We start the new year with this photograph of the Earth and its Moon, taken from Mars.
This composite image of Earth and the Moon, as seen from Mars, combines the best Earth image with the best moon image from four sets of images acquired on November 20, 2016, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter.
Each was separately processed prior to combining them so that the moon is bright enough to see. The Moon is much darker than Earth and would barely be visible at the same brightness scale as Earth. The combined view retains the correct sizes and positions of the two bodies relative to each other.
HiRISE takes images in three wavelength bands: infrared, red, and blue-green, similar to Landsat images in which vegetation appears red.
The reddish feature in the middle of the Earth image is Australia.
Southeast Asia appears as the reddish area (due to vegetation) near the top.
Antarctica is the bright blob at bottom-left. Other bright areas are clouds.
These images were acquired for calibration of HiRISE data, since the spectral reflectance of the Moon’s near side is very well known. When the component images were taken, Mars was about 205 million kilometres (127 million miles) away from Earth.
Previously from Mars…
A previous HiRISE image of Earth and the Moon, acquired on October 3, 2007, by the HiRISE camera on NASA’s Mars Reconnaissance Orbiter.
On this Earth image, we can make out the west coast outline of South America at the lower right, although the clouds are the dominant features. These clouds are so bright, compared to the Moon, that they are saturated in the HiRISE images.
This colour image required a fair amount of processing to make it a nice-looking photographic release.
Actually, the Red-filter image was almost completely saturated. The Blue-Green image had significant saturation, and the brightest clouds were saturated in the infrared (IR) image.
The moon image is unsaturated, but it has been brightened relative to Earth to create the composite. The lunar images are useful for calibration of the camera.
At the time this image was taken, Earth was 142 million kilometres (88 million miles) from Mars, giving the HiRISE image the following characteristics:
- a scale of 142 kilometres per pixel,
- an Earth diameter of about 90 pixels and
- a Moon diameter of 24 pixels.
The phase angle is 98 degrees, which means that less than half of the disk of the Earth and the disk of the Moon have direct illumination. We could image Earth and moon at full disk illumination only when they are located on the opposite side of the sun from Mars, but then the range would be much greater, and the image would show less detail.
On the day this image was taken, the Japanese Kayuga (Selene) spacecraft was en route from the Earth to the moon, and has since returned spectacular images and movies.
NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Mars Reconnaissance Orbiter for NASA’s Science Mission Directorate, Washington.
Lockheed Martin Space Systems, Denver, the prime contractor for the project, built the spacecraft.
The High Resolution Imaging Science Experiment (HiRISE) is operated by the University of Arizona, Tucson, and the instrument was built by Ball Aerospace and Technology Corp., Boulder, Colorado, U.S.