[meteorite-list] NASA Preparing for 2014 Comet Watch at Mars

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?release=2014-026

NASA Preparing for 2014 Comet Watch at Mars
Jet Propulsion Laboratory
January 28, 2014

This spring, NASA will be paying cautious attention to a comet that could 
put on a barnstorming show at Mars on Oct. 19, 2014.

On that date, comet 2013 A1 Siding Spring will buzz Mars about 10 times 
closer than any identified comet has ever flown past Earth.

Spacecraft at Mars might get a good look at the nucleus of comet Siding 
Spring as it heads toward the closest approach, roughly 86,000 miles (138,000 
kilometers) from the planet, give or take a few percent. On the other 
hand, dust particles that the comet nucleus sheds this spring could threaten 
orbiting spacecraft at Mars in October.

The level of risk won't be known for months, but NASA is already evaluating 
possible precautionary measures as it prepares for studying the comet.

"Our plans for using spacecraft at Mars to observe comet Siding Spring 
will be coordinated with plans for how the orbiters will duck and cover, 
if we need to do that," said Rich Zurek, Mars Exploration  Program chief 
scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Comet Siding Spring, formally named C/2013 A1, was discovered on Jan. 
3, 2013, from Australia's Siding Spring Observatory. At the time, it was 
farther from the sun than Jupiter is. Subsequent observations enabled 
scientists at JPL and elsewhere to calculate the trajectory the comet 
will follow as it swings past Mars. Observations in 2014 will continue 
to refine knowledge of the comet's path, but in approximate terms, Siding 
Spring's nucleus will come about as close to Mars as one-third of the 
distance between Earth and the moon.

Comet Ready for Its Close-up

Observations of comet Siding Spring are planned using resources on Earth, 
orbiting Earth, on Mars and orbiting Mars, and some are already underway. 
NASA's Hubble Space Telescope and the NEOWISE mission have observed the 
comet this month both to characterize this first-time visitor from the 
Oort cloud and to study dust particle sizes and amounts produced by the 
comet for understanding potential risks to the Mars orbiters. Infrared 
imaging by NEOWISE reveals a comet that is active and dusty, even though 
still nearly three-fourths as far from the sun as Jupiter is. Ground-based 
observatories such as the NASA Infrared Telescope Facility are also expected 
to join in as the comet becomes favorably positioned for viewing. 

As the comet nears Mars, NASA assets there will be used to study this 
visitor from distant reaches of the solar system.

"We could learn about the nucleus -- its shape, its rotation, whether 
some areas on its surface are darker than others," Zurek said. 

Researchers using spacecraft at Mars gained experience at trying to observe 
a different comet in 2013, as comet ISON (formally C/2012 S1) approached 
Mars. That comet's Mars-flyby distance was about 80 times farther than 
Siding Spring's will be. Another difference is that ISON continued inward 
past Mars for nearly two months, briefly becoming visible to some unaided-eye 
skywatchers on Earth before flying very close to the sun and disintegrating. 
Siding Spring will reach its closest approach to the sun just six days 
after its Mars flyby. It won't put on a show for Earth, and it won't return 
to the inner solar system for about a million years.

At comet Siding Spring's flyby distance, the High Resolution Imaging Science 
Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter could 
provide imagery with resolution of dozens of pixels across the diameter 
of the nucleus. When HiRISE observed comet ISON, the nucleus was less 
than one pixel across. ISON did not get bright enough to make itself visible 
to other cameras at Mars that made attempted observations, but Siding 
Spring could provide a better observation opportunity.

Cameras on the Mars rovers Curiosity and Opportunity might watch for meteors 
in the sky that would be an indication of the abundance of particles in 
the comet's tail, though the geometry of the flyby would put most of the 
meteors in daytime sky instead of dark sky.

