It’s 2pm on Mars, time for 17-year-old Leicester schoolgirl Millie Zedan and Nomathemba Kontyo, 15, from South Africa, to gather with the rest of the US Mars mission team for their daily briefing in Nasa’s Jet Propulsion Laboratory.

“JPL”, birthplace of the first unmanned Moon rocket, nestles in the foothills of the San Gabriel mountains on the northern fringe of Los Angeles. Outside it’s late afternoon. The foothills are swaddled in low cloud, with palm trees silhouetted against a glowering sky.

All this is a world away for the Mars team, though. Inside the $820 million (pound;450 million) mission’s nerve centre, daylight is blotted out and everyone keeps Martian time to synchronise with Nasa’s robotic rovers, Spirit and Opportunity, as they search for signs that our neighbour could once have supported life.

Millie and Nomathemba have been busy all afternoon processing images from the rovers traversing the rust-coloured dust millions of miles away. Next to them is a ledge bearing mascots from 14 other “student astronauts”, from as far afield as Poland, Sri Lanka and Brazil. The 14 all won places at the JPL in previous weeks in the Planetary Society’s “RedRover goes to Mars” international essay competition, which involved writing 1,500 words on how entrants would use one of the rovers to explore a previously visited area of Mars.

“Nasa eggheads,” as Millie calls them, are not the unapproachable stuffed shirts she imagined. “You have this preconception of scientists sitting in jackets, not wanting to talk. But everyone here is casual and relaxed; they walk around in sandals and tell lots of jokes.”

Millie is in Year 12 at Leicester high school, a girls’ independent, where she is taking AS-levels in maths, further maths, physics, ICT and general studies. She says she only entered the competition “to escape my chores”, and is particularly impressed that the team christened one Martian feature “Sleepy Hollow”, after the hit 1999 gothic movie starring Johnny Depp.

“They come up with funny names just to keep themselves amused.” In the same spirit, she is working references to Lord of the Rings star Orlando Bloom into her diary entries on the Planetary Society’s website.

The laid-back, collegiate atmosphere evokes a university campus, but the schedule is gruelling. The scientists work around the clock, although not quite 247, for Martian days - or “sols” - last almost 40 minutes longer than ours. When the crew attached to one rover knocks off, their opposite numbers, working with the other solar-powered craft (on the opposite side of Mars in a separate time zone) clock in for their 12-hour stint. The rovers run on solar power, so daily start times are staggered to coincide with the longer Martian day. While Millie and Nomathemba, assigned to the Opportunity team, might be on a cushy number today, working in the afternoon, later on they’ll be doing graveyard shifts like their counterparts in the Spirit command centre one floor below, due to burst into life in the small hours of tomorrow morning.

It’s not just another eccentricity, then, that many staff sport two watches, one running terrestrial time, the other altered by a local jeweller to tell Mars time.

The briefing runs through the commands Opportunity has executed that day.

Scientists cluster in groups beneath signs denoting specialities such as geology, atmospheric science and soil and rock physical properties.

The mission demands constant give and take between scientists over whose pet projects make it into “the sequence” of commands programmed into Spirit and Opportunity each day. The rovers’ daily power is limited, so the trade-off of pleasing the geologists by going in for a close-up of rock strata, for instance, might be that the mineralogists interested in the rock’s chemical breakdown and composition have to wait another day, explains the students’ minder, KJ Walsh, whose day job is training LA’s science teachers.

Millie, whose home-town University of Leicester spearheaded Britain’s ill-fated rival Beagle 2 Mars mission, denies any bittersweet feelings.

“Science is about trial and improvement - often you can learn more from errors than from things going smoothly,” says the budding astrophysicist.

Surrounded by ultra-powerful computers, sleek flat-panel displays and chic laptops, Nomathemba keeps having to pinch herself. “I never thought I’d come to the US,” she says. At home in Gugulethu, an impoverished township near Cape Town, there are 55 other students in her class at school, textbooks are shared and staff only recently took delivery of the school’s first computers thanks to a generous donation. In a measure of her visit’s importance back home, she was seen off by South Africa’s president, Thabo Mbeki.

As the meeting breaks up the students make a beeline for atmospheric scientist Michael D Smith, who has been plotting Martian temperatures on graphs unlike anything you might get from mapping the weather on Earth.

“It’s a thinner atmosphere, so heat doesn’t transmit as well,” he explains.

If you stood at Mars’s warmest point, its equator in mid-summer, your feet might be at a relatively toasty nought degrees centigrade, adds Dr Smith.

But your head would be at a distinctly frigid - 13C. Dr Smith is also monitoring dust in Mars’s atmosphere. “There was a dust storm in December and it takes a while to settle,” he explains. One of Millie and Nomathemba’s jobs has been to monitor dust build-up on the rovers’ solar panels, affecting their power supply and the mission’s ultimate longevity.

In a race against time before the rovers become inoperable under layers of dust and a waning late summer sun, the team tries not to waste a second.

And though teenagers’ circadian rhythms can be notoriously insistent, everyone’s coped with the gruelling schedule, Mr Walsh says. It’s been “an unbelievable experience and they have used every moment and held up very well”.

Millie Zedan was at JPL from February 20-29. Read her journal at: www.redrovergoestomars.org journalsindex.html