# How the swimming pool got its color

New York Times

NYT Article: EPA science advisors to be fired and replaced with industry-friendly representatives

Once upon a time there was a public pool. Everyone used the pool and enjoyed it, but very soon it became apparent that a few of the pool-goers were relieving themselves in it. The pool quickly turned yellow and smelly.

So the community got together and formed the pool-peeing committee; the goal of which was to cut down on the general amount of pool-peeing that was being done. They would do this by hiring some local experts from the town to measure the pool water regularly, and tell everyone when somebody had taken a leak in it, and to the best of their ability, who was responsible.

Right away, some chronic pool-pissers were caught and yanked from the pool. This was very embarrassing for them, so everyone paid attention when it happened, and so the new standards of pool play more or less caught on and were known by everyone. Soon the water cleared up, and people were able to enjoy the pool again. It wasn’t perfectly clean by any means, but it was much, much better than before, and it was improving every day. This worked pretty well for a long time.

A few people didn’t like the pool-peeing committee. Some didn’t like the idea that someone else could tell them what they could and couldn’t do in a pool. Others were mad because every once in awhile, the committee would accuse them of having peed in the pool when they had only peed a little bit, while Jimmy over there drank a whole 2 liter bottle of Mountain Dew before he swam and let it all out through his bladder, but the committee didn’t catch him and that wasn’t fair. Many had started coming to the pool long after the pool committee had formed, and didn’t understand how smelly the pool had been before the committee came along.

But by far, the people who hated the pool-peeing committee the most were the biggest pool-pissers. The pool-peeing committee was always bothering them, they complained, embarrassing them in front of their friends, and cruelly yanking them out of the pool. All they wanted to do was play in the pool, and didn’t they have a right to do that? So what if a little pee leaks out every now and then. Worse (they argued), if the committee was allowed to yank anyone who peed out of the pool, then pretty soon the pool would be empty and the community center would be bankrupt. Pool-pissers gave a lot of money in entrance fees, they pointed out.

Of course the solution was simply to not pee in the pool (which the rest of the community was able to handle just fine), and to hold it until afterwards, but that really cuts into our pool-playing time, the pool-pissers whined.

So the pool-pissers got together a plan: They would band together and take over the pool-peeing committee— but first they had to convince the other pool-goers that this was a good idea.

“The system is rigged!” the pool-pissers squawked. “The pool-peeing committee gets paid to test the pool! So you see, they all have a stake in the outcome of the pee tests! This is a conflict of interest! They’re on the take!”

A lot of the swimmers began to nod their heads— this sounded really unfair. They started to worry if the pool-peeing committee could be trusted.

“We’re being paid to do our jobs,” said the pool committee. “That’s not a conflict of interest. And we all signed up for this job because we care about having a clean pool. We swim in it too, you know.”

But the swimmers didn’t hear them, or maybe they didn’t care because they were all very worried that something unfair might be happening. And they were right, something very unfair was happening, but it wasn’t what they were thinking of.

“We should kick out these crooked pool experts from the pool committee,” said the pool-pissers. “They don’t know the reality of what it’s like to be a swimmer, like exactly how hard it is to hold your pee. Besides, us swimmers have the biggest stake who stays in the pool. It would be much more fair to put swimmers on the pool committee.”

This sounded reasonable to everyone and soon enough the pool experts were sent away, the pool-testing equipment was thrown out, and the pool committee was re-staffed with “regular swimmers”. Someone noticed that it just so happened that everyone on the new pool committee had been caught peeing in the pool many times, but it was decided that this was okay, because they clearly knew the most about pool-peeing, so it made sense that they were on a committee about pool-peeing. Everyone was very satisfied with this arrangement, and congratulated themselves for having solved the conflict of interest.

Almost immediately, the pool turned bright yellow and smelled like a lot like a subway station, only more so. Nobody was really sure why, or how to fix it. Some swimmers kept saying something about strengthening the pool committee, but it seemed clear that pool committees didn’t work, because we have a pool committee, and look how yellow the pool is.

Many people got very sick, and eventually the community pool lost all of its revenue and had to close after everyone stopped coming to it. The mystery of why the pool turned yellow remains to this very day.

# Some physics about covering the Earth with Internet

Before I read that Elon Musk’s insanely-huge satellite fleet was to orbit at around 1100 kilometers above the Earth, I did some back-of-the-envelope math to figure out what satellite altitude would be necessary to carpet the planet with coverage. From this came an interesting little exercise in hypothetical logistics.

