For the Moon, the arm is about 6-8 times the payload mass. A complete system with drive motor and solar array is heavier. The figure of merit for these systems is the "Mass return ratio". That's how many tons of material delivered over their operating life, divided by tons of mining equipment, including catapults.

For a full discussion, see the Lunar Catapults section of my Space Systems Engineering book.

To verify authorship, click the View History tab on any page of the book and see the edits have the same user name as I have here.

A rotating arm is a flywheel, just a straight rather than circular one.

Electromagnetic catapults (mass drivers) are high peak power devices, since they have to do all the acceleration in a fraction of a second. Therefore they need a big power supply.

For example, getting to lunar orbit velocity over a 300 meter device takes 0.35 seconds. A 4 kg payload needs 5.78 MJ of kinetic energy, therefore 16.5 MW of power at perfect efficiency. If the mass driver is less than 100% efficient, input power needs to be higher.

A mechanical catapult with a rotating arm and electric motor can take as long as needed to come up to speed if it is operating in a vacuum. If you have an hour, the peak power is then 1.6 kW for the same payload. So it is better suited to low annual tonnage.

The original mass driver idea was for building space colonies, and launching half a million tons of material a year. At that kind of traffic rate you can justify the big power supply.

The orbiting cable approach doesn't put high g-forces on things like the other two. So you can carry people and complex equipment up and down. The catapults are suited for bulk materials. So which is better depends on what you are trying to do.

That has pretty crappy performance.

The dark areas of the Moon (Maria) are made of basalt. Basalt fiber is only a little less strong than carbon fiber. Make an orbiting cable from the fiber and just pick up stuff from the Moon. The cable's orbit can be maintained using sunlight and electric propulsion. Oxygen extracted from lunar rock can be the fuel. Performance would be ten times higher than good rocket fuel and fifty times better than trying to burn oxygen with regolith slag.

I used to work at Boeing's space systems division.

Studies I worked on showed 98-99% of space projects could be built from materials already in space - from the Moon and nearby asteroids. The other 1-2% are either too rare to mine in space or too hard to make up there. It is just easier and cheaper to launch from Earth. This would vastly reduce how much you need to send from Earth, and lower the costs dramatically.

Simple products like water and bulk regolith would be first. Water as water and oxygen are needed for life support, and makes up a high percentage of rocket fuel. Bulk rock can be used for radiation, thermal, and impact shielding. Impacts come from micrometeorites and stuff kicked up by rocket exhaust.

Over time you can add other products, including making parts for your space factories. Then the factories themselves can grow without shipping them all from Earth.

"In-Situ Resource Utilization" (ISRU) is NASA's disguise name for "space mining". They disguise the name because certain reactionary members of Congress would lose their shit if they knew what NASA was planning.

> The only thing keeping people in the industry was the benefits.

And the coolness factor. There is no substitute for when walking to the cafeteria to get lunch means going past Space Station modules being built.

The sensor was taking pictures of broccoli in 2020 using a pinhole camera. The finished camera won't be shipped till mid-next year. The camera will be getting its updated refrigeration system before the end of this year, at which point it will be complete. It will then go through final testing, before being shipped to Chile for installation on the telescope.

The Moon is a lousy place for a telescope. Dust on the lunar surface is electrostatically charged and sticks to everything. Some of it is broken glass shards, and also scratches things. During the day the Moon gets to the boiling point of water, and at night to far below zero, so thermal expansion will warp the optics.

Open space is better for telescopes.

The Rubin telescope, like other telescopes, is a really big camera with a really big sensor. The sensor is made of smaller tiles, and the individual pixels are large to collect more light when looking at the sky.

> But in the end what you need is some energy to do his compression.

Gravity is what is doing the compression. The self-gravity of all the material squeezes the center into neutronium (a solid mass of neutrons).

The thing is, current theory says you can't have neutron stars below 1.4 solar masses. At lower masses you get a "white dwarf", which consists of a soup of atomic nucleii and electrons. Above the critical mass, the protons and electrons are compressed to neutrons, making a neutron star.

So the possibilities are our theory is wrong, our mass estimate is wrong, or it is an odd looking white dwarf and both theory and mass estimate are right.

No. Our orbit is 23,455 times the size of the Earth. But there is a second dimension, above and below our orbit.

