Phobos Photobombs Mars

Every two years or so, Mars and the Earth approach each other more closely than usual. We’re in no danger of colliding because both planets are kept in their orbits by the Sun’s gravity. But the orbital geometry gives us an opportunity to get a closer and brighter view of our nearest neighboring planet from our Earth-bound vantage point. The last time this happened was in 2016, and the next time is coming up later this month.

On May 12, 2016, a team of astronomers at the Space Telescope Science Institute used the Hubble Space Telescope (HST) to make several images of Mars close to the time of opposition. At that time, Mars was about 80 million km (50 million miles), or 0.53 AU, away from Earth with an apparent diameter of 18 arcseconds.

We used the Wide Field Camera 3 (WFC3) to make 25 exposures with the UVIS channel, using four filters in a sequence repeated three times. The exposures spanned about 40 minutes, about half of Hubble’s roughly 90 minute orbit. The times in the first graphic are in UT (Greenwich time) and the HST filter names are below each image (the central transmission wavelength in nanometers is part of the filter name, for example, F410M is a medium-width filter centered on 410nm).


We selected different filters so that we could eventually produce color composite images. Exposure times varied from 0.5–20 seconds, taking account of varying filter response and detector sensitivity by wavelength. We processed the images to produce a consistent average brightness (or tonality) across the different filters. Some of the exposures were long enough that the images were partially overexposed and saturated.

We selected the best exposures from each filter in each of the three cycles to produce a color composite with blue, cyan, green, and red color applied to each grayscale (black & white) image according to the wavelength.



Because of Mars’s rotation (with a period of 24 hours 39 minutes, slightly longer than Earth’s rotation period), the globe shifts noticeably even in the short time it takes to change from one filter to another, so features are blurred slightly in any color composite. But the effect is fairly minimal and can be minimized further by a slight shift of each image. [A more complete but much more complex technique that compensates for the shifts due to the planet’s rotation is to remap each image to a flat projection, register and combine all of the images for each filter, produce the color composite, and reproject the resulting image on the globe.]

A bonus from the same set of observations was the appearance of Phobos, one of two moons of Mars, in about half of the exposures. Phobos orbits Mars in only 7 hours 39 minutes, much faster than Mars rotates, and was behind the planet when the Hubble observations began. It is quite small and faint, so it does not show up when the images are processed to show the features on Mars’s disk. But Phobos does appear if the images are made much brighter, as in the next graphic. Even though the filters and exposures are different, the images were processed to produce a consistent appearance of Phobos, even though Mars was entirely overexposed. The disk of Mars has been blocked, but is shown as an outline.


These could then be combined with the color images of the planet to produce a realistic visualization of the motions of the system.


With a bit more careful processing, the shifts between the frames can be interpolated to make a smooth movie.

Stay tuned for the next opposition on July 27th, when Mars will be even closer and brighter. And keep up with Illuminated Universe for the next installment of Hubble’s continuing coverage of Mars oppositions.

For more information about the Hubble images of Mars and Phobos see related news releases at HubbleSite:
Hubble Takes Mars Portrait Near Close Approach
NASA’s Hubble Sees Martian Moon Orbiting the Red Planet


  • Zolt Levay

    Zolt retired as Principal Science Visuals Developer in the Office of Public Outreach at the Space Telescope Science Institute. He currently pursues photography, exploring the relationship between earth-bound landscapes and the larger universe visible in the night sky.

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