# With current technology, could we build a space probe that would catch up to one of the Voyagers?

If so, how long would it take?

The Voyagers are speeding away at about 17 km/s, which is much faster than the 4.4km/s high exhaust velocity max for a Liquid oxygen-liquid hydrogen rocket.

You need to use the Oberth effect to speed up past that using various planetary bodies gravity well. Unfortunately the math to calculate an intercept and timing is non-trivial so I can’t help out there.

By “catch up to”, do you mean just “reach the same distance”, or “pass close enough to it to be able to rendezvous”? If the former, then the New Horizons probe (the one that flew by Pluto) will do it in fairly short order. If the latter, well, there’s no reason we couldn’t make another probe like New Horizons, but aim it in a different direction.

Thanks. I meant the same distance. When will New Horizons get that far?

The main boost mechanism is gravity assist. What engine firing is done at closest approach is for maximizing the change in direction with the smallest amount of fuel. Probes don’t carry enough fuel to make nearly as big an impact on speed.

The Voyagers took advantage of a particular alignment of planets. The particular alignment used for those occurs once every 175 years. But the New Horizon-type systems don’t need such a perfect alignment.

What about ion thrusters like HiPEP, VASIMR or NSTAR? Seems like if we wanted to, they could use those to eventually catch up to the Voyager probes, given enough reaction mass and gravity assists.

Depends on what you mean by “current technology”, and just how badly we really needed or wanted to intercept the space probe in question.

My understanding is that with something like Project Orion, it would be quite do-able, and, assuming we could overcome the engineering challenges involved in such a thing, might not even take all that long to intercept the Voyager in question once it was launched. (We’re talking about a spacecraft with a maximum velocity of maybe 3% of light speed, which may not sound like much, but is hundreds of times faster than the Voyager probes or even New Horizons.) Project Orion is pretty out there, but it isn’t like a warp drive or anything like that; there don’t seem to be any “First, discover a substance with negative mass…” steps involved.

Overcoming the political challenges involved in such a project would–I hope!–be even more formidable than the engineering issues. We’re talking about launching a large spaceship from the surface of Earth by blasting it into space with multiple thermonuclear explosions; unless we had incontrovertible evidence that anthropophagous aliens were about to intercept Voyager 2 and take the famous Golden Record as an invitation to set up a new Rigellian Fried Human franchise on Sol III (“Rigellian Fried Human! It’s tentacle-lickin’ good!”) launching such a mission would be a fairly batshit crazy thing for our civilization to do.

The fact that Voyager used Jupiter, Saturn, Uranus, and Neptune for those gravity assists and that window won’t open up again until ~2151 as you mentioned, is why I mentioned the Oberth effect. You would have to figure out how to carry enough fuel to use the Oberth effect, just as New Horizon did but at a larger scale.

I suppose you could use lots and lots of inner planet accelerations like the Parker Solar Probe used, but in the other direction but it would be hard and you would probably have to extend the Ephemeris past the current used PPN terms.

Isn’t this the exact definition of not current technology?

Well, havent’t the voyagers come to a stop after bumping into the edge of the heliosphere and whatever is pushing back against that? Oh, OK I see recent news.

FWIW, the Dawn probe, which was powered by NSTAR derived ion thrusters, achieved a 41,360 kph total velocity change.

https://dawn.jpl.nasa.gov/news/news-detail.html?id=7177

Starshot could do it.

“The probes would be accelerated to 20 percent of the speed of light in about two minutes… This velocity would get the probe past the orbit Pluto in three days and to the nearest star in 20 years.”

Are you saying that if I’m traveling away from Earth at 4.5km/s and chuck stuff out the back at 4.4km/s that I won’t accelerate? Because I don’t think that’s how Newton’s 3rd Law works. The Tsiolkovsky Rocket Equation is going to put a practical limit on how much delta-V you can get out of a rocket with 4.4km/s exhaust velocity, but that limit should be significantly higher than 4.4km/s or orbital rockets wouldn’t work.

No, I am saying the faster you can toss mass out of the back of the rocket the more efficient it is.

The Tsiolkovsky rocket equation, or “delta vee” will be cruel if you do the actual math though.

Δv = v[sub]exhaust[/sub] * ln(mass[sub]0[/sub]/mass[sub]1[/sub])

If your rocket weighs 1,000,000 kg, with an only unrealistic dead weight of 10 kg and you have exaust that is 4.4km/s you could get up to 50.65 km/s. If you can get it to where your non-fuel total rocket mass is 1% of the total mass with fuel you would only get to 20.26 km/s. If you have 50% fuel you can only get to 3.0 km/s.

The natural log will ruin your day even with the ideal rocket law.

Isn’t this also the exact definition of not current technology?

From what I understand it wasn’t the engineering challenges that stopped Orion, but the results of the engineering challenges that stopped it. Basically to perfect it they would have to detonate a lot of nukes in mid-air and obviously that won’t have very good ecological consequences. If we absolutely did not give a shit about fallout Orion could be built today pretty easily.

Sure, why not? How hard could it be? It’s not Rocket Sci…

Oh, wait a minute…