is identical, the left image is slightly more orange and the right image is slightly more blue.]