NASA has now sent five spacecraft heading outwards from the solar system. We have Pioneers 10 and 11, Voyagers 1 and 2, and the New Horizons mission, headed for a 2015 rendezvous with Pluto and probably another small world beyond.

The Voyagers have already given us an interesting taste of the boundary of the solar system, detecting the first evidence of the "heliopause", or the region where the sun's influence fades away and interstellar space begins. Neither Voyager has crossed the heliopause, but controllers hope that it will happen in a few years, with at least one of the spacecraft still functioning.

The Voyagers are expected to run out of operational life around 2017, when the power levels from their radioisotope generators will fall too low to transmit properly to Earth. After this, they will join the Pioneers as silent emissaries from Earth to the universe beyond.

Eventually, some time after 2020, New Horizons will also fall silent.

These spacecraft have performed marvelously on their missions, and it's incredible to think that we've been working with the Voyagers for more than 30 years!

This begs the question of how far we can go with our current technology. None of these spacecraft were really designed to do much beyond the edge of the solar system. What if we designed a mission specifically to go beyond the edge?

There's an easy objection to raise for such a proposal. What exactly is out there? After Neptune, we have the Kuiper belt of minor planets, populated by Pluto and many other small worlds.

There are other minor planets beyond the Kuiper belt, some in strange orbits. After that, as far as we currently know, there's a lot of empty space. The next feature we encounter on any map of our region of space is the Oort cloud, a huge spherical shell of space that's believed to contain dormant comets. A mission to the Oort cloud would take a while.

It's thought to be roughly one light year from the Sun! Nothing we can build right now can reach it for centuries, assuming that the probe itself could work for that long.

So our solar system looks like an archipelago of islands, surrounded by a vast sea of apparently empty space.

Is the region beyond the planets and minor planets really so empty? We can't really say for sure. Just decades ago, we knew little of what lurked in the Kuiper belt. We still had no firm knowledge of the heliopause, and we still have much to learn about it.

Until something travels well beyond the heliopause and samples the real environment of interstellar space, we won't have any direct measurements of its properties.

We also don't know for sure that there aren't more dark and mysterious objects lurking in very deep space. Some astronomers still hunt for the elusive Planet X, or an "evil twin" of the Sun that sends comets from the Oort cloud streaming inwards on lethal attacks to life on Earth.

We need to map particles, fields, and small objects such as dust in the interstellar medium. We can also gain much by studying the trajectory of objects in this region, to understand the gravitational properties of the outer solar system, and possibly learn more about the force of gravity itself!

Placing an instrument package into very deep space would also give us an interesting perspective on very distant objects in space. Telescopes are regularly flown above Earth's atmosphere to allow them to see more clearly. Imagine how some instruments would perform if they were removed from the influence of the Sun. This is not regularly discussed by astronomers, but it's worth considering as a long-term project.

How could we go beyond the solar system? It would require a spacecraft far more complex and expensive than any unmanned space mission ever launched. But it could be done with our current technology.

The spacecraft would need a good power supply. Solar power won't work when the Sun is just a bright star. Current radioisotope sources would not provide enough power and would fade too quickly.

The only option is to use a small nuclear fission reactor. These have been flown in space before. There are safety and regulatory issues aplenty. The infamous crash of the Cosmos 954 satellite into Canada showed how careful any handling of reactors in space must be.

A properly build reactor could supply ample power for decades, as has been demonstrated on nuclear submarines.

The power from the reactor would do more than just power the spacecraft with electricity. It could also assist with propulsion.

Electric propulsion is now a mature technology, and has been demonstrated on several deep space missions. The recent return of the Hayabusa probe to a pinpoint landing in Australia is just the most recent example.

An electric ion propulsion system could help the spacecraft achieve massive speed by gently accelerating over a long period. It could easily outpace the Voyagers, currently the fastest of all the objects leaving the Sun's domain.

The spacecraft would need a good antenna to communicate with Earth. If power is plentiful, this will probably be straightforward. Some sort of dish antenna, similar to those on Voyager or New Horizons, seems logical.

Instruments would probably not be too different from those carried by other deep space missions. There would be cameras, magnetometers, radio experiments, dust counters, particle detectors, radiation sensors, and so on. Some types of astronomy experiments, for examining objects well beyond the spacecraft's reach, would also be included.

This long wish list of parts comes at a price. It's heavy. Don't expect a mission like this to be launched aboard a single rocket, straight towards it destination.

The spacecraft would need to be assembled in Earth orbit from parts brought up on several launches. It would take a lot of careful work to assemble, inspect and test the various components once they had been integrated. Such work could be performed at a space station, and would probably take months.

Once the spacecraft was declared ready for flight, the ion engines would fire and the spacecraft would begin a slow climb out of Earth's gravity well. This could take weeks. During this time, if anything went wrong, the probe could be steered back to the space station for repairs. The first phase of the mission could thus double as a "shakedown cruise" to prove the spaceworthiness of the probe.

After going beyond the Moon, the spacecraft would be on its own. There will be constant testing and monitoring from Earth. Probably, one or more objects in the inner solar system will be observed as the probe heads outwards. Beyond Jupiter, it could be targeted at Uranus or Neptune, then something in the Kuiper belt.

Afterwards, the mission would be heading onto the main goal. It would begin the journey to interstellar space.

With a good propulsion system, the mission could reach the heliopause in roughly 20 years. Beyond this, it will make the first proper, dedicated survey of near interstellar space.

How long could it run? This ultimately depends on the reactor, and how controllers ration the burning of its nuclear fuel. There will probably be phases of the mission when the reactor will be run leanly, and times when power will be increased for encounters with objects.

The ability to control a reactor is a luxury that isn't available for radioisotope power generators, which steadily release heat at a deceasing rate from radioactive decay. The mission could last as long as 50 years, or maybe longer. As time passed, the spacecraft would get deeper into space, allowing changes in the interstellar medium to be monitored.

Yes, this would be an expensive undertaking, and far beyond NASA's current budget. But it's never too early to begin planning for the next phase of deep space exploration. Without ideas to start with, we cannot get to the reality. Even if it takes decades to plan, we should consider how to take this long mission.

Dr Morris Jones is an Australian space analyst and writer. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.