Why is everyone launching a Mars probe now?

Article by Naoya Ozaki (Spacecraft Applied Engineering Research Division, ISAS)

This month, three different Mars probes left for the red planet, including the UAE’s Hope Mars Mission and China’s Tianwen-1 Mars probe and rover, with the U.S. Perseverance rover to follow later this week. But why is everyone launching a Mars mission now? Has there been a sudden boom in Mars exploration?

Actually, this is the best time for efficient travel from Earth to Mars, allowing us to send a large probe with less fuel. This kind of timing comes about once every two years.

Let’s take a look at exactly why now is the right time.

After departing from the Earth, the gravity of the Sun pulls the spacecraft onto an elliptical orbit around the Sun. It is therefore most efficient to fly in the direction of the Earth’s orbital motion (Figure 1).  This is easy to understand if you imagine throwing a ball. You can throw the ball farther if you make the throw while running forward than if you throw from a stationary position (although of course, if you lose your form, you won’t be able to throw as far).  The same principal applies when we arrive at Mars. If you catch a ball from the front, you can make a gentler catch if you simultaneously move backward. Therefore, if the direction of Mars’s motion around the Sun is in the same direction as the velocity of the spacecraft, we can transfer to the orbit around Mars more efficiently.

This results in the best time to fly on an elliptical orbit between the Earth and Mars is when the Earth is on the perihelion of that ellipse (closest point to the Sun) at launch and Mars is on the aphelion side (farthest point from the Sun) at arrival (Figure 2).  This has been an intuitive explanation, but if you want to know more precise details, please look up the term “Hohmann transfer orbit”. In reality, Mars’s orbit is not perfectly circular (see Mars in Fig. 3), so the spacecraft does not intersect with Mars at exactly 180° as described by the Hohmann transfer orbit, but an orbit example for the MMX spacecraft in Figure 3 shows that the spacecraft still arrives at Mars close to the ideal 180° from the Earth’s departure.

Since the orbits of Earth and Mars cannot be changed, the timing of a launch to take advantage of the Hohmann transfer is fixed. This timing is right now (departing from Earth in July or August 2020), with the next opportunities for an efficient Mars launch being from August to September 2022, and beyond that from September to October 2024 (the timing of the planned launch of MMX!!).  You can see from that list that an efficient time for launch is occurring every two years and a few months. The time between the two planets reaching the same positional relationship is called the meeting period. The meeting period can be estimated with simple arithmetic (try it!). In the case of Earth and Mars, the meeting period is found to be 2 years and 1.6 months. In reality, the timing of the preferred launch will be within a range of several months due to the effect of the deviation of Mars from its circular orbit. Moreover, if you want to fly to Mars in the shortest period of time rather than the most efficient (for example, you want to arrive at Mars quickly for a crewed mission), the orbit will be different. MMX will be launched at the one after next most efficient launch timings.

Stay tuned for the launch to explore Phobos and Deimos in about four years!