If asked which planet in the solar system is closest to Earth, many people would probably choose Venus because it is the closest planet to Earth along its orbit.
Image Source: JACOPIN/BSIP SA/Alamy Stock Photo
In fact, when it comes to average distance, the closest planet to Earth is Mercury, which is also the closest celestial body to all other planets in the solar system in terms of average distance.
The reason is simple: as these planets orbit the Sun, any two planets can move away from each other (imagine two planets on opposite sides of the Sun), and since Mercury is closest to the Sun, its average distance to other planets is almost the same as the radius of the other planets' orbits.
However, even though Mercury is the closest planet to Earth in terms of average distance, there is currently no spacecraft on Mercury, and historically, no spacecraft has ever landed on Mercury.
BepiColombo is a human mission to Mercury that is currently underway. It is a joint project developed by Japan and Europe, launched in October 2018, and is expected to enter Mercury's orbit in December 2025, barring any unforeseen circumstances.
BepiColombo Mission, Image Source: NASA
The BepiColombo mission has been flying for over 7 years, a duration almost equivalent to the travel time of human-launched spacecraft to Jupiter's orbit. However, the distance between Earth and Mercury is only about one-tenth of the distance to Jupiter.
So, the intriguing question arises: since Mercury is closer to Earth, why are there so few probes sent there, and why does it take so long for them to reach their destination?
Up to now, including the aforementioned BepiColombo currently en route, humans have launched a total of three probes to Mercury.
Mariner 10, Image Source: NASA
The first one is Mariner 10 from 1973, which actually took only 147 days to reach Mercury. Its mission ended on March 24, 1975, after only three flybys of Mercury, capturing some photos of the planet.
In the decades following Mariner 10, there were no more spacecraft missions to Mercury. Due to its proximity to the Sun, even telescopes rarely focused on it. For example, the Hubble Space Telescope has been capturing images everywhere but Mercury.
On the other hand, since Mariner 10 only imaged 45% of Mercury's sunlit side, can you believe it? For a long time, people believed that Mercury was tidally locked to the Sun.
The second spacecraft is MESSENGER, launched in August 2004, and entered Mercury's orbit in 2011. This spacecraft flew for over 7 years.
Messenger, Image Source: NASA
Unlike the Mariner missions, Messenger entered orbit around Mercury and flew around the planet for about a year before its mission concluded.
Simply put, it doesn't take much time to fly by Mercury, but entering its orbit is a lengthy process.
Why does it take over 7 years to enter Mercury's orbit?
In reality, entering Mercury's orbit is extremely challenging, which is why there haven't been many missions to the planet.
This task requires a tremendous amount of energy - in fact, the energy required to reach Mercury is even greater than that needed to reach the outer dwarf planet Pluto in our solar system.
The reason is quite straightforward: Mercury is very close to the Sun and is also quite small in size.
Being so close to the Sun means that as a spacecraft approaches Mercury, it is constantly accelerated by the Sun's immense gravitational pull. Additionally, Mercury's small mass means it lacks the gravitational pull to capture a fast-moving spacecraft.
To get a spacecraft into Mercury's orbit, it needs to slow down, and there are two ways to achieve this:
One way is to carry enough fuel for retrograde propulsion, but this method is simply not feasible for entering Mercury's orbit. Even with the largest rockets on Earth, it's impossible to carry enough fuel to slow the spacecraft sufficiently to enter Mercury's orbit.
However, some brilliant scientists have come up with a second method, which involves using a gravity assist to decelerate the probe.
A gravity assist essentially utilizes the relative motion and gravity of astronomical objects to alter the spacecraft's trajectory and speed. It can either accelerate or decelerate the probe, depending on the direction of motion between the probe and the astronomical object.
In simple terms, if the velocity after the slingshot is in the same direction as the motion of the celestial body, it accelerates the probe; if in the opposite direction, it decelerates it.
The gravity assist is actually a solution for insufficient fuel, but its drawback is that it requires more time as you need to continuously leapfrog between other astronomical bodies.
Image: Various Scenarios of Gravity Assist Maneuvers
Currently, for a probe to enter Mercury's orbit, it must undergo multiple gravity assist decelerations. From Earth to Venus, and then to Mercury, continuous gravity assist decelerations are necessary until reaching the speed required to enter Mercury's orbit.
For the BepiColombo mission (different from the Messenger mission), it will perform one flyby at Earth, two at Venus, and six at Mercury – having completed two flybys and decelerations, it is expected to perform four more decelerations to enter Mercury's orbit.
Named after Italian engineer Giuseppe Colombo, a pioneer in utilizing planetary flybys to adjust space mission trajectories, the BepiColombo mission aims to further explore and confirm many theoretical aspects that have been primarily derived from the Messenger mission's findings about Mercury. This mission may provide evidence and insights into Mercury's magnetic field anomaly, as Mercury, being a small planet, retains a magnetic field unlike larger bodies such as the Moon and Mars, which have lost theirs.
In the end
In fact, landing on Mercury is quite challenging because Mercury lacks an atmosphere, making it impossible to use aerobraking for landing. Instead, braking must be done using fuel.
However, carrying more fuel would make the mission even more difficult, so there have been no missions to land on Mercury to date. Nevertheless, NASA plans to launch a Mercury landing mission in 2035 - a mission expected to take 10 years to complete.
