These explorers, which launched in July last year, will be heralds of a busy year of space exploration, launches and astronomical occurrences.

What follows is a preview of some of 2021's most notable expected events. Private companies and the world's space agencies are likely to announce more. The Times's Space and Astronomy Calendar will help you keep up with these dates, and you can subscribe to it here.

Martian meetup

The United Arab Emirates, China and the United States all launched robotic missions to Mars last summer, seeking shortened voyages during the period every two years when Earth is closest to the red planet. The three spacecraft will join a bustling community of explorers either in orbit or on the planet's surface.

The first to arrive will be the Emirati Hope orbiter, the first deep space explorer of the Arab country's small but ambitious space program. The mission will study the Martian atmosphere, sending valuable data back to scientists on Earth. It is to reach its destination on Feb. 9.

China's Tianwen-1 mission is also to arrive at Mars on Feb. 10. After orbiting the planet for a time, it will send a lander containing a rover to the surface in May. Landing intact on the solar system's fourth planet is perilous, and only NASA has done it more than once. A successful landing there would extend China's record of impressive spaceflight achievements, especially after last year's Chang'e-5 mission to the lunar surface then back to Earth with a fresh cache of moon rocks.