Tomorrow, two spacecraft will reach Mars after nearly seven months of traveling together through space — and they’ll both attempt to pull off two separate and incredible feats. One will put itself into orbit around the Red Planet, while the other will land on the surface, hopefully in one piece.

It’s perhaps the biggest moment so far in the first phase of the ExoMars mission, a joint venture between the European Space Agency and the Russian Federal Space Agency, or Roscosmos. On Sunday, the two vehicles — the Trace Gas Orbiter (TGO) and the Schiaparelli lander — separated from one another in preparation for Wednesday’s events.

The main goal of ExoMars is to figure out if there is, or ever was, life on Mars. The TGO will try to answer this question from orbit, by sniffing out the gases in Mars’ atmosphere. It’ll be looking for specific compounds like methane, which on Earth is often produced when biological matter breaks down. Its presence around Mars could indicate life on the planet’s surface, as well. Researchers have long debated whether or not the gas exists around the Red Planet, and the ExoMars mission hopes to finally quell that dispute.

A rendering of the Trace Gas Orbiter as it puts itself into Mars orbit. (ESA)

As for Schiaparelli, its role is just to land intact. Its only mission is to demonstrate that ESA and Roscosmos have the necessary technology to safely land scientific hardware on Mars. That’s key for the next phase of the ExoMars mission, which involves landing a 680-pound rover on the Red Planet in 2021 to explore the surface and look for signs of biological life. The same technologies used to land Schiaparelli will be used to land the ExoMars rover as well.

Tomorrow, things get started a little after 9AM ET, when the TGO ignites its main engine to help slow the spacecraft down. The orbiter will be moving at more than 8,700 miles per hour, so it needs to put on the brakes just enough to get captured by Mars’ gravitational pull. To do this, the vehicle’s engine will burn for two and a half hours, putting the TGO into a highly elliptical orbit around Mars that takes four Martian days to complete. If for some reason the engine burn doesn’t work correctly, the orbiter will fly straight past Mars and its mission will be blown.

While the TGO’s engine continues to burn, Schiaparelli will move closer to Mars and start its descent around 10:42AM ET. The lander then has just six intense minutes to slow itself down enough so that it doesn’t crash into the planet’s surface. And that poses a big challenge, since Schiaparelli will enter the planet’s atmosphere at an altitude of about 75 miles, traveling at nearly 13,000 miles per hour.

But Schiaparelli is equipped to handle the descent. First, it will free fall for three and a half minutes, as the Martian atmosphere slows the spacecraft’s speed down to around 10,500 miles per hour. The lander has a heat shield to keep the vehicle from burning up while entering the atmosphere — a process that creates a lot of intense heating. At about six miles above the surface, Schiaparelli will deploy its parachute, which will help slow the vehicle down to about 155 miles per hour.

At less than a mile, Schiaparelli will ignite a series of onboard thrusters for about 30 seconds to slow down even more. This extra step is necessary because the air around Mars is about 1/100th the density of Earth’s atmosphere and the parachute alone won’t be enough. These tiny engines embedded in the hull of the spacecraft will slow Schiaparelli to 2.5 miles per hour. Eventually, they’ll turn off and the lander will fall the final few feet to the surface. Schiaparelli is equipped with a "crushable structure" designed to absorb the force of that final impact.

Throughout the entire landing, Schiaparelli will be sending back data to the TGO. ESA’s Mars Express probe, which is already in orbit around Mars, will also record the entire event, and an array of 30 radio telescopes in India will be picking up signals of the landing from Earth. Once the entire thing is over, other orbiters like NASA’s Mars Reconnaissance Orbiter will pick up additional data and photos taken by Schiaparelli. All this data will tell researchers how lander’s fall to Mars went.

A rendering of the Schiaparelli lander, post-landing. (ESA)

Once the landing is over, that’s about it for Schiaparelli, since it doesn’t have a way of generating its own power. Just a few days after landing, Schiaparelli will likely use up all of its onboard batteries to send data to the orbiters around Mars before it dies.

But the Trace Gas Orbiter’s mission is just getting started. The orbiter will spend the majority of next year slowing down and adjusting its orbit to get closer to Mars. It’s a process known as aerobraking, and it will bring TGO into a much more circular orbit about 250 miles above Mars’ surface. Then in March 2018, the orbiter will start its science observations — a phase that will last for two years.

ESA plans to live stream all of the big events from mission control tomorrow starting at 9AM ET. Tune back in to see if the ExoMars spacecraft pull everything off.

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