Nasa is making final preparations for a once-in-a-million-years journey as a comet grazes the atmosphere of Mars on Sunday.
Comet C/2013 A1, also known as comet Siding Spring, will pass within about 87,000 miles (139,500 kilometers) of the Red Planet on Sunday – less than half the distance between Earth and our moon, and less than one-tenth the distance of any known comet flyby of Earth.
Researchers say it will provide an unprecedented opportunity to gather data on both the comet and its effect on the Martian atmosphere.
‘Think about a comet that started its travel probably at the dawn of man and it’s just coming in close now,’ said Carey Lisse, a senior astrophysicist at the Johns Hopkins University Applied Physics Laboratory, in a NASA news briefing about Comet Siding Spring last week.
‘And the reason we can actually observe it is because we have built satellites and rovers. We’ve now got outposts around Mars.’
Siding Spring’s nucleus will come closest to Mars around 11:27 a.m. PDT (2:27 p.m. EDT), hurtling at about 126,000 mph (56 kilometers per second).
This proximity will provide an unprecedented opportunity for researchers to gather data on both the comet and its effect on the Martian atmosphere.
‘This is a cosmic science gift that could potentially keep on giving, and the agency’s diverse science missions will be in full receive mode,’ said John Grunsfeld, astronaut and associate administrator for NASA’s Science Mission Directorate in Washington.
‘This particular comet has never before entered the inner solar system, so it will provide a fresh source of clues to our solar system’s earliest days.’
NASA’s five robotic explorers at Mars – three orbiters and two rovers – are being repurposed to witness a comet named Siding Spring make its first known visit to the inner solar system.
So are a European and an Indian spacecraft circling the red planet.
The orbiting craft will attempt to observe the incoming iceball, then hide behind Mars for protection from potentially dangerous dusty debris in the comet tail.
Shielded by the Martian atmosphere, the Opportunity and Curiosity rovers may well have the best seats in the house, although a dust storm on Mars could obscure the view.
‘We certainly have fingers crossed for the first images of a comet from the surface of another world,’ said NASA program scientist Kelly Fast.
Spacecraft farther afield, including the Hubble Space Telescope, already are keeping a sharp lookout, as are ground observatories and research balloons.
‘We’re getting ready for a spectacular set of observations,’ said Jim Green, head of NASA’s planetary science division.
Named for the Australian observatory used to detect it in January 2013, Siding Spring will approach Mars from beneath and zoom right in front Sunday afternoon, Eastern Time.
On Earth, the best viewing, via binoculars or telescope, will be from the Southern Hemisphere – South Africa and Australia will be in prime position. In the Northern Hemisphere, it will be difficult to see Siding Spring slide by Mars.
The comet – with a nucleus estimated to be at least a half-mile in diameter – hails from the Oort Cloud on the extreme fringe of the solar system. It formed during the first million or two years of the solar system’s birth 4.6 billion years ago and, until now, ventured no closer to the sun than perhaps the orbits of Jupiter, Saturn, Uranus or Neptune. It comes around every one or more million years.
It will be the first Oort Cloud comet to be studied up close in detail.
For comparison, the flyby distance of 87,000 miles is about one-third of the way from here to the moon. Siding Spring’s tail could extend from Earth all the way to our moon. Its gaseous coma, the fuzzy head surrounding the nucleus, might stretch halfway to the moon.
No comet has come anywhere near this close to Earth in recorded history.
‘We can’t get to an Oort Cloud comet with our current rockets … so this comet is coming to us,’ said Carey Lisse, senior astrophysicist at Johns Hopkins University’s applied physics laboratory.
By studying Siding Spring’s composition and structure, scientists hope to learn more about how the planets formed, according to Lisse.
Scientists also are keen to spot any changes to the comet or Mars due to the close approach. NASA’s newly arrived Maven spacecraft, for instance, will compare the upper atmosphere before and after it passes.
‘Think about a comet that started its travel probably at the dawn of man and it’s just coming in close now,’ Lisse said. ‘And the reason we can actually observe it is because we have built satellites and rovers.
‘We’ve now got outposts around Mars.’
Scientists initially worried the spacecraft orbiting Mars would be at considerable risk from the comet’s massive trail of dust.
The nucleus itself poses no danger of impact. But the particles in the tail, hurtling through space at 126,000 mph could fry electronics, puncture fuel lines, or destroy computers, transmitters or other vital spacecraft parts.
As Siding Spring’s path became clearer, the threat level was deemed minimal. Still, space agencies are taking no chances. They’re employing the ‘duck and cover’ strategy.
NASA’s three orbiters – Mars Odyssey, Mars Reconnaissance Orbiter and newcomer Maven – will be behind the red planet at the time of peak danger. That’s a 20-minute-or-so period approximately 1 1/2 hours after the closest approach by the comet’s nucleus.
The European Space Agency also shifted the orbit of its Mars Express as did India for its Mars Orbiter Mission, or MOM, the country’s first interplanetary spacecraft that, like NASA’s Maven, arrived last month.
The precautions are prudent, said University of Maryland senior research scientist Tony Farnham, who led a hazard-analysis team.
‘Comets are complex beasts and don’t always live up to our predictions,’ Farnham said in an email Wednesday. ‘If you don’t want surprises, then don’t study comets.’
It will take at least a few days to obtain and analyze the best spacecraft data; but images made from Earth should be forthcoming pretty quickly.
Siding Spring should pass closest to the sun six days after its Mars flyby, then swing back out, bidding goodbye, for at least another million years.
NASA’s Mars orbiters will gather information before, during and after the flyby about the size, rotation and activity of the comet’s nucleus, the variability and gas composition of the coma around the nucleus, and the size and distribution of dust particles in the comet’s tail.
Observations of the Martian atmosphere are designed to check for possible meteor trails, changes in distribution of neutral and charged particles, and effects of the comet on air temperature and clouds. MAVEN will have a particularly good opportunity to study the comet, and how its tenuous atmosphere, or coma, interacts with Mars’ upper atmosphere.
Earth-based and space telescopes, including NASA’s iconic Hubble Space Telescope, also will be in position to observe the unique celestial object. The agency’s astrophysics space observatories — Kepler, Swift, Spitzer, Chandra — and the ground-based Infrared Telescope Facility on Mauna Kea, Hawaii — also will be tracking the event.
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