First, figuring the minimum altitude of a 4,000 station constellation: We can use a formula for the fraction of the Earth visible from a point:

$$A = \frac{1}{2} \left( 1 – \frac{R_e}{R_e + h} \right)$$

…where $$R_e$$ is the radius of Earth, $$h$$ is the altitude of the observer, and $$A$$ is a fraction of the Earth’s surface area. We can divide up the Earth into 4000 pieces and solve for h to know how high a satellite must be to see its portion of the Earth. When we do this, we get…

1.98 miles high.

What.

If you want to blanket the entire planet with internet coverage, and you’ve got 4000 things to do it with, those things only need to be floating about 2 miles off the ground!

Obviously nobody’s putting satellites in two-mile orbits. And we’d like some redundant coverage, so let’s demand that every point on Earth be covered by at least three stations and try again. What do we get? Continue reading

# On Elon Musk’s Ridiculously Huge Satellite Fleet

On Friday, Elon Musk announced SpaceX’s intent to launch a fleet of over 4000 communications satellites, whose primary purpose is to provide a low-latency broadband internet to us Earthlings:

He’s drumming up interest among potential hires for SpaceX’s new satellite branch in Seattle for, fittingly, building satellites.

This is an interesting move. 4000 is a huge fleet— Musk says in the video that this is more than double the number of currently active satellites, but from this database, it looks to be actually more than triple that number.

# Why?

## It’s faster

Currently, to travel a long distance, a packet of data must be routed through thousands of miles of fiber optic cable. Because the fiber is made of glass and is refractive, the speed of light in the cable is about 60% of the speed of light in a vacuum— so this limits how quickly data can go from one place to another on Earth.

Musk plans to route data directly from satellite to satellite through the vacuum of space, bypassing the speed limit of fiber.

Another factor is the number of hops: as a packet makes its way around the internet, it makes pit stops at routing stations, which might hold the data in memory as it waits in line to be sent to the next stop. This takes time, and may happen several hundred times for a long distance communication. This can add up to a significant fraction of a second.

Musk plans to route long distance packets in just a few hops, reducing the delay between endpoints even further. This is possible because each satellite will be in view of many others— it’s possible they’ll transmit data by pointing lasers at each other. For data, it’s the difference between taking the backroads and taking the freeway.

Notably, the proposed fleet would be orbiting at around 1100 km; far lower than the altitude of current high speed internet satellites, which hang out in geostationary orbit. Current satellites 22,000 miles away, which demands a minimum round-trip signal time of about 1/4 second. In contrast, Musk’s satellites will orbit just 3 milliseconds from the surface.

## Money

It seems there is opportunity here to enter a relatively sparse niche. Long distance communication is dominated by fiber, which is expensive to lay. Satellites are expensive too, but in a matter of months that cost will drop enormously— perhaps by an order of magnitude— when SpaceX succeeds in landing its reusable first stage rocket. Elon is in the unique position of knowing exactly how cheap space launches are about to become, and also has the ability to fully reap the benefits of cheap launches without any of the markup that he’ll be charging his competitors.

Musk says he intends to capture about 10% of consumer traffic, and 50% of the “long distance” traffic. This means the network will be focused on backhaul, routing large batches of data between endpoints for telcos and ISPs. It seems he hopes to attract backhaul customers by offering significantly (“order of magnitude”) improved latency over current capability.

10% of consumer traffic is a lot— There are about 2.5 billion subscribers on the internet (that will surely grow), and Musk is therefore hoping for about 250 million SpaceX internet users. That’s incredibly ambitious, although a comparatively small amount of infrastructure (as opposed to laying fiber for the same consumer base) will provide him easy access to the entire globe. If he succeeds, and we suppose a consumer cost of, say, a conservative \$10 a month, that’s a total of 2.5 billion dollars of revenue per month. This would pay off a 15 billion dollar satellite network (Musk’s cost estimate) in just half a year. 10% of all internet subscriber traffic is pretty ludicrous, but if he’s overestimating by a factor of 10, it’s still looks to be easy money. This isn’t counting the backhaul traffic he intends to capture (whose revenue I don’t know how to estimate).

And money is the goal. Musk primarily wants something that will generate cash for his a future Mars colony venture; he seems to believe this will be a cash cow.

## Will I get satellite internet on my phone anywhere on Earth?

Probably not; the radio frequencies SpaceX is likely to get their hands on don’t penetrate buildings. It’s more likely that homes and businesses will need to pay a few hundred dollars for a dish installation on the exterior of the building.

This is maybe something for Musk to worry about; it’s quite possible that in developed countries individual subscribers will soon stop paying for high speed internet at home, and start using their phones, almost in the same way that landlines were outmoded some years ago.

Overall, it appears that Musk is vying to become a major player in the telecom industry. New competitors are rare here, though he did manage to bust into the immensely expensive and entrenched space industry. We’ll soon see how he performs.