China, Emirates, and the United States launched three probes to Mars not long ago respectively. Have you ever wondered why multiple countries launched the Mars probes within a short period of time? Was it a coincidence or a deliberate arrangement?

In fact, July to August this year is the "Launch Window" to reach Mars. Any Probe launched at this period can arrive at Mars with the highest efficiency through the "Hohmann Transfer Orbit". Firstly, the probe orbits around the Sun along with the Earth in a nearly circular orbit. As it launched, the probe accelerates and enters an elliptical orbit, whose aphelion (the position of the orbit farthest from the Sun) is at the distance of Mars' orbit. If a suitable launch window is chosen, the probe will meet Mars when it arrives at the aphelion of its elliptical orbit. At this time, the probe will accelerate again to transfer its elliptical orbit into the orbit of Mars. It can then reach the destination by being captured by the planet's gravity.

This kind of transfer obit is not the fastest or shortest, but it is very fuel-efficient. Hence, it is utilised in missions like planet exploration and satellite orbit transfer. The launch window to Mars opens once per about two years, various countries would grasp this opportunity to go to Mars together.