The transfer window ensures spacecraft use the least amount of fuel needed, as well as shortens the trip.
NASA said: “Just like you have to wait for Earth and Mars to be in the proper position before you head to Mars, you also have to make sure that they are in the proper position before you head home.
“That means you will have to spend three to four months at Mars before you can begin your return trip.
“All in all, your trip to Mars would take about 21 months: nine months to get there, three months there, and nine months to get back.
“With our current rocket technology, there is no way around this.”
source: express.co.uk