By Sabine Stanley, Ph.D., John Hopkins University
Mercury is the smallest planet in the solar system. It is also the closest planet to the Sun. And, if you want to send a spacecraft to Mercury, there is no way to control its speed. Why? Read on to know the answer and to learn more about the planet’s unusual atmosphere and elliptical orbit.
Mercury is a planet of extremes. It’s the smallest planet in the solar system and the closest planet to the Sun. It travels around the Sun much faster than any other planet, 60% faster than Earth, at average speeds of over 100,000 miles per hour. Even getting into orbit around Mercury is unusually challenging. That’s because a spacecraft, as well as comets or anything else, speed up as they get closer to the Sun.
The force of gravity causes everything to speed up as it approaches the Sun and moves deeper into what we call the Sun’s gravity well. Similarly, objects falling to the surface of the Earth speed up as they get deeper in Earth’s gravity well.
So if you want to send a spacecraft to Mercury, the big issue is slowing down. For example, the path the MESSENGER spacecraft followed when it launched from Earth in 2004 to Mercury included one flyby of Earth, 2 flybys of Venus, and 3 flybys of Mercury itself. Now, each flyby uses the gravity of a planet to slow down the spacecraft or change its trajectory. All this gravity-assisted speed reduction took a long time and a lot of extra distance.
MESSENGER began orbiting Mercury 7 years after launch. The Bepi Colombo mission, launched in 2018, is following the same 7-year timeline, arriving at Mercury in 2025.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
Mercury’s Unusual Elliptical Orbits
Once there, another extreme is that Mercury has incredibly long days. In fact, a single ‘day’ on Mercury is longer than an entire year. Mercury’s orbit around the Sun is also less circular—more elliptical—than any other planet. So a single year includes a lot more speed-up and slow-down during one orbit.
We also can’t help but notice the Sun. On average, the Sun is 2 1⁄2 times bigger measured in the sky from Mercury than from Earth. This is because Mercury is on average 2 1⁄2 times closer to the Sun than the Earth. Now, we say ‘on average’ because Mercury’s distance from the Sun changes a lot during its orbit.
At its furthest point, Mercury is 70 million kilometers from the Sun, whereas, at its closest point, it’s only 46 million kilometers away. This means that the Sun’s measured size in the sky would vary at different points in Mercury’s orbit. This is not just an optical illusion. When Mercury is at its closest point to the Sun, the Sun really is about 50% bigger than when Mercury is at its farthest point from the Sun.
This elliptical orbit also gives Mercury seasons that are very different. On Earth, seasons are due to the 23-degree tilt of Earth’s axis, where northern winter coincides with southern summer and vice versa. By contrast, Mercury’s axis of rotation has almost no tilt.
So Mercury’s northern and southern hemispheres experience pretty much the same thing. And yet Mercury as a whole does have seasons of unequal size because Mercury’s elliptical orbit brings the entire planet closer to the Sun for short periods and farther away for longer periods. When Mercury is at perihelion, the Sun is twice as bright as when Mercury is at its furthest point, called aphelion. So there are seasons of the year that are hotter and seasons that are colder.
Within our solar system, Mercury has by far the most elliptical orbit of the 8 planets. We’ll have to go to Pluto and the wild orbits of some exoplanets to see anything comparable.
Learn more about orbiting Earth: up through the atmosphere.
Mercury’s Atmosphere
Mercury has no atmosphere in the sense that balloons, or wings, or parachutes, or other aeronautical devices will not work there. But that doesn’t mean that there aren’t particles floating above the surface, gravitationally bound to Mercury. All planets and even some moons have particles surrounding the planet. On Earth, these particles are mostly nitrogen, along with oxygen, plus much smaller amounts of carbon dioxide.
The particles are gravitationally bound to Earth, and here’s the important part: The density of the particles is large enough that these particles collide with each other, effectively keeping them up in the air. Such particles in collision with one another behave like a gas. These collisions make possible what we typically think of when we say ‘atmosphere’.
Mercury doesn’t have an atmosphere in this sense. But there are still sometimes particles surrounding and gravitationally bound to the planet. It’s just that they almost never interact with other particles. For example, sometimes a high-energy particle from the solar wind can impact the surface of Mercury, knocking an atom off the surface. But this atom has too few other particles to support it. So it might land somewhere. Or, it gets blown off and starts orbiting the planet. This region of orbiting particles is usually known as an exosphere.
Scientists have detected all sorts of elements in Mercury’s exosphere, including hydrogen, helium, oxygen, sodium, calcium, and magnesium. Earth and other planets have exospheres, too. Earth’s exosphere, however, starts far above what we think of as atmosphere, at altitudes where the density becomes low enough that the particles no longer collide anymore.
The difference is that Mercury only has an exosphere. It’s another way that planet Mercury is more like a moon.
Learn more about exploring the Earth-Moon system.
Common Questions about Mercury
Orbits of most planets are eccentric. Earth’s orbit is slightly eccentric, whereas Mercury’s orbit is the most eccentric because it is the closest planet to the Sun.
Orbits are caused by the planet’s interactions with the sun as it moves around by the gravity of the sun. It speeds up or slows down Mercury’s rotation depending on where the planet is on its elliptical orbit.
Orbits are the result of a perfect balance between the forward motion of a body in space, such as a planet or moon, and the exertion of gravity on it from another body in space, like a star.
Instead of a regular atmosphere, Mercury possesses a thin exosphere made up of atoms that were blasted off the surface by forces such as solar wind and striking meteoroids.
