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Comment by wartywhoa23 | Hacker News Reader

It is stated multiple times across the article that the probe would need a means of changing is trajectory, but not even a hint of idea how that could possibly be done is given. So the most important and blocking aspect of the mission is simply skimmed over, and the rest of it is built upon this omission as if it was something trivial to come up with.

Does anyone have an idea how to equip a 1g spacecraft with any means to steer itself at 1/3 speed of light? The kinetic energy at that speed would seem to require something very incompatible with the weight constraint, to my understanding.

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Since momentum is conserved, why not just have a 2 of the 1 g probes strapped to each other with a spring in between. When you need a course correction at 100 AU out (or whatever). The probes calculate how much of a correction is needed, adjusts a screw that tightens or loosens tension on the spring, reorients itself appropriately with a reaction wheel, then the two probes are released from each other, begin pushed apart with the spring. One probe gets the trajectory correction it needs, and the other gets further off course. Maybe with some gravity assists with nearby objects.

https://en.wikipedia.org/wiki/Gravity_assist

also:

Roundtrip Interstellar Travel Using Laser-Pushed Lightsails

https://ia800108.us.archive.org/view_archive.php?archive=/24...

Isn’t that basically how a rocket works? Throw stuff out one side to get the thing on the other side moving. Not sure how this would compare to a rocket engine with hyperbolic fuel.

Yes.

It compares in that it doesn't require said hyperbolic fuel. That fuel is heavy and finite.

Uh, it does - the ‘fuel’ is the other probe.

Notably, this also has a particularly bad ISP?

Also, probes are presumably also heavier and rarer?

You can think of the hypergolic fuel as a type of spring. The chemical energy stored in the bonds of the fuel is what pushes the fuel products apart when it reacts. This is what pressure is, it's nothing more than KE of molecules.

The 'spring action' of fuel is very good because there's a lot more energy (per unit mass) stored in the bonds. Orders of magnitude more than a mechanical spring.

But a mechanical spring will reset itself to its natural position once tension is released

Only in the sense that throwing a knife at someone is the same as shooting a howitzer at them.

Specific impulse.

As far as impulse goes, the spring will probably be pretty inefficient relative to mass.

The point we are trying to make is that there hardly anything that would be less efficient relative to mass.

No. Very much no. The spring system would literally throw away half the mass of the craft for, maybe, a 10m/s delta. Fireworks would be more efficient. A pitching machine attached to a huge pile of baseballs would be more efficient (ie the baseballs could be thrown faster).

They all weigh a lot more than 1g though.

https://en.wikipedia.org/wiki/Specific_impulse

Please read.

Because the specific impulse of the spring is negligible when you’re moving at 1/10c and why would they send a 1g probe if they could accelerate 100kg to that speed? Why do you suppose doubling the weight would be free instead of making the system infeasible?

That's just a really, really ineffective rocket. A spring has nowhere near the energy density of chemical fuel.

What would the "screw" push off of? That rotational force would need to go somewhere or be corrected, else the probes would just rotate. I guess a gyroscope could do that, but what you're describing just sounds... very roundabout, and in terms of force, a few kilos of propellant would have the same effect.

This is infeasible for the reason other have mentioned about specific impulse. But surely you can imagine a set of parallel boards with a coil spring between them and a set of cylindrical guide rods to prevent relative rotation between the boards. A motor fixed to one board turns a screw that engages with threaded nut on the other board, mounted on a thrust bearing, and guide bushing that allows a linear movement, but disallows the rotation degree of freedom. Think of the lead screw on a milling machine or lathe.

Can a gyro work in space? Probably not or all the satellites would be using it instead of gas/ionized gas based propulsion.

Gyroscopes are used in spacecraft / satellites in the same manner they are in aircraft, to measure changes in orientation.

Reaction wheels are used to make adjustments to orientation. See https://en.wikipedia.org/wiki/Reaction_wheel

Thrusters of all sorts can also be used, generally to maintain altitude in satellites, and more generally to provide thrust for space probes.

It's not skimmed over, they cover it near the end in the "Requirements and challenges" section:

> The most challenging phase of the mission may be related to how the nanocraft can transfer from an unbound to a bound orbit and start orbiting around the compact object. All possible solutions should be considered carefully. In the case the transfer is not possible, we may redesign the mission to perform the scientific tests when the nanocraft passes close to the black hole. For example, when the nanocraft is close to the black hole, it may separate into a mother-nanocraft (with a wafer and sail) and a number of small nanocrafts (without sails). The nanocrafts could communicate with each other by exchanging electromagnetic signals. The mother-nanocraft could compare the trajectories of the small nanocrafts to those expected in a Kerr spacetime and send the data to Earth.

Light sales can theoretically be used to not only accelerate away from Earth, but also decelerate at the end of an interstellar journey (see Robert L Forward's work). The practicality of that is another matter.

There’s a really straightforward way to avoid a parabolic trajectory with a black hole. But data retrieval gets a bit difficult.

More seriously, it floors me how often and consistently people forget that the accretion disk is essentially a partial accelerator and crossing or entering it will probably pulverize you to radioactive dust. Possibly before you could hit the event horizon.

Not every black hole has an accretion disk, especially not isolated ones.

So the question there is if it’s worth it for us to study a naked singularity. What would it teach us?

And how do you get into orbit?

Is it worth studying?

Yes. Yes it most certainly is. Whatever we learn will be new.

Naked singularities lack event horizons, not accretion disks. By definition, black holes are not naked singularities.

