Catching Sedna

[mrfox]

Unfortunately, 50 years from now robotics and automation will almost certainly have improved at a faster pace than human space flight, so it is likely that a manned mission will cost much more and be able to do much less. Rockets have hardly changed in the last 50 years, while computers are orders of magnitude smaller, lighter, faster and more energy efficient.

Still, that doesn't mean it isn't worth discussing. Also, a manned mission is definitely more entertaining.

A robotic mission can do more per dollar, but a combined manned/robotic mission would provide the maximum overall return, full stop, given the time constraint. The crew won't be there to do the lifting and grunt work, but to operate the probes and machinery in real time, make decisions, provide feedback, do repairs and modifications, even create tools/machinery/experiments on the fly - all the sorts of other things that limits the operational flexibility of fully robotic missions of today.

[Boltrax]

Why can't I land on the rocket hangar

[Streetwind]

Good thread Streetwind, stickied for September

Whoa, what? I didn't even notice until now!

Thought the thread had already run its course since replies were kind of drying up, but maybe we'll get some new insights still. (The AI discussion is definitely intersting.)

[fenderzilla]

to rendezvous with an object in space, don't you have to match velocities? if we wanted to land a probe on sedna we'd need absolutely insane delta-v to get up to a reasonable relative speed. not that that's hard to image 5 or 6 decades in the future.

[Streetwind]

Indeed you do. If you send a vessel to cross or touch Sedna's orbit, then it will perform a flyby (if Sedna happens to be there at the time of crossing). If you want to capture into orbit though, then the craft needs to have the necessary dV onboard to accelerate itself to solar escape velocity. Sedna's orbital speed is about 99% solar escape velocity. Now of course, we already sent probes to solar esape in the 70's, but that was done with a chain of gravity assists. A Sedna mission would likely be able to get just one assist from Jupiter to help fling it out to meet Sedna's orbit, but more acceleration would then have to take place once there.

However, the problem isn't as big as you think. Here's a dV map I linked earlier in the thread. Presuming ideal conditions (a Hohmann transfer burn at periapsis with no inclination change), going from low Earth orbit to Earth-Jupiter intercept would cost 6300m/s dV. Compared to that, it takes another 2450m/s to go from Jupiter to a Sedna intercept, and some of that dV would be provided by the substantial gravity assist. Finally, it takes about 820 m/s at the Sedna intercept to capture into orbit. Which means the capture burn that accelerates you to Sedna's orbital speed is actually just ca. 10% of the dV budget.

Now of course, you need to add a significant inclination change for Sedna, but that's to be performed on the way (during the gravity assist, likely). The bigger problem is that the travel time for a Hohmann transfer is extreme. You want an accelerated transfer instead, which increases not only the dV expended on the early legs of the trip, but also the dV required for the final capture burn. It's a bit of a vicious circle: How fast do we want to arrive ---> What kind of propulsion system do we need to produce that kind of dV ---> How much power to we need to drive that propulsion system ---> Do we have a power source strong enough that can live through that mission duration ---> How fast do we want to arrive ----> etc.

[juanml82]

A robotic mission can do more per dollar, but a combined manned/robotic mission would provide the maximum overall return, full stop, given the time constraint. The crew won't be there to do the lifting and grunt work, but to operate the probes and machinery in real time, make decisions, provide feedback, do repairs and modifications, even create tools/machinery/experiments on the fly - all the sorts of other things that limits the operational flexibility of fully robotic missions of today.

A manned mission requires a more heavy spaceship and poses the problem that astronauts will suffer radiation poisoning - and that's assuming it uses centrifugal force to simulate gravity and has a decades long reliable life support system.

People were never sent in such long duration missions and those who expended months (and over a year) in space stations did it below the Van Allen belts.

[Rakaydos]

A manned mission requires a more heavy spaceship and poses the problem that astronauts will suffer radiation poisoning - and that's assuming it uses centrifugal force to simulate gravity and has a decades long reliable life support system.

People were never sent in such long duration missions and those who expended months (and over a year) in space stations did it below the Van Allen belts.

And so it comes down to either getting as much science done as possible in a single once-in-forever event, or building a cheaper, easier to design robotic craft and hope you stuffed it with everything it needs.

[NERVAfan]

Indeed you do. If you send a vessel to cross or touch Sedna's orbit, then it will perform a flyby (if Sedna happens to be there at the time of crossing). If you want to capture into orbit though, then the craft needs to have the necessary dV onboard to accelerate itself to solar escape velocity. Sedna's orbital speed is about 99% solar escape velocity. Now of course, we already sent probes to solar esape in the 70's, but that was done with a chain of gravity assists. A Sedna mission would likely be able to get just one assist from Jupiter to help fling it out to meet Sedna's orbit, but more acceleration would then have to take place once there.

New Horizons launched directly into a solar escape trajectory on rocket power alone; it did do a Jupiter gravity assist to speed up later, though. The third stage is also on a solar escape trajectory...

So I'm pretty sure existing rockets could do it for an unmanned probe, theoretically. But...

The bigger problem is that the travel time for a Hohmann transfer is extreme.

Yeah, exactly.

A manned mission requires a more heavy spaceship and poses the problem that astronauts will suffer radiation poisoning - and that's assuming it uses centrifugal force to simulate gravity and has a decades long reliable life support system.

People were never sent in such long duration missions and those who expended months (and over a year) in space stations did it below the Van Allen belts.

Maybe we'll have fusion engines by the 2080s; that could reduce the trip to a reasonable length.

Also, medical technology may have advanced by then to the point that cancer isn't really an issue. (You'd still need some kind of shielding for solar flares though.)

[Laie]

Presuming ideal conditions (a Hohmann transfer burn at periapsis with no inclination change), going from low Earth orbit to Earth-Jupiter intercept would cost 6300m/s dV. Compared to that, it takes another 2450m/s to go from Jupiter to a Sedna intercept, and some of that dV would be provided by the substantial gravity assist. Finally, it takes about 820 m/s at the Sedna intercept to capture into orbit. Which means the capture burn that accelerates you to Sedna's orbital speed is actually just ca. 10% of the dV budget.

Also, what kind of dV would you need to get back? I'm afraid that manned is out of the question -- the crew would spend pretty much their entire adult lives on that mission.

[metaphor]

It's not a great coincidence that we discovered Sedna when it was close to its periapsis. Our current instruments can only detect objects like Sedna out to ~100 AU. Sedna spends approximately 1/80th of its orbit close enough to the Sun to have been discoverable. That means that there's probably ~80 more objects out there with orbits and sizes similar to Sedna's, but not currently close enough to the Sun in their orbits. So by the time we would send a probe to Sedna in the 2080s or so, there will probably already be a few more of these kinds of objects that are nearing their periapses (for example, 2012 VP113, which is half the size of Sedna and in a similar orbit, was recently discovered). Also by that time, our detecting capabilities will have probably progressed enormously, so we could detect such objects out to 1000 AU or more. If there's more objects like Sedna out there and they follow a normal size distribution, the largest one could be about the size of Mars. Now that would be an interesting object to send a probe to.

[Streetwind]

A good point, actually. If there are many more objects of this kind, that also throws up the question of their origin again. It was said it's possible that Sedna was captured from interstellar space, a wandering object ejected from its former parent that fell into the solar system's gravity well and got stuck there (maybe at the expense of something else). But if there really are several dozen like Sedna, that would mean a different origin is far more likely. Maybe the Oort cloud origin. Maybe a different class of objects that we don't know is even a thing yet.