During the month of February, the Martian orbit will welcome three new arrivals from Earth:the UAE's Hope space probe, which successfully completed its orbital insertion on Tuesday February 9; the space probe (equipped with a lander) Tianwen-1, which also successfully placed itself in orbit today (Wednesday 10 February); and the Perseverance rover, which is expected to arrive on the Red Planet within a week. But then, why did the space agencies of three countries (or group of countries) decide to launch their spacecraft at the same time?
It's a busy February for Mars, with three probes from three different countries arriving at the Red Planet in just nine days. But this Martian affluence is not random:it is a question of spatial mechanism. The United Arab Emirates' first interplanetary mission, the Hope probe, reached Mars orbit on Tuesday, February 9. China's first interplanetary mission, Tianwen-1, moved into its own Mars orbit today.
The Chinese probe includes both an orbiter and a lander with a rover on board, which is expected to attempt to land on the surface in May. And on February 18, NASA's first descent vehicle will reach Mars and plunge directly into its atmosphere. If all goes as planned, the vehicle will lose its outer shell and use rockets to stop its descent at the last moment. Then it will hover above the surface to drop off the $2.7 billion nuclear-powered Perseverance rover.
All of these spacecraft appearing at almost exactly the same time is no coincidence, as Jonathan McDowell, an astrophysicist and spaceflight scientist at Harvard University, explains. Mars and Earth are like “runners on a circular track. And the fastest runner (Earth) regularly finds itself level with the outer runner (Mars). So they're here right next to each other, and sometimes they're on opposite sides of the track ". This Earth-Mars cycle takes about two years.
It would take a huge rocket, tons of fuel and much longer to reach Mars from Earth when the planets are farthest apart. But launching a mission when the planets are at their closest — when they're 62.1 million miles apart on average — isn't the most efficient way to get to Mars, either.
There is a point upstream in the two-year cycle of the planets where the journey takes less time and requires less fuel. At this point, which occurs once in the two-year cycle, Earth is a little behind Mars, but still moving faster than its neighbor. This positioning allows a spacecraft to enter a "Hohmann transfer orbit", named after German engineer Walter Hohmann, who devised the underlying mathematics in 1925.
No rocket carries enough fuel to travel all the way between Earth and Mars, a distance that varies between tens and hundreds of millions of kilometers. This means that any interplanetary adventure begins with a brief and intense period of acceleration, followed by a long period of cruising. The job of the rocket engines during this initial period of acceleration is to put the spacecraft into an orbit around the Sun that will meet the orbit of Mars as soon as possible.
The most efficient path between the planets is therefore the solar orbit intersecting with Mars, which can be reached with less expenditure of fuel, and this orbit becomes available once every two years. But space agencies don't have to aim for that exact day. As long as they launch their craft within a window of a few weeks around the date, they can place their spacecraft in Hohmann transfer orbits.
The Hope orbiter was launched on July 19, 2020, Tianwen-1 on July 23, and Perseverance on July 30. Gaps between spacecraft arrivals do not exactly match their launch dates due to minor differences in their rocket technology, space trajectories and destinations. For example, it takes a different angle of approach to dive straight into the planet's atmosphere than to enter a high orbit like Hope did.
This is not the first time that Martian orbital space has been so crowded. The Soviet Union launched four spacecraft to Mars in 1973, but one failed to reach orbit and none of the other three performed as expected when it arrived. Two Soviet spacecraft and one American spacecraft launched to Mars in 1971, and all had at least partially successful missions. Both countries planned additional probes that year, but the American Mariner 8 probe failed during launch, and the Soviet Kosmos 419 never escaped low Earth orbit.
What is different this year is the wide variety of spacecraft reaching Mars and the fact that several additional probes are already active around the planet. NASA has three active orbiters in Mars orbit, the European Space Agency (ESA) also has its own and one orbiter, which is a joint project with the Russian agency Roscosmos. The Indian Space Research Organization also has an active orbiter. NASA's Curiosity rover and InSight lander are also still active on the Martian surface.
