We may have to wait a while - Voyager relied on a rare once every 175 year planetary alignment for gravitational slingshotting. It's only a shame the whole Grand Tour that Voyager had once been intended to be a small part of never happened.
Depending on the mission there's no doubt other alignments that would give the gravity assist.
Do we have any newer propulsion methods that would make a difference in these super-long-running missions? (I am not a rocket scientist... but I'm thinking along the lines of something like a low-impulse ion thruster that uses a relatively small amount of fuel but can run for a very, very long time)
Yes, the new missions now have a Delta-V of something like 11 kilometers per second, which is insane. For instance, the New Horizons which only took 9 years to get to Pluto had thrusters with a Delta-V of only like 0.27 kilometers per second, so most of the trip was done using gravity assists. Missions now can do some crazy things like orbit two separate bodies.
To put it into perspective, Voyager 1 has a current speed of 17 kilometers per second. So the Dawn craft has most of that trip covered without gravity assists if it wanted. AFAIK there's nothing stopping us from loading up some craft with more fuel and larger ion engines except for launch weight. And even then, if we mastered orbital refueling we could surpass that.
As it stands, the craft can carry approximately the same amount of Delta-V that the rocket that got it into space. Equivalent to tons of fuel from chemical rockets.
> To put it into perspective, Voyager 1 has a current speed of 17 kilometers per second
And the Earth has 30 km/s, yet we don't get Voyager "for free" and then some just from LEO. You can't compare speeds in different parts of the gravity well against each other because then you're ignoring the dv required to get from here to there.
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.
This was a mostly political problem with Congress kicking the can down the road for 20 years. According to the article they've restarted production although a bit slower than they wanted.
Of course you can't stockpile radioactive materials for too long. Their very nature limits their shelf life. So constant production is necessary if you want to use it on missions.
I wonder how long that reactor would provide useful energy. U-235 has a half life of 700 million years. If it were to last that long, they'd be dealing with thermonuclear Long Now-type problems.
The Kilopower reactor uses a chain reaction to trigger fission so the U-235 will be depleted much faster than the half-life. Wikipedia says its operational life is 12-15 years.
Do you know if the new designs proposed using thermophotovoltaics and silicon-carbide pellets are more efficient than traditional RTGs in their use of plutonium? Eli Yablonovitch mentions work being done at Berkely: https://www.youtube.com/watch?v=lDxJsa8miNQ&t=3280
There's a quote I thought of when you said this. It's from a very old Wired article (back when Negroponte was a part of it) titled "The Millennium Clock" by Danny Hillis:
"I think of the oak beams in the ceiling of College Hall at New College, Oxford. Last century, when the beams needed replacing, carpenters used oak trees that had been planted in 1386 when the dining hall was first built. The 14th-century builder had planted the trees in anticipation of the time, hundreds of years in the future, when the beams would need replacing. Did the carpenters plant new trees to replace the beams again a few hundred years from now?"
I sincerely hope we do launch more deep space probes in the near future, for our collective far future selves.
The original builder felt strongly enough about the oak beams to plant replacements. The restorers, or whoever hired them, did not. Perhaps they knew that 500 years in the future there would be better alternatives and were not as locked into the original design as the first builder was.
Not all such decisions are short-sighted.
Edit: I think it's been shown elsewhere that it is not the date that you launch the next probe that is important, but the speed at which it travels.
Not only that, but according to the article, when New Horizons "runs out of power in the 2030s, it’ll fall silent more than a billion miles short of the heliosphere’s outer edge."
It would be pretty neat if we launched Voyager 3 & 4 around 2150, following the same 175 year cycle "Grand Tour" of the original Voyagers. I also wonder if the power source in 2150 would still need to be a RTG or if we will have an alternative by then.
Do you mean interstellar probes? The Voyager missions were primarily planetary exploration missions, which NASA and other space agencies continue to run with new probes every few years. The interstellar exploration piece was kind of a secondary feature of the missions taking advantage of high speeds from all the gravity assists along the way.
I don't think we'll see a dedicated interstellar probe for a long time. It just takes too much energy to get out that far in a reasonable amount of time without radical designs or a big leap in propulsion technology.
The propulsion technology for an interstellar mission is available today, there are proposals for probes powered by RTGs or even nuclear reactors and driven by ion engines. Jupiter-Saturn conjunctions that can be exploited for gravity assists happen every 20 years or so: https://en.wikipedia.org/wiki/Interstellar_probe#Proposed_in...