This plot shows the size of asteroid orbits relative to ours on the horizontal scale, and the tilt in degrees relative to the Earth's orbit. You can see the tilts go up to 30-40 degrees.

Any tilt above zero means when they cross our orbit's distance, they can be crossing above or below where the Earth will pass. They can only hit us if they cross our distance exactly on our path, and the Earth also happens to be at that point in our orbit at the same time.

> A rooftop garden isn't going to fix carbon.

Maybe not, but it can eliminate transportation cost and CO2 emissions if the produce is used locally, like in the building or a short distance away.

Exactly. Japan sent a probe to the asteroid Ryugu and brought back samples. The samples indicate its material formed in the outer Solar System. but now it's orbit crosses Earth's (that's why the picked it to visit).

More generally, there are 30,000 known asteroids whose orbits come closer than 1.3 times the Earth's distance to the Sun. That compares to over a million other asteroids. These "near Earth asteroids)" have an orbit half-life of 10 million years, which is very short compared to the age of the Solar System.

So these asteroids need to be constantly resupplied, otherwise there would be none left. The giant planets, which is mostly Jupiter since it is heavier than everything else combined, are the main cause of orbit changes.

The near Earth asteroids are the ones most likely to hit Earth, which is why NASA has a search program for them, and why they just tested changing an orbit by hitting it. What we have going for us is space is really empty. Earth only takes up one part in 500 million of our orbital region. So the chance of any given random rock hitting us is low. On the other hand, there are many rocks. Dust and sand-sized ones hit us every day. That's what meteors in the night sky are.

The asteroid belt has minimal mass (3% of our Moon). So it doesn't affect its own members much, or stop anything else from passing through.

Jupiter is 850,000 times more massive than the entire belt, so it strongly affects their orbits. For example, any asteroid that has a simple ratio to Jupiter's orbit gets pulled in the same direction every time it is closest. This pulls it out of that orbit, creating gaps in the belt. Conversely other asteroids are trapped in resonant orbits to Jupiter - the Trojans and Hildas.

Jupiter is considered the great protector because it removed 99% of the Asteroid Belt in the early days of the Solar System. Those early days produced heavy bombardment, as craters on the Moon and Mars demonstrate. But since then things have been "relatively" benign.

> Granted didnt Phobos slam into Mars too? I don't really know when that was supposed to have happened.

50 million years from now, if we don't mess with it. By then we could have mined it for raw materials, or turned it into an anchor for a space elevator.

We don't know how Phobos and Deimos came to be, but one idea is debris kicked up by an asteroid collision, that then came together by gravity. There are plenty of big craters on Mars that could have been a source.

Coal is already down 57% in the US as a power source. New US Power Plants are around 70% renewables these days.

Oil will take longer. Electric only reached 10% of new vehicle sales this year, and it takes about 20 years to replace the total vehicle fleet. So that means perhaps 1% of the vehicle fleet is electric so far.

> it would make them slightly less money to do anything.

Not correct. Solar and wind are now the cheapest energy sources, which is why they are rapidly growing. Hydropower first came into use in 1882, so it had a big head start, but not for long.

So assuming the usual profit motive, the 1% can make more money investing in renewables, which is exactly what is happening.

I'm easily distracted by the work environment. Too many people talking and walking around. I took early retirement and work half time from home now and get more done.

Some things need to be done in person, but a lot of things don't.

It is five times wider than the orbit of Neptune, the outermost major planet.

We don't know, because of the arrival paradox. If technology is improving, a later mission can travel faster and arrive sooner. So there is no reason to attempt a trip until (1) the trip time is short enough that you won't be overtaken by a faster mission, or (2) technology has reached some limit and isn't improving.

For interstellar trips, neither condition is met right now. For solar system trips, we are working on better propulsion such that any trip over 20 years that started today is likely to be overtaken.

The way science works, they can't release their data until the research articles are published. People involved with JWST from the start of the project have priority this first year of observations

Also, early observations need to be calibrated by looking at known objects. We know how the instruments were designed to perform, but without calibration we don't know how they actually perform. That's one reason some of the early pictures are of Jupiter and other solar system bodies. We know what those look like and how bright they are.

> it baffles me how we know which proteins do what.

Mice have 85% of the same DNA as we do. So-called "knockout mice" have bits of DNA removed, then they see what problem removing it causes.

The other approach is finding an antigen that blocks the particular protein, then see what happens.