Smallsats + solar sails = Photos of exoplanets at 1970s digital camera resolution

It's time to stop hemming and hawing about the monetary and temporal costs of exploring the outer solar system and beyond, say an international group of boffins. We've all the materials we need to do it faster and cheaper by combining modern smallsats with solar sails, and the end result could be actual photographs of exoplanets …

[jmch]

Proper sailing spaceship????

The traditional images I've seen of solar sails are basically like giant parachutes, and as I understand it, they can accelerate only in the direction of the solar wind, directly away from the sun. However it should be possible to use a mast and spars, just like in a sailing ship, to hold the sail at an angle and therefore be able to accelerate in a diagonal (away from the sun but not directly away, having a bit of sideways component). Given the lack of gravity and the minuscule forces involved, it should be able to work with extremely large 'masts' and 'spars' without reaching forces large enough to break them - the limit would be more how much you can fold / unfold them.

I wonder if it would also be possible to tack 'downwind' so to speak i.e. diagonally towards the sun. In real sailing this works because the full sail acts like a wing, using the Bernoulli principle of lower pressure where airflow velocity is higher. Since the solar 'wind' is essentially a stream of particles, however tiny they are, would the Bernoulli principle still work??

[KittenHuffer]

Re: Proper sailing spaceship????

"I wonder if it would also be possible to tack 'downwind' so to speak i.e. diagonally towards the sun." - Technically that would be upwind rather than downwind, as you would be heading towards the source of the wind.

Yes, I believe that it is possible. What you would be doing is to angle your sail such that you would be reducing your orbital velocity. This would then cause your orbit to shrink, and you'd find yourself closer to the sun. It is actually the case that from the Earth more delta-v (change of velocity) is required to get something to hit the Sun than for it to leave the Solar System.

I believe that the proposed sails would be set to work the other way. They would be increasing their orbital velocity, which would raise their orbits, and eventually lead to solar escape velocity.

[Peter Mount]

Re: Proper sailing spaceship????

You might want to look up the Light Sail 2 mission by the Planetary Society. It showed it was possible to perform orbital maneuvers using just the sail.

[Anonymous Coward]

Re: Proper sailing spaceship????

With the sail at an angle you will get an angular force. As it will begin in a solar orbit, to raise the orbit and move away from the sun at all, the sail will be angled to increase the speed. It's possible it could be launched into Earth orbit and use the sail to leave earth orbit and get into solar orbit, but I'd expect a chemical rocket would do that easier.

[Little Mouse]

Re: Proper sailing spaceship????

Surely you can't steer if you don't have something to "push" against? A sailing boat can be steered because the keel & body of the boat meet the resistance of the water. Steering a solar sail would be more like trying to steer a hot air balloon - impossible to do in any direction other than the one that the wind is taking you, even with special sails for steering.

So in that sense, no, it wouldn't be possible to tack into the solar wind, because there's no resistance to space. However the idea of using orbital mechanics to "cheat" and slingshot closer to the sun is quite neat.

(Disclaimer - I am not a sailor and know bugger all about boats really, so am looking forward to being proved wrong and learning something new...)

[ThatOne]

Re: Proper sailing spaceship????

> Surely you can't steer if you don't have something to "push" against

Was about to say the same thing. There is a reason sailing boats have those big keels, thanks to them and their water resistance, the boat is only (mostly) able to go in the two directions of least resistance (ahead and backwards), no matter where the wind comes from. Without a keel they would just drift in whatever direction the wind pushes them.

[jmch]

Re: Proper sailing spaceship????

I did some amateur sailing long ago, so I might be talking bollocks, and would welcome corrections from any sailor and/or physicist who knows better, however....

Sailing boats have big keels, often weighted at the bottom, primarily so that they don't capsize, although it's true that both the keel and the shape of the boat make it easier to steer.

A square sail (think viking ship type) only really works in the direction the wind is pushing, to within a few degrees. A lateen (triangular) sail uses the boom to angle the sail diagonally with respect to wind direction, and the wind 'filling' the sail gives it a wing shape where the air flow of the wind flows faster over the front side of the sail than over the back side. And just like a wing, thanks to Bernoulli's principle, the sail can generate a force on the mast that is almost perpendicular to the direction of the wind. So I believe that theoretically a lateen sail mounted on a round inflatable dinghy with no keel could steer diagonally to the left or to the right of the exact wind direction (because the force on the mast is directly translated to direction of travel).

