They are literally specks of dust, tiny bits of primordial material that wouldn't be visible to the naked eye.
But for spacecraft in orbit around Mars, they could become minuscule agents of destruction.
These dust particles will come hurtling past the Red Planet on Oct. 19, riding on the coattails of Comet Siding Spring. They'll blow by at an incredible 35 miles per second — 25 times faster than an armor-piercing projectile fired from a tank. And there could be millions of them.
At that velocity, they'll have the power to poke a hole in a spacecraft's gas line or crack a glass lens. They could knock out a computer board or take out a few cells on a solar panel.
And that's why scientists and engineers at NASA are nervous.
"They are essentially little cannonballs and bullets flying around, and they could do real damage," said Richard Zurek, chief scientist for the Mars Program Office at NASA's Jet Propulsion Laboratory.
The three vulnerable orbiters cost the agency more than $1.5 billion to build and deploy. Since they're millions of miles from Earth, patching them up if they get broken is not an option.
So scientists are going to play a high-stakes game of hide and seek.
The stream of dust particles will be quite diffuse. Comet modelers have calculated that only one particle will pass through any given square kilometer of space.
"The typical area of a spacecraft is five square meters, so it doesn't sound like much risk," Zurek said. "But these are chances we would rather not take."
C/2013 A1, as the comet is formally known, was first spotted by an observatory in Australia on Jan. 3, 2013. Back then, it was still pretty far away, out past the orbit of Jupiter.
Based on its trajectory, scientists believe it originated in the Oort cloud, an unseen collection of icy bodies at the edge of the solar system.
Long-period comets like Siding Spring are made up of the detritus from the earliest days of the solar system. They are frozen time capsules from the era of planet formation, which is why scientists are eager to study them.
Siding Spring's nucleus is thought to be about half a mile across and contains gas, water and dust that has been in a deep freeze for billions of years. As it flies closer to the sun, the increase in radiation causes the ice to sublimate, releasing ancient gas and dust that follow the comet in a long tail.
The NASA spacecraft orbiting Mars are considerably newer additions to the solar system.
Odyssey arrived in 2001 to study the surface composition of the planet and its subsurface ice, taking measurements to help scientists understand what kind of radiation exposure humans would face if they visited Mars.
The Mars Reconnaissance Orbiter joined Odyssey in 2006. It studies the history of water on the Red Planet.
A third satellite, known as MAVEN, will reach Mars in September. It was designed to help scientists understand how Mars lost its water and atmosphere to space.
Like most of the spacecraft NASA sends into space, these three are made primarily of lightweight materials like titanium and aluminum. That makes them cheaper to launch, but it also makes them sitting ducks for a speeding smidge of dust.
Within a few weeks of Siding Spring's discovery, researchers at JPL's Near-Earth Object Program determined that it would come very close to Mars — possibly slamming into the rocky planet.
A few months and several data points later, however, that possibility was ruled out. The NEO team determined that Siding Spring's nucleus would get as close as 80,000 miles from Mars, or 10 times closer than any known comet has come to Earth.
"This is a once-in-a-several-million-year event," said program manager Don Yeomans. "If it were coming this close to Earth, it would be a scientific bonanza, plus a global celestial display."
When Zurek first heard that a comet was heading toward Mars, he was thrilled. Scientists think that cometary collisions delivered water to ancient Earth, Mars and perhaps other planets, so they are eager for any chance to study them.
"As a scientist I thought this could be really exciting," he said. "If the comet hit the planet, you would see an explosion and all this debris would go shooting up into the atmosphere before raining back down to the surface."
But his enthusiasm quickly turned to dismay.
"I thought, 'Wait, we've got two rovers and two orbiters working away up there, plus another orbiter arriving in a few weeks,'" Zurek said. "Maybe this is a bad thing."
Zurek and his team realized the rovers, Opportunity and Curiosity, would be fine. The Martian atmosphere is less than 1% as thick as the atmosphere on Earth, but it would still be enough to burn up any incoming dust particles, creating a Martian meteor shower.
The orbiters would get no such protection. Though they would be a safe distance from Siding Spring's nucleus, they might share space with the cloud of tiny dust particles that make up the comet's tail.
Three separate teams of computer modelers were called in to figure out how fast the dust behind the comet is likely to travel, and how close it will get to the spacecraft.
All three models predict that the entire planet will spend a few hours engulfed in the outer layer of Siding Spring's coma, a cloud of gas that surrounds the nucleus. The bulk of the comet's dust particles will miss the planet. But beginning 80 minutes after the comet zips by, there will be a roughly half-hour window during which something catastrophic is possible. Remotely possible.
"There's a small probability of an impact, but it's not zero," Zurek said. "And it only takes one to do you in."
JPL staffers pondered various ways to keep the spacecraft out of harm's way. The idea of using large communication antennae to block incoming dust particles was considered. They also looked at rotating the spacecraft so that the side with the least sensitive instruments would be facing the dust stream.
"There were more exotic mitigations we thought about," said Soren Madsen, who spent five years as chief engineer for NASA's Mars Exploration Program. "But they were ultimately not worth implementing."
Instead, engineers settled on another conflict-avoidance strategy: hiding.
"Mars will actually act as a shield for us," Madsen said. "Right behind Mars there will be a hole in the dust cloud."
The plan, he said, is to steer the orbiters into that "safe zone" for 30 to 40 minutes, until the worst of the threat has passed.
Satellites are in constant motion, so they can't simply park behind Mars. But engineers on Earth can adjust the speed of their orbits so that they will be flying on the opposite side of the planet when the risk is greatest.
On Tuesday, the Odyssey orbiter will fire its engines for 5.5 seconds, giving it a gentle boost. It won't change the shape of its orbit, but it ensures it will be in the "safe zone" as the dust zips past.
The Mars Reconnaissance Orbiter performed a similar maneuver July 2 and has another planned for Aug. 27.
MAVEN already had a series of maneuvers planned for the weeks after it gets to Mars. At first, it will move around the planet in a long elliptical orbit, which will be trimmed down by a series of precisely timed engine firings. After Siding Spring's discovery, engineers tweaked those flight plans to make sure the spacecraft will be out of the way for the duration of the danger.
"These maneuvers are low-risk and low-cost," Madsen said.
It may sound like NASA scientists are being paranoid. But they're not the only ones. The European Space Agency had its Mars Express satellite burn its engines June 23.
Now that NASA officials are reasonably certain that the spacecraft will be safe, they can focus on the observations they will get to make during the unlikely encounter between planet and comet.
Using the instruments aboard the three orbiters, scientists plan to take a close look at the size and shape of Siding Spring's nucleus, and to figure out what gases are in its coma and its tail.
They also want to see how gas from the comet will interact with the Martian atmosphere. One hypothesis is that the comet will cause the gases in the upper atmosphere to heat up, causing them to escape into space. This could help researchers better understand why the air is so thin on Mars.
For Jared Espley, a co-investigator on the MAVEN mission, the cosmic coincidence is enough to make him giddy.
"This is an amazing, fantastic opportunity for science," he said. "It was a little unnerving at first, but now that we think we're safe, it's really exciting."
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