> It is stated multiple times across the article...

I was a bit confused by your comment, but I think the article you're referring to is not the OP, but the article the OP was commenting on: https://www.cell.com/iscience/fulltext/S2589-0042(25)01403-8...

> Does anyone have an idea how to equip a 1g spacecraft with any means to steer itself at 1/3 speed of light? The kinetic energy at that speed would seem to require something very incompatible with the weight constraint, to my understanding.

I'm also wondering how such a thing is supposed to communicate back to us over dozens of light years. That also seems incompatible with the weight constraint.

>I'm also wondering how such a thing is supposed to communicate back to us over dozens of light years.

Split particle pairs. We just need to repeal the no cloning theorem, maybe if we promise to not use it for FTL communication the legislators would go for it.

We could call them "Sophons" while we're at it.

>I'm also wondering how such a thing is supposed to communicate back to us over dozens of light years.

Just spit-balling here. Send out the first batch of probes and then 5 years later send another batch of probes. The first batch of probes does their surveying for 5 years, when the later batch of probes start arriving. The data is uploaded to the late-comers, who aren't on an intercept course. Instead they are on a trajectory that causes them to swing around the black hole, and head on back to earth with the data.

> Send out the first batch of probes and then 5 years later send another batch of probes.

What's the separation there, at 0.33 lightspeed? 1.65 light years? Wikipedia says Voyager is 168.35 AU away, and Google says that's 0.00266 light years. Voyager has 23-watt radio focused by a 3.7m dish and its signals are received by a 70-meter dish on Earth.

So you're talking about a 1g spacecraft signaling another 1g spacecraft over 620 times the distance to Voyager, without any of the beefy equipment that exists on both ends of the Voyager link.

Hmm. Seems like you are you multiplying 5 years by 33% of light speed to come up with 1.65 light years? I apparently didn't explain well enough. The 5 years is for the first batch of probes to gather data over an extended period of time (while in orbit around the black hole). The second set of probes is just a roundtrip "fly-by" to collect the data from the first probes and return it to earth. No reason that the return trip probes would have to be very far from the data gathering probes. Maybe you can't orbit close enough to the black hole at these speeds without getting too close to the accretion disk?

Even then, I think you're going to have massive distances between tiny probes moving very fast relative to each other. Maybe not 1.65 light years, but communication over Voyager's 0.00266 light years or even a much smaller distance (e.g. Earth to moon) seems insurmountable for two 1g probes.

Also, the probes are in deep space, right? No solar power. Where are they going to get the energy?

https://en.wikipedia.org/wiki/Betavoltaic_device ?

So skip the communication, and just have the one probe do the gravity assist to get headed back to earth with the data it collected.

The launch system isn't consumed by launch, so launch them as often as necessary to keep the communications gap as small as needed.

Not like a 1 gram probe is going to be expensive compared to the launch system.

People don't realise this, but you can steer perfectly fine with a solar sail. That's because photons transfer momentum not just when they hit the sail, but also when they are emitted after reflection. So just by turning the sail at an angle, you can create a force in any direction perpendicular to the velocity vector. Using a two sail system, you can even accelerate and slow down along a single beam path. So you could theoretically travel to mars with a constant acceleration/deceleration phase (like a flip-and-burn in the Expanse) using only one beam emitter on earth.

How long would it take for a person to get to Mars with a sail powered by an Earth-based laser?

Infinite time since we have no realistic way of making such a laser at this time - and anyone trying it is likely to get nuked before they finish their massive death ray.

Stipulate that we have the laser.

Depends entirely on the power output of the laser, the dispersal length and the size, weight and reflectivity of the sail. Since we are already talking about imaginary technology, any answer between "slower than current chemical rockets" and "some significant fraction of the speed of light" could be valid.

I don't think that is correct. I think there's a rather disappointing limit on the acceleration a laser sail can reach with known materials.

You explicitly asked for unknown materials. Because there is no known material because such a laser does not exist. We have solar sail prototypes that were using the sun's light, but even they are far off in terms of size/weight ratio and reflectivity.

If we’re just making stuff up, why not just have a warp drive or something?

No, I didn't ask for unknown materials; lasers can be built with known materials. Also solar sails aren't lightsails, and the topic here is lightsails.

Simply. You do Monte Carlo with the probes. You fire 1000 and one or two will have perfect trajectory so that no correction is needed.

Did you, by any chance, play Outer Wilds recently?

I don't think 1000, or even 1000000 are enough if you use random directions. Space is <huge>huge</huge>. This has been posted here afew times https://joshworth.com/dev/pixelspace/pixelspace_solarsystem....

> So the most important and blocking aspect of the mission

Idk, i think the fact they are using statistical arguments that there should be a nearby black hole, but haven't actually found any or have any idea where they are, is pretty blocking.

"steering" is a word that can lead to confusion because it leverages the intuition that we have with our ground vehicles.

A change in direction in space requires accelerating the vehicle in some direction, the effect of which is just simple vector addition of the velocity vector of the vehicle.

So if you are going with a huge velocity in one direction and you want to change direction significantly in another direction you have to change velocity (accelerate) a lot in order for the combined vectors to produce a significantly different final velocity vector

Easy fix: change the description to "Interstellar Mission to the General Galactic Vicinity of a Black Hole"

Solar sails. You can fire a shit ton of lasers from the planet (or orbit) at the probes and very,very slowly boost them up to the desired velocity.

gyroscopes?

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