The keel and shape of the boat allow the boat to travel at an angle with respect to the force generated on the mast, which allows the angle of travel of the boat to be larger than 90 degrees with respect to the wind, and that also allows the boat to tack upwind in a series of zigzags.

In space there is no resistance that could allow any shape of spaceship to move in a direction different to that of the force on the mast, but I believe that a specifically designed mast and sail combination could allow angling the sail to produce a force on the mast (and on the spaceship) that is at an angle to the direction of the solar wind.

EDIT - Seems like I answered my own earlier question following some additional thought - ie you could sail in most directions as long as the distance from the sun is increasing, and you couldn't use a solar sail to get closer to the sun

EDITED EDIT - all expressed more succinctly in vulch's comment below

[phuzz]

Re: Proper sailing spaceship????

The keel doesn't need to be that big, or be weighted, it just needs to go through the water better in a forward direction, than it does sideways. Otherwise you're correct.

[Inventor of the Marmite Laser]

Re: Proper sailing spaceship????

Yeah. A wherry is sail powered and a flat bottom..

[Anonymous Coward]

Re: Proper sailing spaceship????

The amount of garbage spouted here about how sails work on sailing ships and how this affects a solar light powered spacecraft is unbelievable :( Both for factual errors and complete irrelevance!

The spacecraft starts in a gravity field travelling very very quickly so its basically in orbit. It then adjusts and balances its speed against gravity and radiation pressure to go to (and past!) a lot of places in the solar system, both nearer and further from the sun. If it wanted, it could use the sail to slow it down so it fell into the sun.

[ThatOne]

Re: Proper sailing spaceship????

Dunning-Kruger at its finest.

_{And you know it, since you posted anonymously...}

[ThatOne]

Re: Proper sailing spaceship????

> theoretically a lateen sail mounted on a round inflatable dinghy with no keel could steer diagonally to the left or to the right of the exact wind direction

The problem here is that the boat itself would turn due to the asymmetric pressure on the sail, and always sail (drift) in the wind's direction. That's what a keel is preventing. Now if you had two masts, you could use one sail to prevent the boat from turning and the other to push you forward, but it would be hugely inefficient, and still you would drift about as much as you'd move forward.

You can sail without a keel, but only in a limited angle to the wind. The use of a keel (which slows you down and increases draft) is necessary to be able to free yourself from prevailing wind directions (yes, you can tack, but this is a huge waste of time).

[John Brown (no body)]

Re: Proper sailing spaceship????

"Now if you had two masts,"

Or gyroscopes?

(no idea really, just putting it out there. Hubble and other sats use them for changing orientation so...)

[ThatOne]

Re: Proper sailing spaceship????

The problem would be the constant push of the solar sail: Gyroscopes work for Hubble and other sats because there are no other forces dictating their orientation, you just need to overcome inertia to move them. On a sail-powered vessel the gyroscope would resist, valiantly, but would eventually lose, much like you can turn a rotating bike wheel, it just takes more force.

[Vulch]

Re: Proper sailing spaceship????

You can't tack properly with a solar sail as there's no way to emulate a keel. Wind powered ships rely on being able to balance the forces caused by the wind against those caused by the water, a light sail at best can have the forces go perpendicular to the light source, never towards it.

[Elongated Muskrat]

Re: Proper sailing spaceship????

The thing to remember, is that space is not a perfect vacuum. The sun is constantly pumping out a stream of particles. Planets with magnetospheres (and the sun's magnetosphere) cause the charged particles emitted from the sun to not move directly "outwards", and uncharged particles are going to continue to move in the direction they're going until they hit something, or pick up a charge (for example by being hit by an energetic photon and spitting out an electron). The result (the "solar wind") isn't just a stream of photons, and even the interstellar medium isn't completely empty, so there's stuff there to tack against, it's just it may not be very effective because the density is very low. Whether it's practical is something I'll leave to someone who wants to do the maths...

[duhmb]

Re: Proper sailing spaceship????

Um, no keel?

