With the arrival of Cassini in orbit around Saturn, we now have a physical presence on a world that is About 1.5 billion km from home, bringing us new insights into our solar neighbourhood. My question is, how far have we gone?

I believe the Pioneer and Voyager missions began in the 1970's, how far have these craft travelled? Have they left the solar system? Do they still operate and communicate with us? How much longer will they be active? Will there be a point where we will lose 'sight' of them? Do they have a significant target (maybe not as spectacular and interesting as Saturn) that they will be sending us data on in the future?

Over the next decade or so we will develop and demonstrate technologies that will make it possible to send probes that can operate electronically for a couple hundred years, and get up to perhaps one percent of the speed of light. So, in a century or two we might be hearing from such a probe as it goes through the thin outer reaches of our Oort cloud.

Such a mission would be pretty expensive to launch, and would need some serious promise of scientific return to justify creating it. I'm not sure that will happen.

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One of the two Voyagers has not only left the Solar System it has also passed through the heliopause - the point where the solar wind from the Sun interacts with the interstellar winds that come from the rest of the stars. If my memory serves me well it was towards the beginning of this year when that happened - there was much debate at the time on where the heliopause actually began.
Heliopause Have a look at this article in UT. & this other link:
Voyager nears edge of Solar System

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It depends on your definition of the solar system. Since the Oort cloud, if there is one, is gravitationally attached to the sun, I consider it and everything inside it part of the solar system. Neither Voyager is anywhere near the Oort cloud yet. The furtherest one is probably not much over 100 AUs.

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Here's an interesting New Scientist article about the solar storms from last fall wizzing past Voyager 1 last week, and potentially pushing out the boundary of the heliosphere soon.

Sun storms reverberate to Solar System's edge

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Guys, guys, this person who posed the question is new to us and perhaps even new to astronomy. Helopause, Orrt Cloud and A.Us likely means very little to this person. Our best tool for measuring, is the simplest to understand. The speed of light. But first we need to supply some points of reference for understanding.

The speed of light is roughly 300,000 Km.s per second. At that speed, light reflectred from the Moon takes a second and a half to reach us.
Light from the Sun reaches Earth in a little under 8 and one half minutes.
Sunlight takes around an hour and ten minutes to reach Saturn, so Cassini's signals take about the same amount of time to reach us.
The farthest (currently defined) planet Pluto is over 4 and a half hours at the speed of light.

The next nearest star Proxiama Centauri is 4 and one quarter years away at light speed. So where does that put our Voyagers space probes?

After over 40 years of traveling as fast as any space craft man ever built so far,
both Voyagers are only a couple days away at the speed of light. Perhaps 4 days out at the most. The signals are of coarse getting weaker all the time, and the generators are losing their ability to produce a decent amout of power, so it is just a matter of a few more years before we lose communications with Voyagers all together.

To gain effective communications for any signifcant amount of time, the probes power generator and transmitters would have to be hundreds of times more powerful then they are now.

The further Voyager is a little over 90 AU away from the sun right now. This is 90 times 8.5 minutes light time, which is about 13 hours light time.

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Whoops, That was a big over estimate on my part.

But that quite clearly shows just how painfully far we are from achieving any fraction of speed of light, let alone matching or exceeding it.
Forty years to travel only 13 light hours. That is depressing.

No wonder there is so little persuit of new probe missions.

Thanks for the info everyone. I guess it looks like the Voyagers will soon reach their limiting distance in terms of our ability to communicate with them. As you say, the signals are getting weaker all the time, and their generators are becoming less effective (I assume these are solar powered?), plus I think that article that Spacemad linked to said that some of Voyagers' instruments had stopped working.

Is there any way we can prolong the life of the Voyagers, either by sending out relay spacecraft for their messages, or is this exercise not worth it because the Voyagers won't be transmitting for much longer? Seeing how it has taken so long for us to travel just 13 light hours, it would seem an awful waste to let them die off and have to start the journey all over again.

Will they still be communicating with us when they reach the Oort cloud?

“I assume these are solar powered?”

They are very definitely not solar powered. At 90 AU the sun is about 9000 times fainter than at Earth orbit (inverse square law plus stuff getting in the way in the plane between the spacecraft and the sun). And solar panels would degrade over 30 plus years from the effects of dust impacts and particle bombardment among other things. Getting by on 1/20,000th the power is just not possible.

The alternative, which has been used for every mission past the asteroid belt, is nuclear power in the form of RTGs—Radio-Thermal Generators, using plutonium for heat and electricity.

Solar power is really impractical past the asteroid belt. Even on Mars the performance of the rov-ers currently on the planet is really compromised by the use of solar power. Their daily activities and lifetimes are both substantially restricted by the use of solar power. Viking 1, which landed on Mars in 1976 operated for 2,245 days; Sojourner landed on Mars in 1997 and operated for 85 days.

Viking 1, of course, was equipped with RTGs, which made possible operations for over years, as opposed to 85 days. The difference in performance is even more dramatic since Viking was per-fectly capable of operating during the night, and Sojourner was not. Spirit and Opportunity, crip-pled as they are, aren’t expected to last a year.

Sorry for this dummy question from a noob but what is a AU ?
Its roughly 153 million kms if i am correct.
300.000 km/s * 8.5 minutes.
Is this correct ?

An AU is an "Astonomical Unit" which is the average distance from Earth to Sun. You are right about it being the about 150 million kilometers [93 million miles]. It is the short side of the triangle when we observe parallax.

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We are developing technology that will get up to much higher speeds.
The Pluto-Kuiper Express [New Horizons] mission will take 9.5 years to get to Pluto [about 35 AU at the time]. It should be further from the sun than the Voyagers about thirty-five years later, even though they had a twenty-seven year head start.

Neither New Horizons, nor the Voyagers will be communicating when they reach the Oort cloud.

Now in the Early stages of development is the Prometheus project. This will enable sending craft out that far MUCH faster. The specifics aren't yet known as to how fast, but currently the project calls for a 100 kilowatt power source that will take many years to degrade, and ion propulsion. It could realistically be used to get a craft going ten to twenty times as fast as the Voyagers.

Solar Sail technology also has some promise for getting out of town quick. As Tom2Mars points out, they can also be used as a solar power concentrator, even at huge distances from the sun.

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How come we resorted to solar power for Spirit and Opportunity if RTGs are available? Are they too expensive, even given the trade-off of a *much* longer operating life? Too dangerous to be launching willy-nilly into the atmosphere? Or is the modern attention span just too short to sustain interest for that long, especially now that Cassini is hogging the limelight? :P

BTW antoniseb you lost me with this one. I think I understand parallax, but which is the "short side" of the triangle you refer to? Is it from the earth at one point in its orbit, to the sun back to the earth at another point in its orbit?

To measure the distance of a star by parallax, you take a two snapshots of the star from opposite sides of the earth's orbit. The slight difference between the two positions of the star (relative to a fixed background), combined with the diameter of the earth's orbit, can enable you to measure geometrically how far away the star is.

The star and the two positions of the earth form a long, thin triangle, with the base being 2 AUs wide (twice the distance of the earth to the sun)

I think that's what antoniseb meant.

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RTGs are expensive, weigh a lot, and are fairly warm. They also caused a lot of controversy when Cassini was being launched because they contain enough plutonium to be chemically toxic if they crash and burn do to a bad launch attempt. We use RTG when necessary. The MERs were intended to be cheap, and to demonstrate cheap technology. I suspect that the MERs were also thought to only have a certain amount of science they could do before they stopped sending new and different information.

As to your parallax question, Sp1ke's answer was right on the money. Thanks Sp1ke!

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