In 2022, the NASA mission DART impacted Dimorphos, the tiny moon of the still-smallish asteroid Didymos. The purpose was to see how much slamming a spacecraft into an asteroid would change its orbit… and it worked! This shows that diverting an asteroid on an Earth-impacting trajectory is at least technically possible.

The impact profoundly affected Dimorphos itself and the environment around it, including its parent asteroid Didymos. The European Space Agency built the spacecraft Hera (named after the queen of the gods) as a follow-up, to fly to the asteroid pair and examine them up close.

The asteroids have an elliptical orbit that takes them out a bit past Mars, so, to help get there, Hera swung by Mars (named for the god of war) on March 12 to make a gravity assist, stealing some of the planet’s orbital energy to boost itself to match the asteroids’ orbit. It passed only 5,000 km from Mars, and also was aimed in such a way that it zipped by its tiny moon Deimos (named after the god of dread) as well, flying past at a mere 300 km. It took images from as close as 1,000 km, and they are super cool.

This image was taken by Hera’s Asteroid Framing Camera, used for navigation as well as science. It doesn’t record color information, so the image is in grayscale. Deimos appears very dark compared to Mars and it’s more potato-shaped (or walnut-shaped, really) than spherical. It’s only about 16 x 12 x 10 km in size, so far too small to have enough gravity to shape itself into a sphere.

Hera also has a Thermal Infrared Imager, built by the Japanese space agency JAXA, which will be used to measure the temperatures of the surfaces of the asteroids. During the flyby it took a series of images as the spacecraft approached Deimos, making this dramatic animation (it repeats twice for a total of three times):

These images are in infrared, and so they show relative temperature; Deimos is brighter than the surface below it, so it’s warmer. Note that in the first image above it appears darker than Mars, because that image is in visible light, the kind we see. Deimos reflects about 7% of the light hitting it — meaning it absorbs 93% of that light — and Mars reflects more like 17%. Because Deimos is better at absorbing sunlight it warms up (just like you might prefer wearing darker colors in winter to help keep you warm), but that means it appears brighter in infrared.

That might be easier to see in this infrared image, where temperature is mapped to colors, so blue/purple is cooler, and orange/yellow warmer:

Like so many things, what you see depends on how you’re seeing it.

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