[meteorite-list] Could Upcoming Comet Flybys Damage Mars Spacecraft?

[Ron Baalke]

http://www.space.com/22859-mars-spacecraft-comet-flybys-dangers.html

Could Upcoming Comet Flybys Damage Mars Spacecraft?
By Leonard David
space.com
September 19, 2013

Two comets will buzz Mars over the course of the next year, prompting 
excitement as well as some concern that cometary particles could hit the 
spacecraft orbiting the Red Planet and exploring its surface.

Three operational spacecraft currently circle Mars: NASA's Odyssey and 
Mars Reconnaissance Orbiter (MRO), as well as Europe's Mars Express. NASA 
also has two functioning rovers, Curiosity and Opportunity, on the ground 
on Mars.

All of these spacecraft will have ringside seats as Comet ISON cruises 
by Mars this year, followed by Comet 2013 A1 (Siding Spring) in 2014. 

Crossing the sublime line

The MRO spacecraft has been on the lookout for Comet ISON, said Richard 
Zurek, MRO project scientist and chief scientist in the Mars Program Office 
at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

On Aug. 20, MRO looked for Comet ISON, which experts say could put on 
a dazzling sky show here on Earth shortly after the icy wanderer zips 
a scant 724,000 miles (1.16 million kilometers) above the surface of the 
sun on Nov. 28.

During last month's observation by MRO, ISON was 1 astronomical unit (AU) 
from Mars and 2.5 AU from the sun. (One AU is the distance from Earth 
to the sun - about 93 million miles, or 1.5 million km.)

Given ISON's distance from the sun, the comet should have crossed the 
solar system's "snow line" by that time, Zurek told SPACE.com. At the 
snow line, many comets brighten as ice more rapidly sublimes into gas 
due to increasing solar radiation. 

"The MRO instruments did not see anything," Zurek said, and evidence suggests 
the instruments "were pointed accurately. Thus, the current conclusion 
is that the comet had not brightened quite enough to be seen at that range 
with the MRO instruments."

Comet ISON's current luminosity is a topic of much discussion among astronomers 
and skywatchers alike. The icy wanderer was branded a "comet of the century" 
candidate almost immediately after its discovery in September 2012, but 
recent observations suggest that it's not brightening as much as expected 
or hoped on its trek toward the sun.

More observations ahead

MRO will look at ISON again, Zurek said, with observations scheduled for 
Sept. 29, Oct. 1 and Oct. 2 (when the comet will be closest to Mars). 
At those times, ISON will be roughly 14 times closer and will likely be 
relatively easy to detect. 

"At the closest passage distance, there is no concern that cometary particles 
from ISON will affect the orbiters or Mars," he said.

NASA's 1-ton Curiosity rover and its smaller, older cousin, Opportunity, 
will also image ISON from the Martian surface later this month, Zurek 
said. However, those plans are still being formulated.

The spacecraft in orbit around Mars and on the planet will give scientists 
a better chance of investigating Comet ISON, though that is not their 
primary function, said Michael Meyer, lead scientist for NASA's Mars Exploration 
Program at the agency's headquarters in Washington, D.C.

"Mars has a better view than Earth does right now," Meyer said. However, 
it is "challenging for orbital and landed assets as they are not really 
designed to do this sort of thing. They are supposed to be looking at 
Mars."

Meyer spoke via Skype Aug. 25 during a New Media Practitioners Professional 
Development Workshop on the upcoming launch of NASA's Mars Atmosphere 
and Volatile Evolution orbiter (or Maven for short). The workshop took 
place at the University of Colorado Boulder's Laboratory for Atmospheric 
and Space Physics (LASP).

Another comet coming

After ISON, scientists will look forward to Comet Siding Spring, Meyer 
said. That comet will make a very close approach to Mars in October 2014, 
skirting just 76,428 miles (123,000 km) from the planet, according to 
the current best estimates.

"That promises to be pretty exciting," Meyer said. "Right now, in all 
honesty, what we know about it and what sort of calculations can be done 
- the error bars are extremely large."

The comet poses risks to orbiters circling Mars, Meyer said, a prospect 
that may lead to re-orienting and maneuvering of the craft to protect 
them from comet particle strikes. But whether it's a 10 percent, 1 percent 
or 0.1 percent risk remains unknown at the moment, he said.

"You can't get too worked up about it until you get some measurements 
as the comet gets closer," Meyer said. "It promises to be quite a show 
- if we're able to look at it." 

Stay tuned

In early August, JPL issued a request for proposals to help characterize 
the cometary environment of Comet Siding Spring, with proposals due on 
Sept. 11.

"The intent is to provide data products useful for risk assessment and 
mitigation-strategy development for the Mars orbiter missions, due to 
possible impacts from dust and ion tail particles as this comet encounters 
Mars," the JPL request stated.

Model simulations are needed to characterize the evolving dust and ion 
particle distributions around Comet Siding Spring, as well as their motions 
with respect to Mars, as this comet approaches the Red Planet.

Because Comet Siding Spring will come so close to Mars, it's likely that 
the planet, along with its associated spacecraft, will pass through the 
coma of the comet, Zurek said. But NASA's rovers will probably be relatively 
well protected, he said.

"As thin as the Mars atmosphere is, it should still shield the rovers 
from infalling particles," Zurek said, "so the risks to be assessed are 
to the orbiters."

Zurek told SPACE.com that scientists won't have an idea of how big a risk 
the comet environment will pose until they make more observations of the 
comet's variability as it nears the sun. 

"We have put out a call for modeling of the cometary environment," Zurek 
said. Such modeling, he said, is dependent on the developing comet activity.

Passage through the comet's coma could result in a wide range of effects: 
anything from a modest enhancement to the background meteoritic flux experienced 
by the spacecraft, which is deemed most likely, to something more substantial, 
Zurek said.

"So stay tuned!" he said.

New arrival

Siding Spring's Mars visitation will also overlap with that of another 
Mars newcomer, NASA's Maven spacecraft. To be launched this November, 
it will arrive at the Red Planet in late September 2014, with Comet Siding 
Spring set to make its closest approach to Mars on Oct. 19.

However, that will likely be too soon for the Maven orbiter to analyze 
the comet.

"Maven will still be in the middle of its commissioning phase at that 
time and will not be ready to take regular measurements," said Bruce Jakosky, 
principal investigator for Maven at LASP.

"Although we'd like to be able to observe the comet as it passes by and 
how it affects the upper atmosphere, our first priority will be spacecraft 
and instrument health and safety," Jakosky told SPACE.com.

The Maven team is working with the Mars Program Office (MPO) to predict 
the likely dust environment as the comet passes by, and how impacts from 
the dust might affect the spacecraft, Jakosky said. The MPO is  coordinating 
the activities, he said, as that group is concerned about Mars Odyssey 
and MRO, in addition to Maven.

Minimizing the risk

"After Maven's launch, we'll be looking in detail at what mitigations 
we can take to minimize any risk," Jakosky said. "We'll look at things 
such as turning the least-vulnerable face into the flow of the dust, putting 
the solar panels edge-on to the flow and so on."

Jakosky said that, at this point, the best analysis indicates a minimal 
risk to Maven.

The number of dust impacts expected, and the effect they'll have on the 
spacecraft should be within the range of what Maven researchers anticipated 
for a normal mission run, Jakosky said. That is, the dust impacts should 
not exceed what researchers had already planned to absorb just from interplanetary 
dust over the lifetime of the mission, he said.

"I do expect that telescopic observations of the comet in the spring and 
more-detailed modeling of the dust environment in the coma and tail will 
help us to refine our analysis," Jakosky said.