Comet Siding Spring closely flew by Mars surface last month. As a result of which it dumped several tons of primordial dust into the thin Martian atmosphere creating a brief but a spectacular meteor shower with thousands of shooting stars per hour if by any chance astronauts were present there to witness it.

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. This was done to avoid any unprecedented event and it turned out to be a very wise decision in the end.

“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 originated during the birth of the solar system 4.6 billion years ago and come from far beyond the orbit of Pluto, halfway to the next nearest star. This was Siding Spring’s first journey to the inner solar system which began almost a million or so years ago.

The comet passed within 87,000 miles from the Mars surface at a relative velocity of about 35 miles per second on October 19.

“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 i.e. the Mars Reconnaissance Orbiter, the Mars Odyssey and the newly arrived Mars Atmosphere and Volatile Evolution (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.

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.

MAVEN’s 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 which are sodium, magnesium, potassium, chromium, manganese, iron, nickel and zinc that spiked immediately following the comet’s flyby and then faded away.

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. And it would have destroyed the spacecrafts within no time so it was a wise decision to keep the spacecrafts on the far side of Mars.