After a 10-month-long, 442 million-mile journey to Mars, it's do or die time for NASA's Maven orbiter — with India's Mars Orbiter Mission following close behind.

Both orbiters are designed to study the Red Planet's upper atmosphere, and both are scheduled to execute crucial engine burns to go into Martian orbit. If the engines don't fire correctly, either or both of the probes could end up veering off into space, as Japan's Nozomi spacecraft did in 2003 — or crashing into the surface, as NASA's Mars Climate Orbiter did in 1999.

"I'm all on pins and needles," Jim Green, the director of the Planetary Science Division at NASA Headquarters, told reporters this week. "This is a critical event."

Maven's moment of truth comes at about 9:50 p.m. ET Sunday, when the orbiter is due to fire its engines for 33 minutes, 236 miles (380 kilometers) above the Martian north pole. The firing is designed to pull the bus-sized spacecraft into a wide-ranging elliptical orbit — which will be adjusted over the following six weeks.

NASA TV is airing coverage of the buildup to orbital insertion starting at 9:30 p.m. ET Sunday. To follow the big event via social media, look for the hashtags #MAVEN and #JourneytoMars. Twitter updates are being posted via @NASA, @MAVEN2Mars and @NASASocial.

The Mars Orbiter Mission spacecraft, also known as MOM or Mangalyaan (Hindi for "Mars-Craft"), blasted off from India's Satish Dhawan Space Center in November 2013, a few days before Maven was launched from Cape Canaveral Air Force Station in Florida. But it took a slightly more circuitous route to Mars — which means MOM's 24-minute orbital insertion burn is set for about 10 p.m. ET Tuesday.

"Confidence is high," V. Koteswara Rao, scientific secretary at the Indian Space Research Organization, told Reuters. "All the operations done so far are successful, and all the parameters measured are normal."

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What the orbiters will do

Maven takes its name from a slightly tortured acronym: Mars Atmosphere and Volatile EvolutioN. The $671 million mission's objective is to help scientists figure out how the Red Planet's environment changed from a warm, moist place into the chilly wasteland it is today.

The orbiter's nine sensors are designed to monitor solar radiation hitting the top of Mars' atmosphere, gauge the rate of atmospheric loss, and map the planet's localized, jumbled-up magnetic fields. The results will be factored into models to reconstruct how the space environment might have stripped away Mars' air over the course of billions of years.

There's at least one big question about Mars' atmosphere that Maven wasn't designed to address: What's the source of the methane that previous orbiters have detected? Are those trace amounts created through geological processes, or could they be coming from microbial activity deep in the Martian soil?

Fortunately, MOM's scientific instruments could shed light on the methane question, and the teams behind the two missions plan to share their results with each other. MOM will also monitor Mars' weather, take color pictures of the surface and map the planet's mineral composition.

Both orbiters will be in position when Comet Siding Spring passes within 82,000 miles (132,000 kilometers) of Mars on Oct. 19. At one time, scientists wondered whether the comet would actually hit the planet, but now mission managers say there's little risk of damage to any of the spacecraft at Mars.

Maven's principal investigator, Bruce Jakosky of the University of Colorado, says the spacecraft should be able to analyze how debris from the comet affects Mars' atmosphere. "I'm told the odds of having an approach that close to Mars are about one in a million years, so it's really luck that we get the opportunity here," Jakosky told reporters.

In addition to its scientific instruments, Maven is carrying a DVD encoded with 100,000 digitized names, 377 student artworks and more than 1,000 haiku poems, including this one: "Martian atmosphere / Awaiting discovery / What secrets lie here?"

What the missions mean

Maven is the latest addition to a fleet of NASA spacecraft working at Mars, including rovers (Curiosity and Opportunity) as well as orbiters (Mars Reconnaissance Orbiter and Mars Odyssey). The orbiters serve as vital communication links for the rovers, and Maven is designed to take over the relay duties in case MRO or Mars Odyssey fails.

Green emphasized that Maven was one link in a chain of NASA missions that would continue with its 2016 InSight lander, its 2020 rover and other probes. The space agency's decades-long robotic campaign is aimed at setting the stage for astronauts' trips to Mars in the 2030s or later.

"For humans to go to Mars, it's not like 'Star Trek.' It's not like 'go where no man has gone before,'" he said. "It's really the planetary scientists that are blazing the trail for us to understand everything about Mars that we need to for humans to be able to land safely on Mars, and explore and journey around the planet."

Scientists and engineers monitor India's Mars Orbiter Mission, also known as Mangalyaan, at the ISRO Telemetry, Tracking and Command Network (ISTRAC) in Bangalore. ISTRAC serves as mission control for India's Mangalyaan mission to Mars. Manjunath Kiran / AFP - Getty Images

India wants to get in on that action as well: The MOM mission costs $74 million — just one-ninth of Maven's budget — and it's meant to demonstrate that ISRO's low-cost approach to space exploration can achieve world-class results. A successful mission would heighten India's prestige in comparison with China and Japan.

"If successful, India would be the first Asian country to orbit a spacecraft around planet Mars, and the first country in the world to achieve it on the first attempt," ISRO Chairman K. Radhakrishnan said last month.

Sending astronauts into space is also on India's agenda. "A number of critical technologies are being developed for a possible human spaceflight in the future," Radhakrishnan said, with a key experiment due to be launched in the 2016-2017 time frame.