Comment by vinaypai | Hacker News Reader

Comment by vinaypai | Hacker News Reader

It is not the fault of the students who wrote the paper, but this is an exceptionally good example of terrible science reporting in popular media. Practically every sentence of the article is wrong including multiple errors in the title.

> Feasible & Inexpensive

The paper says "Building a spaceline would be a huge engineering challenge, stretching the limits of current human capacity - but not exceeding them" which is a far stretch from "feasible and inexpensive"

> Scientists Find

Second year graduate students

> In a paper published on the online research archive arXiv

An as yet un-peer-reviewed pre-publication draft that is being prepared for submisson --- it says that right on top of the first page.

> After doing the math, the researchers estimated that the simplest version of the lunar elevator would be a cable thinner than a pencil and weigh about 88,000 pounds

They they actually said: "Let’s say such a line was made of a cable with a0 =10−7m2: its total mass would then be around 40,000 kg.". So yes, they're right that it would be "thinner than a pencil". A cable with diameter section of 0.36mm is definitely "thinner than a pencil". (They are misquoting them saying thinner than a pencil lead... presumably referencing the lead in a 0.5mm mechanical pencil, not the lead in the regular pencil).

So yes, you could spend billions of dollars to build an elevator that could lift ~100kg off the surface of the moon to GEO, and take months to years to do it any any speed you're likely to achieve with a solar powered elevator tethered by a 0.36mm thick line.

Good points.

For anyone interested in skipping the superfluous and sensationalizing article, here is a direct link to the paper, avoiding the embedded tracking link: https://arxiv.org/abs/1908.09339

> Practically every sentence of the article is wrong including multiple errors in the title.

>> But the Columbia study differs from previous proposal in an important way: instead of building the elevator from the Earth’s surface (which is impossible with today’s technology), it would be anchored on the moon and stretch some 200,000 miles toward Earth until hitting the geostationary orbit height (about 22,236 miles above sea level), at which objects move around Earth in lockstep with the planet’s own rotation.

I see. The difference is they proposed to build their Moon space elevator on the Moon, while others have proposed building the Moon space elevator on Earth... /s

Also, obviously, even if the Lunar tether reaches the _altitude_ of geosynchronous orbit, it will not have the _velocity_ of geosynchronous orbit (it will have the angular velocity of... the Moon). Since the Moon orbits once every 30-ish days and the orbital period of a geosynchronous orbit is 1 day, an object at the end of this tether would not be in geosynchronous orbit. I'll not bother to check and see if that orbit would have an apogee above 100, or 0, km.

In other news, Wet Streets Cause Rain.

We know the altitude of an object with an orbital period of a month: the moon is there. An object going the speed of the end of the tether would drop like a stone.

Objects in geosynchronous orbit are going 3000+ mph, where the end of the tether would be at about a standstill. Still, if you managed to grab on, on your way past, there would be plenty of swing and stretch. It would be hard to keep it from swinging all over, all the time, as the tides had their way with it.

This is actually a huge advantage. As I understand it, there would be no need to accelerated to GEO before docking with the station. The earth side of the teather would travel at about 10% of GEO. This is a large savings in energy expenditure because kinetic energy is m*v^2

True, presuming you could arrange to be at the same place as the tether at the same time. However, with it whipping around loose, how easy would that be?

Maybe a landing platform could be hung from it, helping to stabilize the end.

"stretching the limits of current human capacity - but not exceeding them"

That means "feasible".

As for inexpensive, it may be a poor word choice, but the alternative is launching rocket after rocket in to space, which is one of the most expensive things humans do.

This is not an alternative to a rocket. It goes from geosynchronous orbit height to the moon, so you still need a rocket from earth to the elevator.

It enables the use of smaller rockets rather than no rockets. With a probably optimistic ~billion dollar pricetag for something able to move 100kg. You would need to launch an awfully large number of rockets with tiny payloads to the moon for this to be a net cost savings.

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