Since the relegation of Pluto to dwarf planet status, Mercury is the smallest planet in our Solar System. It is roughly a third the size of Earth, just a little bigger than our moon. It is a barren, rocky planet with a sparse atmosphere, that is constantly being stripped by the Sun. With all that being said, if you still wanted to go there, then how long would it take to get to Mercury? Let’s explore.
Mercury Quick Facts
- Distance: 57,910,000 km (35,983,605 mi)
- Radius: 2,440 km (1,516 mi)
- Temperature: -180 to 430 °C (-290 to 800 °F)
- Day length: 59 Earth days
- Year length: 87.97 Earth days
- Number of moons: 0
- Closest Distance to Earth – 82,134,610 km (May 2015)
- Furthest distance to Earth – 222,000,000 km approximately
How far is Earth from Mercury?
The closest that Earth and Mercury approach each other is when Mercury is at its furthest position from the sun (aphelion) and Earth is at its closest (perihelion), with both planets on the same side of the Sun. This is known as an inferior conjunction and happens roughly once every 105 to 125 days (116 on average). The reason there is a window rather than a precise repetition here is because Mercury has a very ‘eccentric’ orbit.
At it’s farthest, Mercury can be as far as 222 million km from Earth on average. At it’s closest, it can be as far as 82,134,610 km from Earth. This distance was measured in May 2015, and will be the closest approach until Dec 2100 at least. Over the longer term though, the distance of closest approach is slowly diminishing, and will come as close as 82 million km by the year 4487.
How Long Would it Take to Get to Mercury?
This is where things get interesting. While Mercury may be one of the closest planets to the Earth – and at times ‘the’ closest planet (when Mercury is in inferior conjunction while Venus and Mars are on the other side or the Sun from Earth) to send a mission to Mercury can take as long as sending a mission to Jupiter.
If the intention is simply to get to Mercury as quickly as possible, then we have an example of this with the Mariner 10 mission launched in 1973. The purpose of this mission was simply to fly past the planet and take lots of photos. The mission managed three fly-bys over a year. As Mariner had no intention of entering orbit around Mercury, it was able to travel there at great speed without the need to slow down. As such, the transit time was a mere 147 days.
Complexity Adds Time
To conduct a mission with a greater level of complexity than a simple flyby however, the travel time increases greatly. That is because in order to enter a safe orbit around Mercury, the craft needs to be slowed to a safe speed to prevent failure. This is made more complicated by the fact that in the laws of Physics, objects speed up as they approach the Sun.
As such, as the craft approaches Mercury, speed would increase when really you need to lose a lot of that speed to enter a safe orbit. This is where flybys come in handy. Not only for slowing down spacecraft but for making any corrections to trajectory that are required along the way, without the need to waste excess propellant.
Slowly Does It
In the MESSENGER mission, several flybys were employed around both Venus and Mercury until the craft was slowed to a safe speed to enter orbit.
This increased the flight time dramatically. From launch in 2004, it wasn’t until 2011 and several flybys later that the spacecraft was at a slow enough speed to enter a safe orbit.
With the current BepiColombo mission, launch was in 2015 and includes 2 flybys of Venus and a further 6 of Mercury. Then arriving and settling into orbit in 2025.
To sum up, if the goal is simply to get to Mercury fast, then this can be done in a matter of months. If the goal is to carry out complex missions, then this can take years to safely transit.
What is the Shortest Trip?
So far, the shortest trip to Mercury was conducted by the Mariner 10 mission. The trip took 147 days to reach the planet and being the first in a series of flybys. Theoretically, the shortest trip would be be launched and scheduled to arrive when the planets are in their closest approach.
Interestingly, given that a Year on Mercury is only 87.97 Earth days long, it will have nearly completed two orbits of the Sun by the time you arrive!
Missions To Mercury
There have only been two missions to Mercury so far, and there is one conducted by the European Space Agency (ESA) that is currently active. Here are all the missions so far:
The Mariner 10 mission, launched in November 1973, took 147 days to transit and was the first to be directed toward Mercury. The mission consisted of a series of fly-bys over the course of a year:
- 29 March 1974
- 21 September 1974
- 16 March 1975
The same one side of Mercury was exposed to each fly-by, and around half the surface of the planet was photographed across the 3 visits. It was the first time that we got a good look at the heavily cratered surface.
MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging)
The MESSENGER mission was the first and only mission so far to orbit Mercury. Launched in 2004, with a transit time of 1260 days (3 and a half years). MESSENGER again conducted a series of fly-bys before entering orbit around the planet.
The fly-bys took place in the following order:
- 14 January 2008
- 6 October 6 2008
- 29 September 2009
Following these fly-bys the craft was eventually slowed enough to enter orbit around Mercury on 17 March 2011. MESSENGER carried out a number of observations and activities, greatly increasing our understanding of the planet and also of the formation of the Solar System. At the end of its operational life in 2015, MESSENGER was purposefully crashed into the surface of Mercury. This was done at speeds of around 9000 mph.
The craft which measured around 3 meters left a crater roughly the size of a tennis court. It was only scheduled to orbit Mercury for around a year, and outlasted its expected operational lifetime by far. It even discovered evidence of water ice at the poles, which came as a big surprise.
BepiColombo is a current and active mission to the planet Mercury. It is only the third mission to be sent to the planet and was launched in 2015. It is a joint mission between the ESA and Japan.
The mission is named after the mathematician Giuseppe ‘Bepi’ Colombo, who first suggested the use of fly-bys around neighbouring planets as a means to correct a spacecrafts flight path to another planet. In this case, it was for the Mariner 10 mission, with Colombo suggesting a fly-by of Venus would allow for more than the single fly-by planned for Mercury.
This mission is scheduled to conduct a series of fly-bys of Venus, then Mercury over a number of years. Before arriving at a safe velocity and path to orbit Mercury in December 2025.