Robert Frost, a NASA employee, has shut down a conspiracy theorist on the popular question-answer site Quora. He perfectly explained how NASA sends probes and rovers to Mars. I wanted to share it here because his answer is so informative and enlightening.
Here’s how NASA sends probes and rovers to Mars
The question on Quora was: “The distance to Mars is 115.71 million miles. Therefore, to reach Mars in 9 months, you would have to travel at approximately Mach 46. Is this what NASA did, or is travel to Mars actually impossible?”
Answered by Robert Frost (see notes 1), Instructor and Flight Controller at NASA
There’s so much wrong with that question that it will take a minute or so to parse it.
First, you have indeed found the average distance to Mars (115.71 million miles or 225 million kilometers), but since Mars and Earth are both orbiting the Sun, the distance between the two planets is constantly changing. At their closest, they are 34 million miles (54.6 million kilometers) apart and at their farthest, they are 249 million miles (401 million kilometers) apart. We’ll get back to that.
Second, I have no idea how you came to the conclusion that the spacecraft would have to travel at Mach 46. I can’t see any math that would produce that result. We’ll get back to that. Also, the term Mach isn’t relevant here, since Mach is a measurement of the ratio of the flow velocity to the local speed of sound. Spacecraft travel in the vacuum of space.
The Mars Science Laboratory mission (the one that carried Curiosity to Mars) left Earth orbit at 22,500 miles per hour (36,210 kilometers per hour) – with respect to Earth. It traveled a total distance of 352 million miles in 253 days. That’s an average speed of 57,971 miles per hour (93,295 km/hr). That is faster than the rocket left orbit because that rocket also had the speed of the Earth around the Sun as part of its total velocity.
Third, you know damn well that NASA, the European Space Agency, the Soviets/Russians, and most recently the Indian Space and Research Organization, have sent robotic spacecraft to Mars and NASA has put rovers and landers on the surface. It is obviously not impossible. Accusing those nations and the people that worked on those programs of fraud is definitely a violation of Quora’s Be Nice Be Respectful (BNBR) policy.
With that handled, let’s talk about the actual journey to Mars.
There are different types of Earth-Mars mission trajectories. They don’t all start when Mars and Earth are close. There are multiple factors involved, including whether or not the spacecraft is to come home, whether a gravity assist from Venus is available, and the capabilities of the launch vehicle. However, for the typical one-way mission to get a probe or rover to Mars, we do indeed launch when Mars and Earth are fairly close.
Mars and Earth are at their closest to each other when Mars is at opposition. However, we don’t actually want to launch at this point. We want to launch before this point.
We want to use a minimum energy transfer orbit in order to use the least amount of fuel. A Hohmann transfer orbit does this (see note 2 below this post). Our spacecraft starts at Earth’s orbit. A Hohmann transfer orbit uses a burn at the starting point (periapsis) that increases the aphelion of the orbit such that it occurs at the orbit of Mars. This will be 180 degrees later in the orbit.
So, our goal is to time the launch such that Mars will be at that same location when the spacecraft gets there. Since Mars is in a larger orbit, it takes longer to move the same angular distance as the Earth. That means we need Mars to be ahead of Earth when we launch our spacecraft.
We calculate the period of the orbit that our spacecraft will be in. That turns out to be about 520 days. Our spacecraft is traveling in half of an orbit, so our trip will be about 260 days. Mars has an orbital period of 687 days. In 260 days, Mars will travel an angular distance of 136 degrees. That means the optimal time to launch the spacecraft is when Mars is 44 degrees (180-136) ahead of Earth in its orbit, as shown below. That means we launch the spacecraft about three months before Mars and Earth are at their closest.
Note: Earth, Mars, and the Spacecraft are orbiting counterclockwise in the above diagram.
Editor’s note: the video below, published by NASA MAVEN Mission to Mars channel, perfectly explains this.
Here is a table showing the numbers for a sample of Mars missions:
Journey to Mars: Curiosity Rover
And here’s an oldie but goldie – a NASA animation published before the Curiosity rover has been sent to Mars – showing how the journey would be.
1. Robert Frost
Robert Frost works in NASA’s Flight Operations Directorate. He’s trained astronauts on the guidance, navigation & control of the International Space Station and is currently a flight controller for visiting vehicles. In addition to space exploration and instructional design, he likes to write about movies, comic books, and history. He has a weekly Quora blog: The Greatest Films
2. Hohmann transfer orbit
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits of different radii around a central body in the same plane. The Hohmann transfer often uses the lowest possible amount of propellant in traveling between these orbits, but bi-elliptic transfers can beat it in some cases.
The orbital maneuver to perform the Hohmann transfer uses two engine impulses, one to move a spacecraft onto the transfer orbit and a second to move off it. This maneuver was named after Walter Hohmann (18 March 1880 – 11 March 1945), the German scientist and engineer who published a description of it in his 1925 book Die Erreichbarkeit der Himmelskörper (The Attainability of Celestial Bodies).
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