After two billion seconds on this planet, you tend to think about the time you've occupied and what you may occupy. Our ability to sense scale is largely limited to what we can sense and the physical and temporal spaces we navigate. Historians usually work with somewhat larger scales, but I'm guessing they don't have a good feel for - say - what 917 AD actually means. Science looks at larger and smaller time scales. I've spent a lot of time worrying about processes shorter than a femtosecond as well as those that are billions of years long. I write down the scientific notation, but confess I don't have a physical feeling for what it means.
I'm particularly interested in what we can do about the future. Dealing with global warming as a practical matter to mitigate huge biological and financial losses would see to be the big challenge (hopefully we'll see enough progress!). On the scientific side there are projects that span decades. Initial work on CERN's Large Hadron Collider began in the late 70s and it should run through at least the mid 2100s. Solar system exploration often looks three or more decades out. Voyagers 1 and 2 managed to capture the public's imagination early on and they continue to provide amazing results - over 44 years from their launch. (you can find out how far they are away and how fast they're moving at this JPL site)
The genesis of Voyager came when a Caltech grad student was working on a Summer project at the Jet Propulsion Laboratory in 1964. Gary Flandro was studying techniques to send probes to the outer solar system. Even during the space race the scale of space made this look completely blue sky. Gary realized an alignment of Jupiter, Saturn, Uranus, and Neptune would take place in the late 70s and that it might be possible to make a "grand tour" of these planets if you used gravity assist - a mechanism that transfers a bit of the kinetic energy from a planet to a spacecraft - to speed up the journey. Such alignments are rare on the scale of a human life - about once every two hundred years. The project was funded and, with help from Carl Sagan, caught the public imagination with a golden record of humanity attached to the spacecraft as well as one of the most iconic images humanity has recorded - the pale blue dot.
Exploration of the planets of the outer solar system was the prime mission. The fly-bys were smashing successes, but the two spacecraft kept working. Instrumentation was primitive by today's standards and optimized for conditions expected around Jupiter and Saturn, but it was possible to track the solar wind and a few other things.
In 2004 Voyager 1 crossed the boundary of the heliosphere - a huge region surrounding the Sun inflated by the solar wind. There was celebration as something made by humans had crossed into interplanetary space 94 times as far from the Sun as we are. There was a shock in 2007 as Voyager 2 traveling in a different direction, crossed the boundary about ten percent closer to us. The nature of the boundary was also different. It was, and is, a time of delight as so many new questions and hypotheses formed.
I'll skip the details, but there are two protective regions that deflect charged particles that help make life as we know it possible on Earth. The smallest and most important is a comet-shaped region around the Earth generated by our planet's magnetic field. This magnetosphere deflects charged particles streaming at us from the Sun as well as some cosmic rays from deep space. Without it much of our atmosphere would have blown away and radiation levels would be high on the Earth's surface.
The much larger heliosphere defects cosmic rays from outer space. We've learned that the Sun has been traveling through a dust cloud known as the local interstellar cloud for about 60,000 years. We're currently at an edge and soon we'll be on the outside (this may have already happened!) Then we travel for about 2,000 years before entering another dust cloud called the G cloud. Depending on its density the heliosphere may shrink. Cosmic ray fluxes would increase and that raises an interesting set of questions.
[speculation alert] Between two and three million years ago some crust samples show a lot of Iron-60. The isotope does not occur naturally on Earth - rather it's the product of supernova explosions. We may have spent time passing through a cloud laden with the "ash" of these huge explosions. The heliosphere would have shielded us from charged particle radiation levels that would have done a number on life on Earth, but some would have made it through. Who knows - perhaps this had an impact on evolution? In any event there are a huge number of questions.
A number of fantastic missions to the outer solar system on the drawing board - missions that may find the first extraterestial life. I probably won't be around to learn about the findings, but am an enthusiastic supporter. One of the most interesting is a follow-on to the Voyagers - probes that dip their toes into interstellar space. A study project is underway in the US and the Chinese are talking about a mission. Details on the US proposal are amazing. It's modern day cathedral building with details that must work forty and fifty years out when most of the designers are probably gone and the world will probably be a different place. Thinking about it makes the hair on your back stand up.
In addition to the scientific and engineering challenges, there's also a management challenge. How do you form a cohesive team of experts who will have to excite and train new people along the way? These projects tend to be small and the loss of one expert can be a disaster. A great book that covers the New Horizons mission to Pluto and beyond shows how it can be done: Chasing New Horizons by Alan Stern and David Grimspoon.