The Hubble Space Telescope, whose cosmic images have graced coffee-table books and calendar pages, has now given earthlings an idea of what an interplanetary spectator might have seen as Comet Siding Spring approached its close encounter with Mars on Oct. 19.
The composite image is among a smattering of initial images and data that spacecraft at Mars and space-based observatories in the inner solar system have sent back to Earth after the comet buzzed Mars – flitting to within 87,000 miles of the red planet.
Data from five US, one European, and one Indian spacecraft at Mars are of special interest. Siding Spring, discovered in January 2013, comes from the Oort Cloud – a vast halo of comets and potential comets that extends from roughly 5,000 astronomical units (AU) from the sun to 50,000 AU or more. One AU is 93 million miles, the average distance between Earth and the sun.
Given the fleet of orbiters and rovers at Mars, Siding Spring's close encounter with the planet is expected to give scientists their most detailed look yet at an Oort Cloud comet.
For instance, the HiRISE camera on NASA's Mars Reconnaissance Orbiter returned images of the nucleus this week, the first clear look at an Oort Cloud comet's core. Observations from ground-based telescopes suggested that the nucleus was about 2,000 feet across – small compared with some other comets that originate much closer than the Oort Cloud.
For comparison, the nucleus for Comet Churyumov-Gerasimenko, which currently hosts the European Space Agency's Rosetta spacecraft, averages about two to three miles across. Based on the HiRISE images, researchers now estimate that Siding Spring's nucleus may be closer to 1,000 feet wide.
NASA's MAVEN orbiter, which arrived at Mars Sept. 21 to undertake a year-long study of the planet's upper atmosphere, captured the comet's halo, or coma, with its imaging-ultraviolet spectrograph. The spectrograph keyed in on solar ultraviolet light that scattered off hydrogen atoms, mainly from water molecules that the icy comet releases as the sun heats it.
This halo of hydrogen extended up to 93,000 miles from the comet, suggesting that some of the atoms may have slammed into Mars' upper atmosphere. If that's the case, the comet's hydrogen atoms would have been traveling 125,000 miles an hour, according to the MAVEN research team.
The interaction is of interest because it may shed light on processes in the upper atmosphere that cost Mars a warmer, wetter climate early in its history. MAVEN's mission is to make measurements that will allow scientists to reconstruct that climate history.
The MAVEN team bracketed Siding Spring's flyby with observations a day and a half in advance, and for two days after the comet's departure, notes Bruce Jakosky, an astrobiologist at the University of Colorado at Boulder and MAVEN's lead scientist.
“The data are now on the ground, and we're looking at them,” he writes in an e-mail. “It may be a little while before we understand what they are telling us though.”
The reason: The science team is still fine-tuning the craft's instruments. The team will need to understand how the instruments are behaving in their new environment before they can fully interpret the information gathered, he explains.
As for Hubble's image, the portrait of Mars and its visitor has to be assembled from different images because the light from Mars alone would have swamped the glow from the comet.
It's a conundrum familiar to photographers.
Expose to show the comet clearly, and Mars becomes a reddish blob with no detail. Expose for detail on Mars, and the faint comet virtually vanishes. In addition, each is moving against the background stars at substantially different speeds. Expose for a “stationary” Mars, and – if it appeared – the comet would be a blurry streak.
The only way to get a clear group shot was to image each individually between Oct. 18 and 19 and combine the two.
Ansel Adams probably would have approved.