Mission to Mercury: BepiColombo is one of very few spacecraft ever sent to the Solar System’s innermost planet.Credit: ESA/S. Corvaja

Last week, the BepiColombo spacecraft successfully began its seven-year journey to Mercury — only the third-ever mission sent to the planet. The day before, scientists said that NASA’s next Mars rover — intended to be first to gather and return rock samples to Earth — should visit as many places on the red planet as possible.

So which of our Solar System’s planets has proved most, and least, popular with space scientists — and why?

Over the past few decades, dozens of missions to other planets have been initiated. By now, every one of our Solar System’s worlds has been paid at least a flying visit by a spacecraft.

Attention grabbers

Earth’s closest neighbours, Mars and Venus, have received the bulk of attention since the first planetary missions began in the 1960s (see ‘﻿Planetary visits’).

In the early years, this was a matter of convenience, and a natural progression from sending spacecraft to the Moon, says Elias Roussos, astrophysicist at the Max Planck Institute for Solar System Research in Göttingen, Germany.

But Mars stole the limelight when it became clear that there was once water on the planet’s surface, Roussos says.

That discovery has made Mars a favourite target for exploration, notes Andrew Coates, planetary scientist at University College London — and an important aim of many current and future missions is to look for signatures of ancient life.

Several craft are orbiting the red planet right now, and four rovers have landed successfully on its surface. They will soon be joined by two more — the ExoMars rover and the Mars 2020 rover are both set to launch in 2020.

Earth’s evil twin

Although Venus might not attract the same interest as it did in the early days of spaceflight, there are still lessons to learn from the planet, says Coates.

Venus’s environment was once a much more Earth-like planet, but it suffered a runaway greenhouse effect that resulted in very hot temperatures and an atmosphere filled with carbon dioxide. “It’s like Earth’s evil twin”, Coates says. “We don’t want what Venus is now to be our future.” Japanese spacecraft Akatsuki is currently orbiting the planet to find out more about its climate.

Mercury, meanwhile, has been neglected among the inner planets. That’s because it’s so challenging to get to, says Coates. Its close proximity to the Sun means that visiting spacecraft need to be slowed down considerably before they are able to enter the planet’s orbit.

Beneath the surface

But Coates suggests that it’s the outer planets that will see more exploration in the future — particularly the moons of these worlds.

Previous missions have found that Saturn’s moon Enceladus and Jupiter’s moon Europa harbour subsurface oceans, making them prime candidates for hosting life. Two orbiters, the JUpiter ICy moons Explorer (JUICE) and EuropaClipper, will check out Jupiter’s moons in more detail, though it will take some time. JUICE is not set to launch until 2022, and will only get to Jupiter in 2030. “You have to be patient with space work,” says Coates.

Further out, Uranus and Neptune have only received a single, fleeting visitor — Voyager 2 — while Pluto had to wait until 2015 for New Horizons to fly past. And although it’s understandable that the closer planets have received more attention, says Roussos, to truly understand the Solar S﻿ystem we will need to explore these faraway worlds in more detail.

That means that scientists cannot dismiss sending orbiters and other craft to Neptune or Uranus just because it is technically challenging, says Roussos. “What we should do is say that there is interesting and basic and fundamental science that we want to do at these planets,” he says. That, he adds, should also motivate the evolution of new technology to get us there.

“Our ultimate goal in exploring the Solar System revolves around how the Solar System formed and evolved, including understanding where life may have taken hold,” says Lori Glaze, acting director of the Planetary Sciences Division at NASA Headquarters in Washington DC. “The details may change and get tweaked but the big picture remains.”