[en.es]

Space is BIG

Incredible that even the SGL point mentioned is only 1/460 of the way to the nearest star or alternative planet.... We need warp drives or TARDISes or something (icon because burning stuff will not get us there!)! :)

[KittenHuffer]

Re: Space is BIG

If we had either of those devices then why would we stop at reaching the SGL point just to take photos of exo-planets? Why not just visit the exo-planets instead?

[HamsterNet]

Re: Space is BIG

547Au is 0.0086 Lightyears and thats going to take a year.

Just our nearest star is 4.37 Lightyears.

[BackInFiveMinutes]

After a (very quick) rough calculation, it seems travelling at 33Km/s a craft could get tol Proxima Centauri in around 125 years. To me that seems doable considering Voyager has been going for, IIRC, 40 years.

Any real reason for not shooting one off to 2nd nearest star?

[BackInFiveMinutes]

Yeah my calculations were waaaaayyy off. Thanks to Google Bard.

My second estimate of the time is around 38400 years, which actually sounds about right to me.

[Roland6]

Could send one off to chase Voyager I - potentially overtaking it in 40 years - although with the speed differential it will be a case of blink and you'll miss it style of passing.

[Anonymous Coward]

Can we crowdfund it?

This seems such an obvious 'Duh' just-do-it project. At a cost of $20M, can we just crowdfund it? Or get Elon or Jeff or whoever to reach into their back pockets?

[Eclectic Man]

I wonder

Could a refrigerator-sized payload with a solar sail balance the gravitational attraction of the sun against the solar wind to hover over the solar pole? This could provide permanent views of solar polar activity (one at each pole, of course), and provide solar scientists with lots of information. They would always be in direct line of sight to the Earth for communications and control.

Just a thought. Any comments?

[Claptrap314]

Re: I wonder

The problem is not "putting" an object at such a location so much as "stopping" it there. That's a LOT of kinetic + potential energy to shed. It would probably be easier to put something into near solar orbit than actually stable.

But, ignoring that, yes--if the observatory's mass is low enough relative to the area of the solar wind it is catching.

[Andy E]

How do you stop?

Once the craft gets to the SGL focal region does it need to stop to take the photo? Or is the SGL focal region so big it can take one or more photo's as its passing through?

[ThatOne]

Re: How do you stop?

It better hurry taking those snapshots since there won't be any means to stop it, or even slow it down a little bit. No solar pressure out there, and definitely not from the direction needed (opposite)...

[Anonymous Coward]

Re: How do you stop?

And it's all very well having a 25km resolution, but how do you even *aim* the camera/sun-lens combination at a particular exoplanet to get an image?

Bear in mind that even if the camera weren't in orbit around the sun, the target exoplanet is in orbit around *its* sun, and the two suns are moving relative to each other.

[Claptrap314]

Re: How do you stop?

Meh. It's just a bunch of math to work out a bit of physics. Yes, "pointing" is a matter of position, and position is going to have to be defined very precisely. But ultimately, not particularly challenging to achieve. Remember, this is not about searching for exoplanets, it is about photographing ones that we are highly certain are there.

[John Brown (no body)]

Re: How do you stop?

Not only that, the article states detecting signs of habitation. Based on the 25km per pixel resolution and the example image given, I think we'd be hard pressed to detect signs of habitation on a similar photo of Earth, let alone an exoplanet.

[Doctor Syntax]

Re: How do you stop?

Motion blur?

[tojb]

Re: How do you stop?

solar orbit at that range is almost stationary in terms of angular and linear velocity... Pluto takes 248 years to go 360 degrees. So I guess it would be a case of slowly gaining angular velocity by relfecting incoming light at an angle, and slowly losing radial velocity by pulling out of the gravity well. Sounds painful but possible.

[Arthur the cat]

smallsats – around the size of a refrigerator

If that's an American style double door fridge it's huge. You could get a lot of science in that. (Or several of your enemies' corpses.)

[Little Mouse]

Clouds & Continents?

"High" resolution (i.e. greater than 1*1 pixels) photos of exoplanets would definitely be cool, but I think that concept art should be taken with a huge grain of salt.

Given enough time, surface features could theoretically be resolvable, even with the planet in question constantly rotating & slipping in and out of shadow, etc. But individual clouds...?

[Roland6]

a solar-sailing smallsat could reach Jupiter in a year

So potentially, one could get to Jupiter ahead of JUICE which isn't expected to arrive in the Jovian system until 2031...

