WHY WERE THERE SO MANY MISSIONS TO MARS LAUNCHED LAST SUMMER?

July 2020 was a particularly busy month for space agencies. Three missions were launched to the Red Planet: the Emirates Mars Mission, China’s Tianwen-1, and NASA’s Mars 2020 mission. (ESA’s ExoMars probe launch was postponed until 2022.) All three probes are arriving this February. But why were there so many leaving Earth at one time? The reason was the so-called ‘Hohmann transfer orbit’.

To journey from Earth to Mars, a spacecraft is launched into an elliptical orbit. To save fuel (and time), the best ‘launch window’ occurs when the path between the two planets is at a minimum. This happens around the time that Earth and Mars line up with the Sun. This is known as ‘opposition’, because Mars is then exactly opposite the Sun in the sky. This alignment happens every 26 months or so. To take advantage of this particular planetary alignment, the missions needed to start in July 2020 and end this February, minimising the distance each probe has to travel. Other factors, such as the capability of the launch vehicle, the mass of the spacecraft and the desired time of arrival at Mars also help determine the exact launch date.