By Sabine Stanley Ph.D., John Hopkins University
When a spacecraft travels to Mars, it reaches the red planet after about six months. Once it arrives, it faces low gravity and a thin atmosphere, so thin that even if a fast wind is passing by it is hardly felt. However, some Martian dust storms cover the whole planet. How is it possible to have storms, when winds cannot move objects?
The orbit of Mars, the red planet, is roughly 75 million kilometers from the orbit of Earth, and about 11/2 times Earth’s orbital distance away from the Sun. As the orbits are not perfect circles, sometimes Mars and Earth come about 25% closer to each other. Spacecrafts, however, travel a path six times longer than the average distance between the orbits. But why?
Traveling to Mars
A spacecraft cannot leave Earth and go directly to Mars. It took the Mars Insight mission over six months to do that. Traveling directly would take too much fuel because the Sun’s gravitational field would also have to be overcome. The fuel-efficient solution for spacecraft traveling away from the Sun is a Hohmann transfer orbit.
Initially, the spacecraft starts orbiting the Earth until it gains enough velocity to escape Earth’s gravitational field by firing the engine. The ship starts orbiting the Sun after orbiting Earth, and then orbits Mars. Relative to the Sun, the spacecraft aims to leave Earth’s orbit as close as possible to the Sun and enter Mars’s orbit at the farthest distance of its elliptical path. When that distance is reached, the spacecraft makes one final burn to change its velocity enough to be captured by Mars’s gravity and begin orbiting Mars.
When the spacecraft wants to come back, the same procedure will happen, this time from Mars to Earth. Timing is critical on this trip.
Learn more about water on Mars and prospects for life.
Martian Year and Day
Earth and Mars have to be aligned in a certain way to make the journey possible. This happens every 26 months – three months longer than a Martian year. One day on Mars, called a sol, is 24 hours, 39 minutes, and 35 seconds. This similarity is a perfect coincidence, as scientists working with Mars missions can easily adjust their time with Mars.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
What Is Mars Like?
Mars has a 25° axial tilt, similar to Earth’s tilt of 23.5°, and it has seasons. However, it almost lacks an atmosphere, and the surface pressure is 200 times lower than that of the Earth. The thin atmosphere is made up of 95% carbon dioxide. The deepest crater on Mars, Hellas Crater, has the highest surface pressure of 12 millibars.
The very low pressure makes it very difficult to feel winds, even though they sometimes reach a speed of 70 kilometers per hour. However, the winds are strong enough to push dust, which is found excessively on Mars.
Learn more about what the biggest exoplanets reveal.
Martian Dust Devils
A dust devil is a hurricane of dust that can be seen from the orbit as dark spots. The dust devils can grow big enough to cover a continent-sized area and last several weeks. Usually, Mars has one huge dust storm every Martian year, when it is as close as possible to the Sun. The dust storms can grow bigger and cover the whole planet.
The Mars Reconnaissance Orbiter detected a global storm on May 30th, 2018. It took the storm two weeks to cover 25% of the planet, and by June 19th, it covered the majority of the globe. After four months, the storm was over, but it left its marks. The Mars Opportunity rover, which had been exploring the region of Meridiani Planum on Mars since 2004, permanently lost its contact with Earth through the storm.
Learn more about how our Sun defines our solar system.
Global Dust Storms
Mariner 9, the first mission to orbit Mars, also saw a global storm in 1971. Such storms have been detected on Mars from 1971 up to 2019, but there may have been more. The nuclear-powered Curiosity mission had no problem with dust as it could not blind the solar panels, like in Opportunity. When the storm hit, Curiosity’s view was blocked. Sometimes the storms can completely block out the Sun. How do these storms happen?
The heat from the Sun can cause winds that move very fine dust around. Due to the low gravity and very thin atmosphere, the dust easily stays in the air and eventually creates a self-regulating mechanism for the storm. It shields the lower atmosphere from more solar heating, and as the solar heating decreases, the winds die down, and, eventually, the dust falls as well.
The storms might create dangerously high static electricity. The energy can sometimes break up carbon dioxide and water molecules, creating hydrogen peroxide that later snows down on the surface.
Mars, this small rusty neighbor, has its ways of creating excitement despite the low gravity, temperature, and pressure.
Common Questions about Mars
No, humans cannot live on Mars since there is no oxygen, the surface pressure is too low, and the atmosphere is too thin and made of 95% carbon dioxide.
Generally, Mars is a cold planet since its atmosphere is too thin to retain warm temperatures, even though it is made of 95% carbon dioxide. The Martian poles retain water ice and dry ice in the winter.
Despite the short distance, it takes around six months to get to Mars from Earth. A spacecraft has to leave Earth’s gravity first and then be bound by Mars’s gravity until it can finally land on the new planet.