The European Space Agency and the Russian Federal Space Agency, Roscosmos, won't be sending a rover to Mars until 2020, both agencies announced today. The rover's launch consists of the second phase of the ExoMars mission, which is intended to determine if Mars contains — or has ever contained — alien life. But technical challenges have caused the European collaboration to delay the rover's voyage to Mars — a journey that was supposed to take place in 2018.

Back in March, the ESA and Roscosmos commenced phase one of the ExoMars mission by sending two robotic spacecraft on a seven-month voyage to Mars. These spacecraft are the Trace Gas Orbiter and the Schiaparelli EDM Lander and they're scheduled to arrive at Mars in October of this year. Once there, the Trace Gas Orbiter will circle the Red Planet and measure the types of gases in the atmosphere, while the Schiaparelli lander will try to land on the Martian surface.

The rover will dig up samples of dirt on Mars

The second phase of the ExoMars mission involves sending a Russian-led surface platform and a European-led rover to the Red Planet. The rover is designed to explore the planet and dig up samples of dirt to look for signs of life. But now it looks like that won't happen for another four years. "Taking into account the delays in European and Russian industrial activities and deliveries of the scientific payload, a launch in 2020 would be the best solution," the agencies said in joint statement.

The agencies didn't go into detail about the technical reasons for the delay. But it's likely that the delay's length — two years — is at least partially the product of the planets' "behavior." To keep the cost down, missions to Mars tend to take place during a short window of time when the orbits of both Earth and Mars are close enough to each other. After 2018, the next window is 2020.

Phase one of the ExoMars mission launches to the Red Planet