Four comet images we thought we'd never see, and what they mean

Tuesday, October 28, 2014, 12:11 PM - As more results filter in from NASA regarding the close flyby of Comet Siding Spring, here are four amazing images from the encounter, and what they'll mean for science.

When Comet Siding Spring swung by Mars on October 19, coming within just 140,000 kilometres of the planet's surface, it was an unprecedented opportunity to image and study one of the oldest remnants of the formation of our Solar System. The scientists and engineers with NASA's Jet Propulsion Laboratory certainly didn't waste this chance, perhaps the only one of its kind that they'd have in their lifetime (or longer). So, even as they programmed their orbiting satellites to dodge the comet's debris trail, they left themselves ample leeway to have the orbiters observe the comet and gather as much data as possible. It's taken a few days for the satellites to settle back into their proper orbits and to download the copious amounts of information they collected, and here's a small collection of what we've seen so far.

The comet is tiny but bright

NASA's Mars Reconnaissance Orbiter (MRO), which normally takes incredible high-resolution images of the surface of the Red Planet, aimed its powerful telescopic camera at the comet, and snapped pictures to capture the nucleus (top two images) and the 'coma' of gas and dust surrounding it (bottom two images).

Credit: NASA/JPL-Caltech/University of Arizona

With each pixel of the image measuring less than 140 metres across, this puts the nucleus at just under 500 metres in diameter. That's surprising, since previous estimates - made my measuring the comet's brightness with Earth-based telescopes - were more than double that size!

What does that mean? As Karl Battams pointed out on the Planetary Society blog, based on all the comets we've seen so far, the assumption has been that they are all the same charcoal black in colour, and it's just the gas and dust blown off as it generates the coma and tail that are the only bright parts. The Rosetta spacecraft is currently giving us some very closeup images of Comet 67P/Churyumov-Gerasimenko, which has gone around the Sun nearly 700 million times, ranging in its orbit from just beyond Earth's orbit to out beyond Jupiter and back. However, it could be that this charcoal-coloured surface is the result of repeated sweeps through the inner part of the solar system. Since this is Comet Siding Spring's first journey through the inner solar system, and it is very likely an Oort Cloud comet (originating from far beyond the orbit of Pluto), it is composed of materials that can be considered 'pristine' from the formation of our solar system. It's surprising brightness for its size, therefore, may be due to this pristine nature. It's possible that the comet's surface doesn't have this same charcoal colour. It might be a 'lighter-coloured' black, or more 'gray' or possibly even white.

The comet's coma was 'colourful'

As MRO snapped the above images, its Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument was gathering in the light from the comet's coma to study its properties.

Credit: NASA/JPL-Caltech/JHUAPL

According to the JPL website: "The images show the inner part of the cloud of dust, called the coma, that is generated around the nucleus by the warmth of the sun. The solid nucleus itself is not resolved. CRISM observed 107 different wavelengths of light in each pixel. Here, only three colors are shown. Researchers think the appearance of color variations in the inner coma could be due to the properties of the comet's dust, possibly dust grain size or composition. The full spectra will be analyzed to better understand the reason for the color variations."

What does this mean? We'll have to wait for the full analysis to know the real answer to this question, but knowing the properties of the particles - their size and what they're composed of - will give us more information about the conditions that were present 4.5 billion years ago, when the cloud of dust and gas collapsed to form our Sun and then coalesce into the planets and other objects orbiting it. Knowing that information will give us powerful insights into the objects and events we observe out in the galaxy and the universe and could possibly bring about some discoveries we don't even anticipate yet.

The comet's coma brushed Mars' atmosphere

This image from NASA's newly-arrived MAVEN spacecraft may not look like much, but it's t 

Credit: NASA/Laboratory for Atmospheric and Space Physics/Univ. of Colorado

From the NASA JPL website: "The image shows sunlight that has been scattered by atomic hydrogen, shown as blue in this false-color representation. Comets are surrounded by a huge cloud of atomic hydrogen because water (H₂O) vaporizes from the icy nucleus, and solar ultraviolet light breaks it apart into hydrogen and oxygen. Hydrogen atoms scatter solar ultraviolet light, and it was this light that was imaged by the IUVS. Two observations were combined to create this image, after removing the foreground signal that results from sunlight being scattered from hydrogen surrounding Mars."

What does this mean? This cloud of hydrogen happens with all comets, as they all undergo the same process detailed above. However, MAVEN confirmed that this cloud of hydrogen surrounded the comet's nucleus out to a distance of around 150,000 kilometres. Since the comet came within  around 139,000 kilometres of the planet, and the planet is less than 6800 km wide, that means that Mars was fully exposed to the comet's coma. Given the relative velocity of the two, the gases of the coma would have hit the upper atmosphere of Mars travelling around 200,000 km/h, possibly disturbing it. What would that do? Since nothing like this has ever happened (at least that we've witnessed), there's really no way to be sure. We'll have to wait for followup observations and studies to know for sure.

Hubble immortalizes the chance encounter

Unlike the fanciful artist impressions that we were treated to ahead of the rendezvous, this one is 100 per cent bonafide ... if you forgive some adjustments due to brightness.

Credit: NASA, ESA, PSI, JHU/APL, STScI/AURA

This image shows Comet Siding Spring and Mars as they made their closest pass by each other, superimposed on the field of stars in the background. However, due to the relative brightness of everything in the image, this wasn't just one picture snapped at that precise moment in time. It's a composite image, combining pictures taken by the Hubble Space Telescope and by the Palomar Digital Sky Survey here on Earth. Since Mars was around 10,000 times brighter than Siding Spring in Hubble's eye, the telescope had to take two separate images, adjusting the exposure so that it could capture both Mars and the comet at their best. Since both objects are much brighter than the background field of stars, the 'backdrop' for this view of the encounter was provided by astronomers with Palomar, after it was adjusted to match Hubble's resolution. Thus, while the individual elements had to be brought together to produce this image, it does show the real comet, near the real planet Mars, at the right time when they were closest to each other, and among a real view of the real stars that were visible in Hubble's view at the time.

What does this mean? Well, there's likely no deep scientific meaning for this image, beyond the incredibly cool fact that we have so many space-viewing assets at our disposal that we can accurately produce this amazing view of a comet close encounter.

MORE AMAZING STORIES ABOUT COMETS AND NASA'S Jet Propulsion Laboratory

See the 'Center of Our Universe' deep inside NASA's Jet Propulsion Laboratory
Comet Siding Spring viewed by robots in orbit and on surface of Mars
Rosetta gets a whiff of Comet 67P, and it stinks of rotten eggs and pee
'J' marks the spot for Rosetta comet mission landing in November