Manned mission to Ceres

[KelvinZero]

Im not normally a fan, but how about a rotovator?
(edit: just did the math and got 6km radius for a 1g, 0.25km/s system. nah... just landing on rockets where you want and leaving when you want is far more attractive)

Another interesting question is what to use instead of rovers (including robotic rovers). I imagine some sort of bouncing thing. Or maybe just very large but light wheels so that flipping over is not a problem?

Also, how about some sort of reaction wheel for rovers and suits. I realise you can't get angular momentum for free, but the important thing for a suit is that you are helmet-side-up at the instant you return to the surface after a step. Maybe you would also have some sort of cold gas thruster in the occasion that the wheel becomes saturated? Or maybe all you need is a shock absorbing cage around the helmet so you can land on it as often as you like.

How much mass to get a cereanaut back to orbit? Perhaps this could be an option of every suit in addition to the lander?

[Robotbeat]

Not if you're using solar-electric propulsion, like was used to reach Ceres in the first place and is currently the favorite for Mars missions.

Ion engines have very weak thrust. Even on Ceres T/W woul be less than 1. Chemical would be need for ascent from Ceres surface as well as soft landing.

Well, not really chemical. You could, actually, land and/or launch with cold gas or warm gas thrusters. Even a bottle of superheated steam would technically work.

But I agree you're not going to be doing that landing with a 3000s Isp ion thruster. Not happening.

[Nomadd]

Not if you're using solar-electric propulsion, like was used to reach Ceres in the first place and is currently the favorite for Mars missions.

Ion engines have very weak thrust. Even on Ceres T/W woul be less than 1. Chemical would be need for ascent from Ceres surface as well as soft landing.

I was always amazed at the tiny size of the tanks on the lunar ascent module. A plate of beans for dinner the previous night would probably be enough to take off from Ceres.

[MATTBLAK]

With only 2.8% percent of a G on Ceres, it's possible...

[Paul451]

Ion engines have very weak thrust. Even on Ceres T/W woul be less than 1. Chemical would be need for ascent from Ceres surface as well as soft landing.

I think the assumption with Ceres is usually that ISRU once you get there is going to allow simple chemical rockets in the region. SEP/NEP is for long distances.

General aside:
The consensus seems to be that SEP is probably sufficient for a Ceres mission; while Ceres itself is a little far out, most of the acceleration happens inside of Mars' orbit where SEP is more mass-efficient than NEP. However nuclear may be more appropriate than solar for operations at Ceres. In which case, it would make sense saving mass by using the same reactor for your electric-drive on the trip out. You have to carry the thing anyway.

Im not normally a fan, but how about a rotovator? (edit: just did the math and got 6km radius for a 1g, 0.25km/s system. nah...

Why 1g? Humans handle 3-4g during launch easily.

[KelvinZero]

Im not normally a fan, but how about a rotovator? (edit: just did the math and got 6km radius for a 1g, 0.25km/s system. nah...

Why 1g? Humans handle 3-4g during launch easily.

Just an example. You just divide by acceleration so 3g gives about 2km and so on.

I think I would have considered it further if the dimensions were similar to typical artificial gravity spacecraft. There was a big thread saying AG is not needed, but suppose a spinning AG vehicle design happened to trivially solve the landing on Ceres? That turned out not the case, and then on top of that you have all the extra disadvantages such as only landing at equator, and tricky unproven pickup only at very precise points in time. Rockets are just way more practical for exploration.

I can definitely see someone arguing rotovators for trading with a colony there, or something like that.

[Paul451]

(edit: just did the math and got 6km radius for a 1g, 0.25km/s system. nah...

Aside: The thing is, 6km is not that difficult a tether length, 2km even easier. We've already played with similar lengths. (Not well. But... {waves hand} ...it's down to practice and engineering, not materials or fundamental technology.)

all the extra disadvantages such as only landing at equator, and tricky unproven pickup only at very precise points in time.

Meh. We can only launch from Earth from specific locations during certain windows. With a synchronised rotovator (where the tether-spin is in some exact proportion of the circumference, so it "touches" the same set of points) you have the locations for your settlements and "space-ports". Given how easy it is to move around on the surface, those should be sufficient as your entry and exit points.

I can definitely see someone arguing rotovators for trading with a colony there, or something like that.

For cargo trading (launching ice/fuel/air, for example), you also have the option of a horizontal rotating arm on the surface. The stupidly low gravity allows you to build a long rotating arm to throw cargoes into orbit, without the tip velocity (or droop from gravity) exceeding the material limits. The simplest would be a non-rigid tether, of course, but spin-up/down would be harder. You have to winch in and out each time. Winches loves teh vacuum.

