# QuestionSolar sail acceleration

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#### Catastrophe

##### "There never was a good war, or a bad peace."
When you consider how sparse deep space is, I really don't believe that it is likely that there would be enough damage for it to really be relavant
Just check on exactly what is in "empty" space, and what it could do to a flimsy sail.
So flimsy that even large "dust" particles can puncture it.

Cat

#### billslugg

That is how much energy is imparted to the sail. Let's see how the numbers work out:
The formula for the force of a light beam is Force in Newtons = Watts/speed of light

100 GW = 1e11 watts
Force = 1e11 / 3e8 = 333 newtons
How much acceleration would this create?
F = ma
Mass = "a few grams" we'll say 3 grams = .003 kilogram
a = 333newtons / .003 kilograms = 110,000 meters per second^2
1 g= 9.8 m/s^2
a = 11,300 g's
This agrees with their specification of 10,000 g's

Helio

#### Catastrophe

##### "There never was a good war, or a bad peace."
billslugg, what is your answer to space particles, from dust to rogue planets?

Cat

#### billslugg

The amount of matter in interstellar space is exceedingly small, dust and rocks would not likely be a problem. Here are two ways to look at it:

Make a scale model of the universe such that the Sun is the size of a 1/2" diameter grape. The nearest star (grape) would be located 493 miles away. Everything else would be empty space.

Insterstellar space contains about 1 million atoms per cubic meter. (Mostly hydrogen). If our 5 meter diameter solar sail managed to collect every atom it swept out between here and the nearest star, it would collect only 7.8e23 atoms, or 1.3 grams of hydrogen.

There just isn't much there.

#### Catastrophe

##### "There never was a good war, or a bad peace."
billslugg - Would you be prepared to risk it? I certainly would not.

There might not be a high concentration, but you are talking about very long journeys.

One large piece of dust/matter - and you are dead.
Anyway, that is assuming that large sails are viable - and they are not.

Cat

#### billslugg

Once again you asked a question, I gave the answer and you don't like the answer. If you already knew the answer why did you ask the question?

#### Catastrophe

##### "There never was a good war, or a bad peace."
OK, I take it that if it were feasible, you would risk a very long journey (even weeks instead of decades) on the mathematical probability that you would not have your flimsy sail destroyed. EOS.

wem shrugs

Cat

#### Helio

One large piece of dust/matter - and you are dead.
Anyway, that is assuming that large sails are viable - and they are not.
Yes, that is a risk. Astronomers are able to quantify, to some extent, just how much dust is out there and by region. The light from background stars will see their light scattered as a function of the no. of dust and gas particles.

It may help to note that NASA has always been okay with taking the risk of sending probes directly through the asteroid belt -- containing trillions of particles -- rather than avoid it. This is not intuitive, and it surprises many to hear this, at least until one does the math that shows just how much empty space is there even in the belt.

#### Helio

So, back to the Multisperse hypothesis....

We have three main laser batteries, one on Earth, one at Jupiter, one past the dark side of the Sun.

We use less wattage initially, to reduce the no. of layers of sails that get wasted. We geyser it out to Jupiter, it does ~ 180 with the turn assistance from the J-lasers, then it gets the next geysering to get it to the Sun for the next U-turn—hence the third battery — whereupon it provides the next geyser toward battery 1 (Earth).

Perhaps several loops will be needed to keep wattage down.

But the last round will get the max. wattage it can handle, allowing for shield shedding as needed. The final geyser push comes from the Jupiter battery. Perhaps another battery in the KBO belt for final push and trajectory tweaks.

Given the speeds, several batteries of lasers may be needed to push the probe faster around each turn.

Multiple probes are highly recommended. The batteries might serve other purposes, including sailing the solar sea or for power generation where needed.

[Long layover in Denver...on iPhone. ]

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#### Catastrophe

##### "There never was a good war, or a bad peace."
Come on Helio, "Yes, that is a risk"

Is that a risk you would take going to:

(a) Mars
(b) Neptune
(c) alpha Centauri
(d) Andromeda Galaxy?

Cat

#### Helio

Come on Helio, "Yes, that is a risk"

Is that a risk you would take going to:

(a) Mars
(b) Neptune
(c) alpha Centauri
(d) Andromeda Galaxy?
Sure, but to be fair, I would not require you to pay for it all. I’d pitch in a few hundred.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Sure, but to be fair, I would not require you to pay for it all. I’d pitch in a few hundred.
Sorry, my friend, it was you doing the travelling, not I!

Cat

Helio

#### Helio

Sorry, my friend, it was you doing the travelling, not I!
There are likely many probes that this system would serve, from the realms of the outer planets to deep into the Oort.

Given enough experience in proving and tweaking the concept, then the stars could be the destination. I assume a string in series would be launched, partly to serve as repeaters for the feeble signals.

