Comet Siding Spring's close flyby of Mars last month dumped several tons of primordial dust into the thin martian atmosphere, likely creating a brief but spectacular meteor shower with thousands of shooting stars per hour had any astronauts been there to see it, scientists said Friday.
The comet dust also posed a much more serious threat than expected to an international fleet of spacecraft in orbit around the red planet and roving about its surface. While engineers did not think the comet posed a major hazard, the orbiters were maneuvered to put them on the far side of Mars during close approach. Just in case.
As it turned out, that was a smart decision.
"After observing the effects on Mars and how the comet dust slammed into the upper atmosphere, it makes me very happy that we decided to put our spacecraft on the other side of Mars at the peak of the dust tail passage and out of harm's way," Jim Green, director of planetary science at NASA headquarters, told reporters during a teleconference. "I really believe that hiding them like that really saved them, and it gave us a fabulous opportunity to make these observations."
Siding Spring, more formally known as Comet C/2013 A1, originated in the Oort Cloud, a vast realm of icy relics left over from the birth of the solar system 4.6 billion years ago that extends from beyond the orbit of Pluto halfway to the nearest star. It was Siding Spring's first trip into the inner solar system, a journey that began a million or more years ago when the gravity of a passing star, perhaps, nudged it onto a different trajectory.
On Oct. 19, the comet passed within about 87,000 miles of Mars at a relative velocity of some 35 miles per second, or 126,000 miles per hour. Had the comet flown by Earth at that distance, it would have been just a third of the way to the moon.
"We believe this type of event occurs once every eight million years or so," Green said. "So it is indeed a rare opportunity for us to observe this."
Three NASA orbiters -- the Mars Reconnaissance Orbiter, the Mars Odyssey and the newly arrived MAVEN -- along with the European Space Agency's Mars Express and India's Mars Orbiter Mission all trained their cameras and instruments on the comet or the martian atmosphere to study the possible effects of Siding Spring's passage.
MAVEN, an acronym for Mars Atmosphere and Volatile Evolution, was built to study the martian atmosphere. Its Imaging Ultraviolet Spectrograph instrument detected major changes as dust from the comet slammed into atoms and molecules in the upper atmosphere, high-energy collisions that caused the thin air to glow. The spacecraft's Neutral Gas and Ion Mass Spectrometer detected clear signs of eight ionized metals -- sodium, magnesium, potassium, chromium, manganese, iron, nickel and zinc -- that spiked immediately following the comet's flyby and then faded away.
The Mars Reconnaissance Orbiter trained its high-resolution camera on Siding Spring and while the instrument was not able to resolve the nucleus, brightness variations indicated the comet was rotating once every eight hours.
By analyzing photos taken at different distances, and given the comet's trajectory and sun's illumination, Siding Spring could be larger than a mile across or just a few hundred yards.
"So we have an exciting time ahead to untangle that," said Alan Delamere, a co-investigator for MRO's camera.
Another instrument on the Mars Reconnaissance Orbiter uses a 130-foot-long antenna to probe the subsurface of the planet with radar. The surface appears sharply defined in images taken just before the comet's passage. But images in the immediate aftermath are slightly blurred, the result of atmospheric ionization caused by comet dust dust that affected the radar beam as it passed through the atmosphere to the ground and bounced back to the orbiter.
The amount of dust and its effects on the atmosphere were a surprise. Green said initial modeling indicated Mars would just skirt the edge of Siding Spring's dust tail. More recent photos taken by the Mars Reconnaissance Orbiter, however, showed the comet's trajectory was slightly different than expected. And the dust tail was larger than initially believed.
"The analysis seemed to indicate Mars would miss the dust tail in a significant way," Green said. "In other words, as the comet flies by the dust tail is following the trajectory.... it still would not have reached Mars to any significant amount. The surprise was indeed the dust tail seemed to be larger. The other surprise, the comet wasn't quite in the same position we thought it was."
Most of the particles were very small, tiny fractions of an inch across. But given their extreme velocity, they had a noticeable effect.
"With the amount of dust that came in, it's very possible that these are not just micron size, but they can be quite large, perhaps up to a centimeter size," Green said. "And anything that is of any size could easily destroy a spacecraft given it's high velocity and hitting in the right location. So, we were speculating the spacecraft would survive (in the dust tail's path), but I think it's pretty obvious they wouldn't have based on the tremendous response of Mars' atmosphere to the comet tail."
By measuring the glow of magnesium ions -- material from the comet that had electrons stripped away in high-energy collisions with particles in the atmosphere -- scientists could make a rough estimate of how much dust must have been deposited as the planet encountered Siding Spring's tail.
"And the answer we're coming up with is a few tons," said Nick Schneider, a leader of the Imaging Ultraviolet Spectrograph team at the University of Colorado at Boulder. "Based on this mass, we can make a rough estimate of what the meteor shower would have looked like, and it's looking like that meteor shower must have had thousands of shooting stars an hour, possibly what's called a meteor storm, although we're still working on the numbers.
"Numbers aside, it must have been a spectacular meteor shower on Mars," he said.
Meteor storms with more than 1,000 per shooting stars per hour are rare in Earth's sky and whether Siding Spring's display at Mars was a storm or a very intense shower, "I don't think anybody on the phone has ever seen that," Schneider said. "It's extremely rare in human history."
Asked what an astronaut on the surface might have seen, he said "it would have been truly stunning to the human eye."
"Now, we've got all these high tech robots around, but I have to say it might be the most sensitive science instrument of all having a human lying outside with dark-adapted vision looking up at that sky and to see many shooting stars happening at once," he said. "I think it would have been really mind blowing."