Mars Express Spacecraft's New Phobos Video Shows Rare Phase Angle

The European Space Agency's Mars Express mission recently wrapped up its tracking of Phobos, the innermost moon orbiting Mars. Using the spacecraft's camera, the mission captured images of the oddly shaped moon from many angles, the collection of which have been assembled together into a video. Visible in the images are a number of the moon's features, including large craters and other marks.

Phobos joins the moon Deimos in orbiting Mars; it is larger and was first spied in the late 1800s by Asaph Hall, an American astronomer. The ESA's new video shows the moon from many angles, highlighting its unusual rock-like shape with protrusions and divots, pockmarks and furrows.

The video was generated from 41 images captured with the Mars Express spacecraft's High Resolution Stereo Camera's Super Resolution Channel last month. The images each have around a 21MP resolution and were captured from a distance of around 1,491 miles. The subtle motion of the moon visible in the video was caused by the spacecraft's oscillation, according to the ESA.

The images happen to highlight a 'very rare' phase angle, which is the angle between the Sun, the camera, and the moon. The phase angle starts at 17-degrees, eventually reaching near zero degrees halfway through the video, and then increasing back to 15-degrees by the end of the video. This can be seen in the illumination and darkening of the images in the animation.

Observing Phobos' phase angle at near-zero is described as incredibly rare, something that can only happen up to three times per year. The ESA says the odds of capturing this event again won't happen until at least April 2020. By snapping images during this time, researchers can see many different aspects of the moon's landscape highlighted by the light changes.