There are going to be plenty of technological and physiological hurdles to jump before we land the first astronauts on Mars. But once they're safely on their journey, another kind of challenge may rear it's head. What happens when you can no longer see Earth?

Even the quickest Mars trip is going to be long. If you get the timing right, as Dennis Tito plans to, you could manage a round trip to Mars in 501 days. Most missions, especially if they land on the surface and do a bit of exploring, will take longer.

Leaving aside the not-so-tiny medical problems with long-term spaceflight like muscle and bone wastage, eye problems and increased radiation levels -- keeping six people happy on a Mars mission is going to be hard. For example, sleep deprivation, as discovered by the Mars 500 simulation participants, can be a problem. Of course, Mars 500 also showed that we can do experiments here on Earth to work out how to overcome these problems.

One thing it's harder to test for is what happens to someone when they can no longer see their home planet.

Earthshine by Manu Gopinath from Manu Prasad Gopinath on Vimeo.

This bittersweet animation, by Manu Prasad Gopinath, captures some of the loneliness the first astronauts to land on Mars might feel at being so far away from home.

But the truth is we don't know what astronauts will experience when they finally lose sight of Earth. The moon is as far as anyone has ever been.

For reference, from the moon, the Earth looks like this:

From Mars, like this:

David Green is a lecturer in human and aerospace physiology at King's College London. I spoke to him earlier this year for an article about the Mars 500 simulation, but he's worked with spacefaring astronauts too. He told me:

“The crewmembers who’ve been to the moon, they all say how disconcerting it was as they got further and further away from Earth. These guys knew that they were going to an object that wasn’t that far away. If you're going to Mars, at some point the Earth is going to go to the size of a pin, and then you’ll lose sight. What that will do to somebody, who knows?”

It's a problem that's been considered before. In her book Packing for Mars, Mary Roach quotes the following passage from the book Space Psychology and Psychiatry:

"In the history of human beings, no one has ever been on the situation when Mother Earth, and all of her nurturing and comforting aspects… has been reduced to insignificance in the sky… It seems possible that it will induce some state of internal uncoupling from the Earth. Such a state might be associated with a broad range of individual maladaptive responses, including anxiety and depressive reactions, suicidal intention, or even psychotic symptoms such as hallucinations or delusions. In addition, a partial or complete loss of commitment to the usual (Earth-bound) system of values and behavioural norms may occur."

Roach calls it the "earth-out-of-view-phenomenon", and we can guess at what it might be like, or even whether it exists at all. (In Roach's book, cosmonaut Sergei Krikalyov dismisses the idea, saying: "Psychologists need to write papers".)

But we're really going to need to go to Mars (or some other faraway part of the solar system) to find out for sure.