Last night I drove from a small village in the Cotswolds (UK) back to my home in Norfolk. The journey took a good three hours, sticking to the national speed limit at all times (of course) and, ignoring the rather torturous windy English lanes and city bypasses, I had to head in a general easterly direction.
My home was due east when I left and was due east just as I arrived. Yet in our everyday lives, we don't realize how handy that is. If we want to go somewhere, we know where it is and head straight in that direction.
But imagine if the place you want to get to keeps moving!
That's the problem space scientists are constantly faced with when trying to send spacecraft to the planets, just like the recently launched NASA Mars Science Laboratory and ill-fated Russian Mars Phobos-Grunt mission.
You only have to look at the night sky over a period of a few hours to notice that things are moving. Granted, it's slightly different in this example as it's the rotation of the Earth causing the apparent motion. But even so, if you see the moon overhead, point a rocket at it and blast off, by the time you get there it will have moved!
The real problem is the time it takes to get to get anywhere in space, and with the Earth and planets all moving independently it makes for plenty of head-scratching to work it out.
The key to solving the problem is to work out how long it will take to get you there and where the object will be at that time. It's a bit of a chicken-and-egg situation though as until you know either, you can't work out the other. Confused?
Fortunately we do have a very good understanding of how bodies in the solar system move thanks to Johannes Kepler and Isaac Newton who, between them, managed to define the laws that govern their motion.
Thanks to their hard work and modern computer modeling, it's now a computation that takes just seconds to run.
However, to add another level of complexity, most spacecraft don't fly direct to their destinations -- instead they take a rather convoluted route to get there, often flying past other planetary bodies. The reason? Fuel economy.
To get a spacecraft to the planets in a reasonable period of time (a few years, say) they have to go fast, very fast. It would take an incredible amount of fuel to accelerate spacecraft to the high speeds required so, instead, a technique called a "gravity assist" -- or "gravitational slingshot" -- is used.
It's a pretty nifty idea where a spacecraft flies close to a planet or other body en route to its final destination. By flying close to another planet, the planet's gravity accelerates the spacecraft, changing its velocity by at least twice the orbital velocity of the planet without burning any additional fuel.
While this is a beautiful solution to the problem it does mean that not only do we have to work out where the target planet will have moved to but also calculate any planetary way-points en route and where they will be!
The Cassini mission is a great example of how accomplished we are at traveling around the solar system.
Following its launch in 1997, Cassini flew by Venus twice and then returned to Earth before a flyby of the the mighty planet Jupiter, each time getting accelerated to greater and greater speeds and all planned and executed with incredible accuracy before it finally arrived at Saturn in 2004.
It's a testimony to the work of scientists like Kepler and Newton and their groundbreaking work so many years ago that it's still at the very forefront of science and helping us to get our robotic (and future human) missions to the planets, ultimately unlocking the mysteries of the solar system.