In theory, using a long enough non-rigid (winched) cable, you could even launch humans with a horizontal throwing arm. That'd be a fun system to design though.

[Hop_David]

Im not normally a fan, but how about a rotovator?

Ceres is a great place for a Clarke/Artsutanov style space elevator. The small gravity well and high angular velocity make this much less challenging.

Ceres synchronous orbit is 706 km above Ceres' equator. A 1922 km tether would be long enough to maintain tension. There's not much stress on the tether so the taper ratio is 1 even for ordinary materials like Kevlar.

An ion engine could dock with a Ceres elevator. One of my favorite day dreams is lots of xenon dissolved in Ceres' subterranean oceans.

The speed at the top of a 1922 km Ceres elevator would be about .45 km/s. Not enough for injection to a Ceres to Earth transfer orbit but it would help.

A Ceres elevator 15,000 km tall would suffice to hurl a ship down to 1.52 A.U. perihelion, the neighborhood of Phobos and Deimos.

[Robotbeat]

Argon works nearly as well as Xenon.

But there ARE electric thrusters which can operate on water.

[Hop_David]

Argon works nearly as well as Xenon.

But there ARE electric thrusters which can operate on water.

And there may be an abundance of water on Ceres. I'm hoping those white spots are water that has made it to the surface after meteorite impacts punctured the surface.

Besides great ISP, ion engines also seem to have great durability. I had assumed that was due at least in part to using noble gases as the reaction mass. Would an ion engine that uses more chemically active propellent be as long lived?

[Hanelyp]

Would an ion engine that uses more chemically active propellent be as long lived?

Taking "ion engine" to mean the more general class of electric thrusters, that depends on the engine.  Electrodes tend to be a difficulty for longevity.  Some electric thrusters energize the propellant by induction instead.

[KelvinZero]

Meh. We can only launch from Earth from specific locations during certain windows.

Im interested about what that means for Ceres. Would a manned mission there be restricted to certain locations? My google search suggested you need about a 1/3 km/s if you are not exploiting the equator, 0.25km/s at the equator but someone should check that.

I guess i just don't see the Rotovator as an enabling technology for Ceres, I mean it isn't the thing that will make us say "Hey! we can do Ceres!". The only key requirements seem to be SEP, long term lifesupport, and the confidence for really long missions.

What about radiation? Mars durations seem ok from discussions here though apparently not within NASA guidelines.

[Robotbeat]

By the way, electromagnetic launch sleds, like are used in the newer supercarriers, can get about 100m/s in operation. You may be able to improve that to 200m/s, then you just need another 70m/s or so to circularize the orbit (assuming you're launching from the equator, which gives you about 90-100m/s advantage).

[Paul451]

I guess i just don't see the Rotovator as an enabling technology for Ceres, I mean it isn't the thing that will make us say "Hey! we can do Ceres!".

It's not really an enabler for a first manned mission. I think it was proposed more as a long term thing. (Ie, "what if a Ceres settlement is trading with other settlements?")

However, a 5-10km tethers really aren't that bad, technologically speaking. It might be a way of landing robotic explorers without needing chemical rockets. That means you can do a bunch of Curiosity-Class robotic landers with just SEP. (Too early to trust a 2-years-in-vacuum tether and winch to work reliably enough for a manned mission. But you could probably say the same about any chemical-rocket lander.)

The only key requirements seem to be SEP, long term lifesupport, and the confidence for really long missions.

It would be nice to have some data about the availability and nature of the ice. Allowing you to pre-send ISRU infrastructure. (Hopefully Dawn, soon. But I suspect more detail will be required.) That makes your mission a little easier if you know you can get at least water and air when you arrive. Even better water, air, and fuel.

[A_M_Swallow]

Not if you're using solar-electric propulsion, like was used to reach Ceres in the first place and is currently the favorite for Mars missions.

Ion engines have very weak thrust. Even on Ceres T/W woul be less than 1. Chemical would be need for ascent from Ceres surface as well as soft landing.

With a bit of thought the chemical thrusters can form part of the RCS. Choosing the right thruster will permit a 5 second burn to get the spacecraft to orbit.

[AlanSE]

I will try to do somewhat of an objective comparison between a GEO tether and chemical propulsion for landing on Ceres. I will assume the equator.

