When Italian astronaut Samantha Cristoforetti returned to Earth, she was in no doubt about what she had been missing the most - a big salad with a lot of fresh tomatoes.

It's a common craving for anyone living for months aboard a space station. Fresh lettuce doesn't stay fresh for long.

The trip to Mars would be predicted to last almost two-and-a-half years, which is a long time to go without your greens.

Fresh foods like tomatoes, blueberries and red lettuce are a good source of antioxidants. Not only might they have a positive impact on morale, they could also provide some protection against radiation in space. As a result, NASA has been keen to work out the best ways to grow crops in space.

In fact, while Cristoforetti was on board the International Space Station fresh lettuce had already been grown aboard, but crew still weren't allowed to eat it.

It was only after the Italian returned to Earth - after food safety tests were carried out on the first plants - that NASA relented and the crew finally tucked into some red romaine space lettuce, complete with dressing.

Lettuce seeds had been sent to the space station more than a year earlier, along with a collapsible growing unit known as "Veggie."

The unit includes a flat panel of red and blue LEDs as well as weaker green ones. Plants only really absorb and use the red and blue parts of the visual spectrum - which is why they tend to be green.

The inclusion of green LEDs simply helps make the plants look attractive, while rooting "pillows," which contain the seeds, and "wicks," which help direct plant growth, were used to counter the problem of growing in a microgravity environment.

But what crops would you grow? With thousands of plant varieties available, it's a question NASA's trying to answer with the help of a valuable human resource - schoolchildren.

"We don't have time to check out every leafy green, every plant, we just don't," said Trent Smith, who is project manager for Veggie. "There are thousands of plant varieties and we can't check all of them ourselves, so that's where these students are giving us a lot of help."

Around 115 schools in and around the US city of Miami were given growing stations similar to Veggie, with similar lights and a fertilizer mix that mimics the one that would be supplied in space. Schools were then asked to share their results via social media, with the Twitter handle @GrowBeyondEarth, as part of the ongoing project.

NASA is also looking at how to grow food when man arrives on the Red Planet, with US President Barack Obama last weekreiterating his support for the US space agency's journey to Mars.

"We have set a clear goal vital to the next chapter of America's story in space: sending humans to Mars by the 2030s and returning them safely to Earth, with the ultimate ambition to one day remain there for an extended time," he said in a column written for international newscaster CNN.

When astronaut Tom Watney found himself stuck on Mars in sci-fi movie 'The Martian,' he was faced with the tough task of growing enough food to last until he was rescued.

The solution Matt Damon's character came up with was ingenious, yet simple. It mainly involved mixing potatoes, Martian soil, and human waste.

While it appears plausible on a small scale, growing food to any meaningful level would - in real life - present a wealth of questions, the "soil" being one of them.

"Soil, by definition, contains organics; it has held plant life, insects, worms. Mars doesn't really have soil," said Ralph Fritsche, the senior project manager for food production at Kennedy Space Center.

Mars is covered not with soil, but with "regolith" - crushed volcanic rock with toxic chemicals and no organic components. The space center chose soil from Hawaii to simulate Martian soil, based on the spectral data from Mars showing what the rock and dust there was made from.

They grew lettuce plants in three conditions: one in simulant, one in simulant with added nutrients, and one in normal terrestrial potting soil. The "Martian-grown" lettuce tasted similar to normally grown varieties, but had weaker roots and seeds took longer to germinate. The tests have only just begun, and will take about nine months, with plans to grow radishes, kale, snow peas, dwarf peppers and tomatoes.

And what about those toxins? Heavy metals such as lead, cadmium and copper. A study by scientists at the Netherlands' Wageningen University - which is also looking at growing food on the moon - found that edible plants could be grown in simulated Martian "soil," despite high levels of metals.

"For radish, pea, rye and tomato we did a preliminary analysis and the results are very promising," said ecologist Wieger Wamelink. "We can eat them and I am very curious as to how the tomatoes will taste." 

The team even organized a meal for sponsors of the research, using vegetables grown in Mars and moon soil simulants.

They also plan to look at the vitamin content and the amount of flavonoids, which play a big role in determining the taste, as well as alkaloids, which may be poisonous. But the crucial test, as to whether plants take up toxins in the altered conditions of Mars, will have to wait.

"It is still unknown if the take up of heavy metals is the same on Earth as it would be under the lower gravity conditions found on Mars and the moon," said Wamelink.

"It is likely that only research 'on site' will solve this question."