[FrogsAndChips]

Re: a solar-sailing smallsat could reach Jupiter in a year

"smallsat velocities up to 20-25 AU/yr will be achievable in 5-7 years", so no, don't expect one of these sails to reach Jupiter before 2030.

[Roland6]

Re: a solar-sailing smallsat could reach Jupiter in a year

Looks like the US will be taking second place again then :)

[Atomic Duetto]

Disaster Area

The first sundiver craft really needs to be sponsored by Hotblack Deisato.

[John Brown (no body)]

Re: Disaster Area

I'm not sure Hotblack Desiato Estate Agents are THAT big a business that they can sponsor $10m project :-)

[Anonymous Coward]

Click to enlarge

The irony in the "Click to enlarge" option of the potato quality image...

[Sceptic Tank]

Disposable Camera

I take it that the mission will be good for one planet only? You will have to fly to the exact opposite point of the sun to the object you want to photograph, yes? I hope they remember to remove the lens caps.

[ThatOne]

Re: Disposable Camera

Just for one planet indeed, if only because, not being able to brake, that probe* will zoom through the SGL focus region at whatever speed it accelerated to to get there... I have no idea how big the focus point of that gravitational lens might be, but given the probe's speed and the fact they say it will need a year to take that exoplanet picture, it won't have a lot of time. I even wonder if it will have enough time.

I'm not sure this is well thought out.

_{* It's definitely not a "satellite"! A satellite's definition is "something orbiting something else".}

[John Brown (no body)]

Re: Disposable Camera

* It's definitely not a "satellite"! A satellite's definition is "something orbiting something else".

So, a probe then? (oo'er Missus!)

From the subjects POV, we a re the aliens taking the phots with our probe --------------->

[M man]

Re: Disposable Camera

Am I an idiot in thinking you would just send say 200 of these into solar orbit at the SGL focal region.

and combine them as they orbit past the appropriate place pointing at the planet.

the orbit time would be huge but with enough satalites you would eventuty get a continuous stream of 3D data for a huge strip of the sky?

might we be able to detect exo planet we just cat see now?

in fact could we use it to photograph solar object on the other side of the sun?

[Roj Blake]

How Do You Slow Down Though?

It seems to me that the problem isn't so much getting to an area of space where you can use the Sun as a gravitational lens, so much as how you stay there once you've arrived.

[ThatOne]

Re: How Do You Slow Down Though?

You don't, as I said earlier. You simply can't: The sun is not in the right direction so no sail, it is not brighter than any other star, so no free energy to power an ion engine, and given the speed you'll have you can't hope bringing enough fuel for chemical engines.

You will rush through the SGL at maximum speed and will be through and away before you know it...

[Jimmy2Cows]

solar sail smallsat craft sitting at the SGL focal point

The keyword there is "sitting". All well and good getting a smallsat out there, but it's got to maintain that SGL focal point, at least long enough to achieve a viable image. 12 months minimum to collect enough photons. The SGL focal point is very sensitive to the relative positions of the smallsat, the sun, and the thing you want to image. If that focal point can't be maintained, sending anything there isn't much use.

So kudos for a way to get their. Now really impress us with a way to stay there.

[Draco]

There have been lots of directly imaged planets

Wikipedia has list of almost 3 dozen directly imaged extrasolar planets. (Or more than 2 dozen if you want to argue from the other direction)

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

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Sundiver is of course also an excellent science fiction novel by David Brin.

[Inventor of the Marmite Laser]

OK. So you get to the solar focus point, going like the very clappers. Hopefully you'll want to hang around to maybe - you know - do a bit of science and somehow try to image different targets.

How the blazes do you slow down enough to "park" in orbit?

[Anonymous Coward]

How do you slow down?

Just eject your camera from the main body of the spacecraft back towards the sun.

[HamsterNet]

For theose who want to know how this woudl work.

Here is a nice vid from an actual astrophysicist on how a gravity telescope using the sun would work.

https://www.youtube.com/watch?v=jgOTZe07eHA

(Cool Worlds lab, part of the Department of Astronomy at Columbia University)

No, you don't slow down.

Yes, there is time to take the picture.

It requires launching a series of craft per image.

[John Brown (no body)]

Re: For theose who want to know how this woudl work.

Ah, so a bit like the airborne horse camera(s) shot then?