NASA’s Hubble Space Telescope, Mars Rover Opportunity and two Mars orbiters – the MAVEN and the Mars Reconnaissance Orbiter – have captured stunning images of Comet Siding Spring as it made its closest approach to the Red Planet on October 19, 2014.
This composite Hubble image captures the positions of Siding Spring and Mars on October 19, 2014. Image credit: NASA / ESA / PS / JHU / APL / STScI / AURA.
On that date the comet, also known as C/2013 A1, passed by Mars at approximately 139,500 km, about one-third the distance between Earth and the Moon. At that time, the comet and Mars were approximately 240 million km from Earth.
The Hubble image shown above is a composite of several separate images of Siding Spring and the Red Planet.
NASA’s Hubble Space Telescope took the image of Mars on October 18. The comet image is a composite of Hubble exposures taken on October 18-19. The images have been added together to create a single picture to illustrate the angular separation, or distance, between the comet and Mars at closest approach. The separation is 1.5 arcminutes, or 1/20th of the angular diameter of the full Moon.
A view of Siding Spring taken by Mars Exploration Rover Opportunity’s Pancam on October 19, 2014. Image credit: NASA / JPL-Caltech / Cornell University / ASU / TAMU.
The solid icy comet nucleus is too small to be resolved in the Hubble picture. The comet’s bright coma, a diffuse cloud of dust enshrouding the nucleus, and a dusty tail, are clearly visible.
This is a composite image because a single exposure of the stellar background, the comet and the planet would be problematic because the objects are all moving with respect to each other and the background stars. Hubble can only track one planetary target at a time. Also, Mars is actually 10,000 times brighter than the comet, and the exposure here has been adjusted so that details on the planet can be seen.
The second image was taken by the panoramic camera on NASA’s Mars Exploration Rover Opportunity about two-and-a-half hours before the closest approach of the comet’s nucleus to Mars.
This image was taken by the Mars Reconnaissance Orbiter on October 19, 2014. Image credit: NASA / JPL-Caltech / University of Arizona.
“It’s excitingly fortunate that this comet came so close to Mars to give us a chance to study it with the instruments we’re using to study Mars. The views from Mars rovers, in particular, give us a human perspective, because they are about as sensitive to light as our eyes would be,” said Dr Mark Lemmon, a member of NASA’s Opportunity team and a researcher with Texas A&M University.
The third image was taken by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter from a minimum distance of about 138,000 km, yielding a scale of about 138 meters per pixel.
Telescopic observers had modeled the size of the Siding Spring’s nucleus as about 1 km wide. However, the HiRISE image shows only 2-3 pixels across the brightest feature, probably the nucleus, suggesting a size smaller than half that estimate.
This UV image of hydrogen surrounding Siding Spring was taken by the MAVEN spacecraft on October 17, 2014. Image credit: NASA / Laboratory for Atmospheric and Space Physics / University of Colorado.
The fourth image shows hydrogen surrounding Siding Spring in ultraviolet light. It was taken by NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft on October 17, two days before the comet’s closest approach to Mars.
The bulk of the scattered sunlight shows a cloud that was about a half degree across on the ‘sky’ background, comparable in size to our Moon as seen from Earth. Hydrogen was detected to as far as 150,000 km away from the comet’s nucleus. The distance is comparable to the distance of the comet from Mars at its closest approach. Gas from the comet is likely to have hit Mars, and would have done so at a speed of 56 km per second. This gas may have disturbed the Mars atmosphere.
