Assuming all goes to plan, the craft will arrive at the Red Planet on 19 October. It carries a small lander named Schiaparelli that will touch down on the surface, giving the European Space Agency and Roscosmos much-needed landing practice for a future ExoMars rover, due to launch in 2018.

Neither agency has a great track record when it comes to Mars. The only ESA mission, Mars Express, successfully put a spacecraft in orbit in 2003. But Beagle 2, the British-built lander it was carrying, failed to phone home, though it was recently rediscovered on the surface. Russia has fared even worse, losing its 2011 Phobos-Grunt mission to a botched launch. The Soviet Union attempted more than a dozen Mars missions, but none was a complete success.

As such, NASA has dominated Martian exploration so far. Its most recent probe, MAVEN, is currently analysing the planet’s upper atmosphere, but ExoMars 2016 project scientist Håkan Svedhem says TGO will give us a new perspective nearer the surface. “It has instruments that are much, much more sensitive than in the past,” he says, it can detect molecules at a level of parts per trillion.

Seeking methane

Connecting gases in the air to their origin on the surface is key to figuring out whether Mars is as dead as it seems, and TGO’s top priority is methane. The gas breaks down in sunlight after a few hundred years, meaning any found on Mars must have been produced recently, either by active volcanoes or gas-belching microbes. “If there is methane, it needs to be supplied continuously from somewhere,” says Svedhem.

Previous sightings of methane by NASA’s Curiosity rover, along with other orbiters and telescopes, have proved confusing, as the gas seems to be more short-lived than expected. To get to the bottom of this mystery, TGO is equipped with two suites of spectrometers designed to sniff out the planet’s atmospheric gases down to tiny amounts, a camera to photograph potential ground sources, and neutron detector to map water ice down to a metre below the surface.

Those tiny amounts – the trace gases that give TGO its name – will crack the planet’s methane secrets. If the methane is accompanied by a whiff of sulphur dioxide, and traced to geological features on the surface, active volcanism is the most likely cause. Methane laced with higher levels of the isotope carbon-12, which is preferred by life on Earth, would point to a biological origin – though we would still be far from confirming that there is life on Mars.

“Detecting methane by itself doesn’t tell you whether it is produced biologically or geologically – you need to look at the whole suite of atmospheric behaviour,” says Bruce Jakosky of the University of Colorado, Boulder, who leads the MAVEN mission. “That would be considered the home run for TGO, to define the source of methane.”

Even geological methane could get astrobiologists humming. “It would suggest there are areas of heat and chemical exchange,” says Nicholas Heavens of Hampton University in Virginia. Both are ingredients for potential life.

Take an aerobrake

Whatever TGO finds, don’t expect results straight away. Although the spacecraft will likely snap some pictures and make a few measurements to confirm everything is still working after its interplanetary trip, the mission’s actual science phase won’t start until the end of 2017.

That’s because the probe reaches Mars travelling thousands of kilometres per hour, and has to spend around a year slowing down by gently grazing the atmosphere, a process called aerobraking. ESA had a practice run in 2014 with its Venus Express craft, just before that mission came to an end. “We’ve got quite good experience now from doing it with Venus Express,” says Svedhem.

The Trace Gas Orbiter (top left) with the ExoMars rover (bottom right) / ESA/ATG medialab

This delay means the 2018 ExoMars rover, which is due to touch down in 2019 in a region called Oxia Planum, won’t be able to directly follow up on TGO’s discoveries. Like Curiosity, the rover is bristling with instruments, including a drill capable of looking for signs of life down to two metres below the surface.

“Unless something very dramatic happens, we don’t expect the findings from TGO will affect the decision on the landing site for the 2018 mission,” says Svedhem. “But it will surely do that for later missions to come.”

Instead, the Schiaparelli lander strapped to TGO will act as a dry run for the rover. The lander is battery-powered, so will only last a few days on the surface, but is equipped with weather sensors and a camera to get a snapshot of the Martian environment. Its more important job is testing out the rough road to the surface.

Human visitors

Schiaparelli will break away from the orbiter on 16 October, a few days before it arrives at Mars, and enter the atmosphere at 21,000 kilometres per hour. In a little under 6 minutes, a heat shield, parachute and thrusters will slow the probe to walking speed. The same technology writ large should ensure a safe landing for the future rover.

That’s not to say TGO’s data won’t assist the ExoMars rover once it arrives, or indeed NASA’s planned 2020 sequel to Curiosity. “If there are genuine sources of interesting gases discovered, it will be down to those rovers to hopefully get near them, or characterise environments that are potentially like them,” says Heavens.

If this gang of explorers does turn up hints of biological activity, we’ll have some important decisions to make over the future of Mars exploration. It seems likely we will set foot on Martian soil in the coming decades: NASA has loose plans to send humans there by the 2030s, and other agencies and private organisations are vaguely promising to go. The potential for contamination of native life might prompt a rethink, says Heavens. “There will have to be a different set of thoughts about whether we should actually send crewed missions to Mars.”

Or it could drive us forward. “I think it would become a strong motivator to bring samples back to the Earth or send humans,” says Jakosky. “No contamination is total, we study microorganisms here on Earth even though we’ve contaminated it with people.”

First, ExoMars must actually get off the ground. The mission has a 12-day launch window should weather or another issue scupper efforts next Monday, but Svedhem is feeling positive they won’t need it. “We’re quite confident that it will work out,” he says. “Everything looks really very good now, everything is ready now and I think we will really go on the 14^th.”

More on these topics: Mars solar system space exploration