It's entirely a political problem. There is no will to fund such a mission, and what's worse is that in the US funding is approved year by year.
In nuclear engines the fuel and propellant is separate. I keep wondering how much gas a rocket needs to take with it in order to have something to push against. Nuclear fuel is incredibly energy-dense, but the density of the gas used for propulsion in a nuclear rocket can't be much different from what's used in conventional rockets.
The JIMO mission was supposed to have 12 tons of xenon on board, a substantial amount of the yearly production (~ 70 tons). You do wonder how Elon Musk's Starlink madness will affect the xenon price.
The StarLink satellites use krypton thrusters instead of xenon [0]. They need more electic power to run, but produce higher specific impulse [1]. And krypton is cheap.
There's a (partly funded!) project to send a 'chain' of probes to α Centauri[1], as long as they don't need to slow down when they get there, they can shoot right through and relay results back, as I understand it.
With launch costs plummeting it may be possible to launch a bigass booster and give an interstellar probe a real kick even without a magical planetary alignment.
The question then becomes: what instruments would you put on this that would tell us something the Voyager probes have not? What is your mission beyond "making something go even further away from us"?
As per the article, exploring the trailing edge of the heliopause would be interesting and is something that hasn't yet been done. But yes, presumably instrumentation has also improved over the last 50 years, so hopefully we could get better data as well.
Most of the thrust of rocket is loss to get out of the earth gravity well.
I believe that this is wrong.
The Earth weighs 5.96e+24 kg and has radius 3.37e6 meters. The Sun weighs 1.98e30 kg and our orbit has radius 1.496e11 meters. That means that the potential well for getting away from Earth is about 11,800,000 joules/kg while for getting out of the Sun's gravity well is about 882,800,000 joules/kg. Assuming that I did the math right, that's about 7.5 times as hard.
It therefore takes a lot more energy to climb out of the Solar System than it does to climb out of Earth's gravity well. Voyager got a LOT of energy from those gravitational slingshots.
You're trying to compare apples to apples here, when the situation we have is actually apples to oranges ;).
A probe has to launch from the surface of the earth. But counterintuitively the probes start out already in solar orbit, even before they're launched -- because the earth is in solar orbit. More than half the energy required to achieve escape velocity is needed just to get into a roughly circular relatively low orbit around the gravitating body you're trying to escape from, and by virtue of being launched from the earth the probes get that velocity for free. Furthermore, the earth's orbit isn't really "low" with respect to the sun. We're fairly far out there, so the fraction of the energy needed to go from earth orbit to a solar escape trajectory is even less.
Put another way: a probe launched from the earth gets no help leaving earth's gravity well. But once it does it gets a huge automatic gravity assist from the earth itself as it enters solar orbit.
There's a saying among KSP players: "Orbit is halfway to anywhere".
It takes a lot of energy to escape Earth's gravity well. It takes a comparatively tiny amount to escape the Sun's, as a sibling poster pointed out. It would be actually harder to visit, say, Mercury, as you now have to shed all the energy Earth has given you for free, in order to "fall" into the Sun's well.
Voyager wanted to visit multiple planetary bodies – changing orbital parameters is not cheap. But if all it wanted was to get out of the system, burning straight out would probably be cheaper (in Delta-V terms). The closer to Earth the better, for the Oberth effect.
This is correct. On my astrodynamics written qual, one of the questions was whether it would be more energy efficient to solve Earth’s trash problem by launching garbage into the sun or into deep space. Surprisingly, the answer turned out to be deep space, and it wasn’t that close.
> Wouldn't it be possible if we launched from the moon instead?
It would be cheaper in fuel terms, and require less thrust, but it'd also require us to manufacture propellant on the lunar surface and fly everything we can't build there from Earth (and landing on the Moon is purely propulsive).
> By atomic composition, the most abundant element found on the Moon is oxygen. It composes 60% of the Moon's crust by weight, followed by 16-17% silicon, 6-10% aluminum, 4-6% calcium, 3-6% magnesium, 2-5% iron, and 1-2% titanium.
You have oxygen. You have aluminum. You can now make a solid rocket. There's some magnesium there too if you want to use that instead.
It's kind of hilarious that the hardest thing to find in the inner solar system, off Earth (and, of course, the Sun), isn't precisely water but hydrogen.