But, you realize I’m “playing” science more than applying science; it’s more supposition than hypothesis.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Helio, I am sorry that I disagree. I believe that any large scale exploitation is completely impossible. Gram sizes may be possible, but of limited utility, I suggest.

Cat

#### Helio

Helio, I am sorry that I disagree. I believe that any large scale exploitation is completely impossible. Gram sizes may be possible, but of limited utility, I suggest.
I’m not near that pessimistic. Going to the Moon was pure fantasy not that long ago.

Multiple laser banks at less power stationed properly at great distances would work. Multiple disposable sails to dump heat also might be needed.

I think we can get past the tough technology requirements, but the cost to do it will be very unpopular unless an Earth II is very likely the destination.

Perhaps the JWST and the LMT (Magellan Telescope) will get a few candidates of worth.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Helio, are you aware of this?

Essential reading for anyone who thinks that this is a practical means of space travel.

"A necessary requirement for the Breakthrough Starshot mission is keeping the mass of each sail-equipped Starchip to just a few tenths of an ounce (a few grams). . . . . . . likewise the solar sail itself, which is expected to span up to around 13 feet (4 metres), will need to weigh in at less than 0.035 oz (1 gram). It will also need to be extremely thin, as otherwise the sail would absorb far too much heat and be vaporized by the barrage of laser light . . . . . . "
the barrage of laser light to drive a few grams??

"Mass is the bane of accelerating objects to great speeds. To significantly increase the velocity of a heavy object takes a tremendous amount of energy. So, if the goal is to reach a distant star in a reasonable amount of time, say within a generation, a spacecraft must be extremely tiny and, therefore, robotic. Plus, it still requires an insanely energetic boost to get up to speed."
" . . . propelling a lightsail-equipped nanocraft, or Starchip, [sic] would require hundreds of individual lasers ,spanning roughly 200 acres (1 square kilometer). The array would also need access to enough energy to fire a coherent 100 gigawatt laser beam for several minutes during each . . . launch.
That's roughly the amount of power generated by all the nuclear power plants in the U.S. in a given year."

Another problem occurred to me, which is not mentioned. 200 acres of individual lasers surely would not be very manoeuvrable. Would it be able to follow said Starchip, even for a few minutes (as the Earth turns)? Also, would it not be usable only at a fixed latitude - that at which it was built?

Not to mention stopping when it gets there!

Cat

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#### Catastrophe

##### "There never was a good war, or a bad peace."
Helio, sorry, I should have put the background first:

Probably the most practical and informative approach you can find is in the book "Extraterrestrial" by Avi Loeb, John Murray (Publishers), 2021. Author is Chair of Harvard's Astronomy Department, and all the rest - one of the world's top astrophysicists.

He was approached by a billionaire (as happens a lot at the moment) over a special project. This guy wanted to fund a mission to the Centauri system, to arrive there during his lifetime - unmanned of course. Conventional chemical propulsion would require over 100,000 years. Avi came up with the idea of using a light sail. However, this is totally impracticable for anything over a few grams. They only wanted to take pictures and similar as they passed by. Obviously it would take too long to stop.

The system uses a 100 gigawatt laser beam. It is stated that everything they propose be within existing technological bounds. This is no joke. It is a serious mission. To avoid burning the sails, they had to absorb less than 1/ 100,000 of the (laser) light striking them.

There is an article published in Astrophysical Journal, October 2015, by Avi and James Guillochon on lightsails. It was decided to announce the Starshot Initiative, as they call it, on April 12 2016, On the stage were included Stephen Hawking, Freeman Dyson and Yuri Milner.

I hope this will help, but if you are thinking about manned trips using lightsails - forget it - unless you want to completely ignore the science.
Cat

#### Helio

Helio, sorry, I should have put the background first:
The astrophysicists is as much suggesting that it could be done — enough force to reach 0.2c— as implying that almost imaginary things are required to do so. It’s certainly not a position of impossibility, but kinda close to it.

billslug has done a nice job showing why something straightforward can’t work. I’m arguing that acceleration stages will be necessary for anything close to doable.

The success of Apollo leaned heavily on rocket stages. Correct me if I’m wrong, but I think the weight of more fuel and increase in rocket weight prevented a single-staged rocket to carry a heavy payload from getting into orbit. If so, we may have a fair analogy.

#### billslugg

Please note that the Breakthrough Starshot proposal recognizes that "several orders of magnitude" improvement in "at least 30 technologies" are necessary to make it work. They recognize it is impossible given current technology.

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#### Catastrophe

##### "There never was a good war, or a bad peace."
IMHO jet fighters in cloud cuckoo land are more probable . . . . . . . . . but maybe in 1000 years time, who knows, the intelligent insects that replace mammals may find it easy? Perhaps their wings may be large enough to use as sails?

Cat