Rendezvous at the red planet

The Mars space probes prepare for the fly-by of the Siding Spring comet

October 08, 2014

Mars can expect a visitor: on 19 October of this year the Siding Spring comet will hurtle past the red planet at very close range. Although a collision is out of the question, the tail will very probably penetrate into the planet’s atmosphere. This presents the space probes and rovers, which are currently exploring Mars, with a unique opportunity to observe this rare spectacle from very close up. The ASPERA-3 particle detector, for example, which is investigating the atmosphere from on board ESA’s Mars Express space probe and includes the participation of the Max Planck Institute for Solar System Research, will attempt to identify components of the comet’s tail in the hours after the fly-by.

Comet in sight: The Siding Spring comet, photographed by the Hubble space telescope in spring 2014.


When the comet with the scientific designation C/2013 A1 appeared in the sights of a telescope at the Australian Siding Spring observatory on 3 January 2013, it was already heading for Mars. Even a collision with the red planet seemed initially possible. Precise observations over the months that followed brought the all-clear signal: although there would be no collision, the comet would fly past Mars at a distance of a mere 132,000 kilometres; this corresponds roughly to the separation between the Earth and the Moon.

And the cosmic guest could not have chosen a more favourable flight path – from a scientific point of view at least. No other planet is monitored by as many on-site measuring instruments: five space probes are currently studying Mars from orbit, two rovers are making their way over its surface. During the fly-by, all these instruments will direct their gaze towards this unusual research object and record every detail of the unique rendezvous.

The same applies to the ASPERA-3 particle detector on board ESA’s Mars Express space probe. The spectrometer – like the space probe itself – is one of the older instruments: it has been measuring charged and neutral particles in the atmosphere and environment of our neighbouring planet since 2003.

Unlike, for example, the situation with MAVEN, NASA’s new space probe, for example, which reached Mars only a few weeks ago, the safety measures planned to protect this space probe against the cometary dust that is rapidly approaching are not quite so strict. “ASPERA-3 will be active during the entire fly-by,” says Markus Fränz from the Max Planck Institute for Solar System Research.

“We expect that the comet’s tail and Mars’ atmosphere will mix to some extent,” adds Fränz. The scientists estimate that Siding Spring will input around 100 kilograms of foreign material per second into Mars’ atmosphere. This corresponds to the quantity of particles that Mars is continuously losing into space. “These low particle densities make the measurements extremely challenging,” emphasises Fränz.

Close encounter: On 19 October, the Siding Spring comet will fly past Mars at a distance of only 132,000 kilometres.


The researchers nevertheless hope to be able to observe how the composition of Mars’ atmosphere changes as a result of the hurtling past of the guest. In addition to other ions and molecules, ASPERA-3 is expected to detect water molecules, for example, which are not normally present in Mars’ upper atmosphere. “Deciphering the composition of Siding Spring’s tail in this way provides us with a further piece of the puzzle, so that we can understand how our solar system formed,” Markus Fränz explains.

The long-period comets, which include Siding Spring, originate from the so-called Oort cloud, a region in deep space way beyond the orbit of Neptune. Far removed from the Sun, this part of the universe has changed very little since the planetary system was born. Which substances are present there and in which proportions? Which processes formed the comets and other frozen chunks?

“Each comet is slightly different,” Fränz believes. “By accurately examining as many as possible, we can obtain a better and better idea of where they were formed.” When it reaches Mars, Siding Spring could solve part of this mystery.


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