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It’s a busy February for Mars, with three probes from three different countries arriving on the Red Planet in just nine days. But this Martian festival did not happen by accident – it has to do with the mechanics of the orbits of Earth and Mars.
The United Arab Emirates’ first interplanetary mission, the Hope spacecraft, reached Mars orbit on Tuesday (February 9), as Live Science Space.com sister site report. China’s first interplanetary mission, Tianwen-1, is expected to enter its own Martian orbit on Wednesday (February 10). The Chinese probe includes both an orbiter and a lander with a rover on board, which is should attempt to land on the surface in May. And on February 18, NASA’s first descent vehicle will reach Mars and dive straight 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 lower the rhino-sized, nuclear-powered Perseverance rover worth $ 2.7 billion, using a celestial crane.
It’s no coincidence that all of these robots appearing at almost exactly the same time, said Jonathan McDowell, an astrophysicist and space flight specialist at Harvard University.
March and Earth are like “runners on a circular track,” he said. “And the really fast runner [Earth] regularly runs the runner just outside [Mars]. So sometimes they’re right next to each other, and sometimes they’re on opposite sides of the track. This Earth-Mars cycle, which means the Earth completely overlaps Mars, takes about two years.
It would take a huge rocket, tons of fuel and a lot more time to reach Mars from Earth when the planets are far apart, McDowell told Live Science. But launching while the planets are at their absolute closest – when they are an average of 62.1 million kilometers apart – is not the most efficient way to get to Mars either.
There is an earlier point in the two-year cycle of the planets where travel 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 continues to move faster than its neighbor. This positioning allows the spacecraft to enter a so-called “Hohmann transfer orbit,” named after the German engineer Walter Hohmann, who developed the underlying mathematics in 1925.
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Here is how it works:
No rocket carries enough fuel to burn all the way from Earth to Mars, a distance that varies from tens to 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 coasting trade. The job of the rocket motors during this initial period of acceleration is to place the spacecraft into an orbit around the sun that will come into contact with 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 the least expenditure of fuel, and this orbit becomes available once every two years.
But space agencies don’t have to nail that day exactly. As long as they launch for a window of a few weeks around the date, they can place their spacecraft in Hohmann transfer orbits. Wait more than a few weeks, however, and the journey starts to get much more difficult very quickly.
The Hope Orbiter launched on July 19, 2020, Tianwen-1 on July 23, and Perseverance on July 30. The discrepancies between spacecraft arrivals do not exactly match their launch dates due to minor differences in their rocket technology, space trajectories and destinations, McDowell said. (For example, it takes a different angle of approach to dive directly 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, McDowell said. 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 upon arrival. Two Soviet spacecraft and one American spacecraft were launched to Mars in 1971, and all have had at least partially successful missions. (Both countries planned additional probes that year, but the U.S. Mariner 8 probe failed at launch and the Soviet Kosmos 419 never escaped low Earth orbit.)
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What’s different this year, McDowell said, is the great diversity of spacecraft reaching Mars and the fact that several additional probes are already active around the planet. NASA has three active orbiters in Martian orbit, the European Space Agency (ESA) has one of its own and an orbiter which is a joint project with Russia’s Roscosmos, and 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.
Despite this relatively crowded situation, McDowell said he doubts any of the probes are even tens of thousands of miles apart, even though neither country has verified its paths. with others in advance.
“Space is big,” he says.
Originally posted on Live Science.
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