Mercury, Pluto probes: Life and death at opposite ends of solar system

One robot spacecraft ended its life on Mercury this week, while another set its sights on Pluto. 

After nine years in space — four of them orbiting the closest planet to the sun — the MESSENGER spacecraft plunged into the surface of Mercury on Thursday, completing the first successful reconnaissance of the swiftest planet.

Mercury is one of the least explored planets, only seen a few times before during three quick flybys by Mariner 10 more than 30 years ago, which only saw one side of the planet. MESSENGER (MErcury  Surface Space ENvironment GEochemistry and Ranging) circled the planet, revealing the entire surface of Mercury, a world that strongly resembles our moon.

In fact, Mercury is just slightly larger than Earth's moon, but with unusual features, such as wrinkles that show how the planet has shrunk over time. There are odd holes in the surface from volcanic materials erupting out of the ground and boiling off into space.

Most surprisingly, there is ice at the north pole.

You would think that a planet so close to the sun, with a daytime temperature greater than 300°C — hot enough to melt lead — would be pretty inhospitable to a snowball. But if you visited the bottom of some deep craters at Mercury's north pole, you would never see the sun. You would also be extremely cold because there is no air to provide insulation against the deep chill of space. Dig into the dirt and you would find enough ice to fill Lake Ontario.

So, you could literally make a snowball on a hellish world. That ice was likely put there by comets and asteroids that struck all the planets, including Earth, in the early days of the solar system's birth, and adds credence to the idea that much of the water that fills our oceans came from space.

The journey to Mercury was as interesting as what the spacecraft found when it got there. Even though the direct distance from Earth to Mercury is about 77 million kilometres, MESSENGER covered 7.9 billion km over six years, circling the sun 15 times to get there. Talk about going around the block to get next door.

The extra-long route was necessary because any journey towards the sun is "downhill", as the powerful gravity of our star pulls you in faster and faster. The spacecraft had to slow itself down along the way to ensure that it wouldn't just whiz by Mercury. It did that using a clever technique called gravitational braking, where it used the gravity of the planets to reduce its speed.

If a spacecraft in orbit around the sun makes a close pass by a planet, it can use that planet's gravity to either speed up or slow down. If it passes the planet on the outside of its orbit, the speed of the planet is added to that of the spacecraft, so it accelerates. This technique has been used to slingshot many spacecraft to the outer planets, including the New Horizons mission to Pluto.

But, if a spacecraft passes on the inside of a planet's orbit the gravity of the planet pulls the spacecraft back, so it slows down. MESSENGER did this six times, passing by the Earth once, Venus twice, and Mercury itself three times, before it lost enough speed to be captured by Mercury's gravity and go into orbit. That's an astounding feat of planetary navigation.

The MESSENGER mission was designed to orbit the planet for one Earth-year, but everything was running so well that it was was extended to four.

Finally, after exhausting its manoeuvring fuel, the spacecraft was sent into a death plunge onto the planet's surface. This was to ensure there would be no space junk orbiting the planet, but also to perform one final experiment. MESSENGER hit the surface at 14,000 km/hr, which should produce a new crater 16 metres wide. Any future missions to Mercury could spot that crater and study the freshly excavated material.

New Horizons

Meanwhile, the New Horizons mission got another tantalizing view of Pluto and its moon Charon, orbiting each other out at the very edge of the solar system.

Details of these icy worlds are beginning to emerge, as New Horizons prepares for the first close encounter with the dwarf planet on July 14th. Stay tuned for much more on than mission.

Robots are the unsung heroes of space exploration. They go where no one has gone before, often where no one will ever go. They do the primary exploration of our neighbouring worlds. They map alien landscapes, sniff atmospheres, land and taste soil in the search for life. They are an extension of our senses that allow us to discover our own planet in its full environmental context.

After surviving the most hostile environments imaginable and performing well beyond expectations, these robotic emissaries do not come back. They drift away silently in deep space, die on the frozen surface of another world, or, like MESSENGER, end up making a smoking hole in the ground. But the years worth of data they provide will never be forgotten.

So good-bye MESSENGER; on to Pluto!