Juno has launched this year and is on route to Jupiter, to arrive in 2016. I think we have sent fewer probes because we want to send new probes, not same old probes again and again. And new probes take time to design.
The probe is 2.7 million miles from Jupiter, headed further out (its orbit is highly eccentric). That's nearly 11 times the distance from the Earth to the Moon.
At closest approach, Juno will be 2,200 miles from Jupiter, less than 1% of the distance between the Earth and Moon.
Space exploration is expensive, and a very large part of that expense is mass. Putting a 2.7 million mile scope on a probe that's going to be spending much of its time far closer in to interesting targets costs mass, and fuel, and other sensing apparatus you can't add. So there's that.
It's possible that there are higher-resolution capabilities for the probe as well, or that post-processing or higher data rates will improve current images.
It's also possible that visual reconnaissance wasn't a high priority for this mission, though I'm not familiar with the sensing platforms included.
Given orbital eccentricity, Juno can acquire and store high-quality data, and transmit that at leisure during the outbound parts of its orbit.
The problem isn't that the probes get too far out; the problem is that they run out of power. The Voyager probes were launched in the 70's and their RTG's won't provide enough power to keep them running for much longer.
Most of the escape velocity of the voyager probes came from a gravitational slingshot around the gas giants. I don't think additional rocket power would help much.
That's not how mission planning works. They don't start with something roughly like Juno and decide which planet to send it to. Rather, the planet and rough scientific mission is picked first, then the probe is designed around that.
I guess you could just mean "There is an RTG shortage, so we can't send anything past Jupiter", but I don't think this is actually how the decision was made.
This is not true. The only reason the previous probe (Huygens) only lasted a few hours is because it was battery powered (and the battery wasn't particularly large). This probe will carry a power source powered by the radioactive decay of Plutonium 238 (virtually all the radiation being alpha particles, which are easily blocked by the casing of the power supply and thus will not hurt the electronics or ground crew) that can provide 50-100 Watts of power for years or even decades.
One of the biggest problems is that NASA's Plutonium-238 stockpile is running low, and is only barely being replenished (it was an artifact of nuclear weapon production, at the time). Each of Voyager 1 and 2 used something like 15kg of it for their RTGs. Because they were so heavily powered, the half-life won't power the craft down for decades.
The availability of Pu-238 is why European missions can't go past Jupiter without coordinating with NASA -- politically is impossible for them to produce nuclear spacecraft, but solar power becomes ineffective farther from the sun.
But given that NASA has a limited Pu-238 stockpile, they're only stocking new craft with the minimum necessary to hit the key science objectives.
That mission features a complex trajectory that will use a lot of gravity assist to get to Jupiter. It will actually take eight years to get there, while Juno needed five and the Voyagers only two. Why do recent missions choose to do that? Is that a cost saving measure?
Surprisingly, according to the Wikipedia article on RTGs (Radioisotope Thermoelectric Generators, the power source for the Voyager probes), by 2001, the probes' RTGs were down to 67% of capacity, instead of the 83-and-change percent expected. [1]
So, while still impressive, they're unfortunately not working as well as they could be.
The delta-v for a mission like this is pretty incredible. Voyager 1 is still traveling at about 17km/s away from the sun. That's not much less than when it left Earth despite traveling so far away from the Sun... only because of the special gravity assists it did on the way out. There are some things that even money can't solve :)
Compared to a planet, a spacecraft is almost negligible in both weight and capability to accelerate in space. Ion engines are pretty cool though; Maybe we can do it today but it will take a lot longer to get there. Of course, if it takes too long, we may not have a viable spacecraft by the time it gets there. Furthermore, even if do have a viable spacecraft, we might not have the knowhow to work with the technology that we sent in the first place.
As an aside, what has stopped us from sending more probes? Do we simply have all the data we need? It seems like cost should not be an issue, as something like the Voyager re-made with modern technology would be cheap to develop and even cheaper to manufacture.
"Capabilities" seems a little ambiguous to me. Is it what we have done or what we could do? Using those boosts is a way to achieve the same results, only cheaper. Yes, it's slower than building a big ass rocket, but it seems people that sign the checks are in no hurry.
Also if I'm not mistaken, Voyager probes were not even intended to live so long or leave the system.
If there were a reason to send a probe to another star as soon as possible and no matter the cost, there would be a way, maybe not fast, maybe very expensive, maybe something weird, but it would be done.
The Voyager missions were able to use a rare and convenient alignment of the planets that would slingshot them into deep space. Juno is orbiting Jupiter itself, which is not a hard thing to do in our solar system. JUICE is attempting flybys of several of Jupiter moons, to end up orbiting around Ganymede. This is a much more delicate orbital insertion and has much stricter requirements for the gravity assists on its way there.
I don't think any of us are in a position to be too critical of the engineering done for the Voyager probes. The number of machines running continuously for longer than I've been alive with no maintenance is pretty short; the number of them out past the orbit of Pluto even smaller.
Both Voyager probes power themselves with radioisotope thermoelectric generators (RTGs), which convert heat from decaying plutonium into electricity. The continual decay process means the generator produces slightly less power each year
Two months before the Voyager probes were shipped for launch it was found out that Jupiter's magnetic fields and radiation environment were a lot stronger than anticipated, in excess of what the spacecraft were designed to tolerate. They ended up wrapping many of the cables on the spacecraft with aluminum foil bought from a local supermarket as a protective measure.
First we’d need to have a deep space planetary observation probe with a high dV propulsion system, an RTG and TWTA backed high gain antenna. Chucking a standard cubesat at it would be no more use than launching a washing machine. At this point the probe wouldn’t catch up with it until it’s at least as far away as Voyager is now.
> So what does that conclusion mean about when the probe will be so far away that we are below the Shannon limit?
I think the practical limit right now is that the Voyagers are losing power.
"The radioisotope thermoelectric generator on each spacecraft puts out 4 watts less each year. [...] The two Voyager spacecraft could remain in the range of the Deep Space Network through about 2036, depending on how much power the spacecraft still have to transmit a signal back to Earth."
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