Chemical

Let's assume propellant ejection velocity of 3,000 km/s to be realistic. The equator of Ceres is spinning at roughly 95.5 m/s. The orbital velocity at the surface is 360 m/s, making for a difference of 264.5 m/s neglecting gravity drag. That should be pretty small anyway since the gravity is so little. We could presumably use solar-electric or nuclear-electric ion drives to get to orbit to begin with.

Mass ratio comes out to a ballpark of around exp(-264.5/3000) = 0.9156, meaning that the mass of propellant to payload as a ratio is about 0.092

GEO Tether

Yes, we can assume a taper ratio of 1, but that doesn't mean the tether is massless. The longer the span is, and the higher the tension is, the heavier the tether will be. I will denote specific strength by "sigma".

M_tether = M_payload g L / sigma
(M_tether/M_payload) = (1922 km)*(0.025*9.8 m/s^2)/(2514e3 m^2/s^2) = 0.1873

/end comparison

Interestingly, the mass penalty would be almost the same for both options if the chemical rocket both sets down and lifts back up. If you assumed in-situ resource use for the trip back up then clearly rockets will win for this particular comparison. But that's not very fair.

Obviously the rotovator would perform vastly better (because of shorter length), but if you spun up with propellant in the first place you'll defeat the entire point. In fact, the only way I can see this making sense is if you're already using the tether for artificial gravity. Honestly, that's a bit larger than what would normally be practical for artificial gravity. Too much risk to dangle on a several-km tether for many years. There might be propellant-less ways to spin up the tether as you get close to Ceres, but I don't know what these would be. Actually, you could use a slender 1,000 km tether to make direct contact with the dwarf planet while your craft is in GEO. Then you could use this anchor to slowly spin up your rotovator. You still have all kinds of wonky problems with firing an ion engine at the fulcrum of a rotating structure to lower orbit, but as an academic argument it certainly shows that direct momentum exchange activities are viable if you plan to be spending much time or doing enough operations there to make it worth the time.

[Nilof]

Hmm, what about battery-powered low-isp electrothermal thrusters? Normally these are not practical because they have a maximum mass ratio which is too low for most applications, but in this case, they might be a practical option for a hopper using local water as propellant.

[KelvinZero]

It's not really an enabler for a first manned mission. I think it was proposed more as a long term thing. (Ie, "what if a Ceres settlement is trading with other settlements?")

Fair enough. I was just looking at "suppose we wanted to go now"

Longer term, I like the idea of a magrail on the equator. It not only provides access to the rest of the solar system but also local transport and sharing of solar power between day and night. Even more extravagantly, I think all these dwarf planets will become ocean worlds under protective eggshells of ice once we have human industry buried there producing waste heat. There is a bit of math to back that up. It wouldn't take extravagant power levels.

Initially I was wondering why this thread was in advanced concepts but I think there is one good reason: there are moon firsters, mars firsters and asteroid/ARM firsters. No one is seriously proposing Ceres first so this is just a fun academic exercise unless we spot a monolith there or something. If we ever go to Ceres we will already have another world under our belt. We probably won't even think of it as a manned mission in the apollo sense but as extending our domain.

[Nilof]

Initially I was wondering why this thread was in advanced concepts but I think there is one good reason: there are moon firsters, mars firsters and asteroid/ARM firsters. No one is seriously proposing Ceres first so this is just a fun academic exercise unless we spot a monolith there or something. If we ever go to Ceres we will already have another world under our belt. We probably won't even think of it as a manned mission in the apollo sense but as extending our domain.

I think this is mostly because Ceres is the hipster planet that few people actually know about. Up to now we didn't even have any sharp pictures of it. No spacecraft we could build could reach it at a reasonable expense. We are still speculating on its geology.

Dawn has indeed changed the equation noticeably, and Ceres definitely looks much more reachable. Though certainly not easier than Phobos or Deimos.

Longer term, I like the idea of a magrail on the equator. It not only provides access to the rest of the solar system but also local transport and sharing of solar power between day and night. Even more extravagantly, I think all these dwarf planets will become ocean worlds under protective eggshells of ice once we have human industry buried there producing waste heat. There is a bit of math to back that up. It wouldn't take extravagant power levels.

Ceres is indeed a very fun world to think about colonizing. The centigravity environment makes it rather unique. I imagine that colonies there would have some rather interesting architecture. The gravity is just barely low enough that having a dedicated floor in a room may or may not make sense in all situations.

[Stormbringer]

Happily: Ceres is due for a promotion and so is Pluto:

http://blogs.discovermagazine.com/crux/2015/02/25/pluto-ceres-planet-again/#.VPXHIOGoPcQ