"A third aspect for investigation could be what effect the infalling particles 
have on the upper atmosphere of Mars," Zurek said. "They might heat it 
and expand it, not unlike the effect of a global dust storm." Infrared-sensing 
instruments on Mars Reconnaissance Orbiter and Odyssey might be used to 
watch for that effect.

Assessing Possible Hazards to Mars Orbiters

One trait Siding Spring shares with ISON is unpredictability about how 
much it will brighten in the months before passing Mars. The degree to 
which Siding Spring brightens this spring will be an indicator of how 
much hazard it will present to spacecraft at Mars.

"It's way too early for us to know how much of a threat Siding Spring 
will be to our orbiters," JPL's Soren Madsen, Mars Exploration Program 
chief engineer, said last week. "It could go either way. It could be a 
huge deal or it could be nothing -- or anything in between."

The path the nucleus will take is now known fairly well. The important 
unknowns are how much dust will come off the nucleus, when it will come 
off, and the geometry of the resulting coma and tail of the comet.

During April and May, the comet will cross the range of distances from 
the sun at which water ice on a comet's surface typically becomes active 
-- vaporizing and letting dust particles loose. Dust ejected then could 
get far enough from the nucleus by October to swarm around Mars.

"How active will Siding Spring be in April and May? We'll be watching 
that," Madsen said. "But if the red alarm starts sounding in May, it would 
be too late to start planning how to respond. That's why we're doing what 
we're doing right now."

Two key strategies to lessen risk are to get orbiters behind Mars during 
the minutes of highest risk and to orient orbiters so that the most vulnerable 
parts are not in the line of fire.

The Martian atmosphere, thin as it is, is dense enough to prevent dust 
from the comet from becoming a hazard to NASA's two Mars rovers active 
on the surface. Three orbiters are currently active at Mars:  NASA's Mars 
Reconnaissance Orbiter (MRO) and Mars Odyssey, and the European Space 
Agency's Mars Express. Two more departed Earth in late 2013 and are due 
to enter orbit around Mars about three weeks before the comet Siding Spring 
flyby: NASA's Mars Atmosphere and Volatile Evolution (MAVEN) and India's 
Mars Orbiter Mission.

Orbiters are designed with the risk of space-dust collisions in mind. 
Most such collisions do not damage a mission. Design factors such as blanketing 
and protected placement of vulnerable components help. Over a five-year 
span for a Mars orbiter, NASA figures on a few percent chance of significant 
damage to a spacecraft from the background level of impacts from such 
particles, called meteoroids. Whether the Siding Spring level will pack 
that much hazard -- or perhaps greater than 10 times more -- into a few 
hours will depend on how active it becomes.

This comet is orbiting the sun in almost the opposite direction as Mars 
and the other planets. The nucleus and the dust particles it sheds will 
be travelling at about 35 miles (56 kilometers) per second,  relative 
to the Mars orbiters. That's about 50 times faster than a bullet from 
a high-powered rifle and double or triple the velocity of background meteoroid 
impacts.

Cautionary Preparations

If managers choose to position orbiters behind Mars during the peak risk, 
the further in advance any orbit-adjustment maneuvers can be made, the 
less fuel will be consumed. Advance work is also crucial for the other 
main option: reorienting a spacecraft to keep its least-vulnerable side 
facing the oncoming stream of comet particles. The safest orientation 
in terms of comet dust may be a poor one for maintaining power or 
communications.

"These changes would require a huge amount of testing," Madsen said. "There's 
a lot of preparation we need to do now, to prepare ourselves in case we 
learn in May that the flyby will be hazardous."

JPL, a division of the California Institute of Technology, Pasadena, manages 
the NASA's Mars Exploration Program for NASA's Science Mission Directorate, 
Washington. For more information about the flyby of Mars by comet Siding 
Spring, visit http://mars.nasa.gov/comets/sidingspring/ .

For more about the Mars Exploration Program, visit http://mars.jpl.nasa.gov  .


Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.webster at jpl.nasa.gov

2014-026