Dr. Phil Metzger Profile picture
Aug 23, 2019 3 tweets 2 min read Read on X
I'm reading old sources to understand how & why moons have (always) been classed as a type of planet, how culture forgot this, and how culture intruded in science. Here's an interesting text on the types of planets from an 1897 textbook (Advanced Physiography by John Thornton.)
BTW, if wants to help the research and can translate Latin to English (better than Google Translate😭), this chapter 16 of Philosophia Naturalis, written in 1651 by Johannes Phocylides Holwarda, tells crucially how the "planet" taxon changed in the immediate wake of Galileo. 1/2
2/2 Here's a small sample showing how hard it is to translate texts similar to this one using automated methods. First, parts of the font don't copy-and-past correctly. Second, after fixing those by hand (as best I can), the automated translation is unreadable. Ackkkk!

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More from @DrPhiltill

Mar 18
I’m not so sure. The link to the prior estimate is a paper that measures the “blast zone”, which is the region around a lander where the reflectivity of the surface has changed. We have never known exactly what causes this change. Is it from gas blowing the dust texture flat? /1
2/ Or is it from blowing dust plowing across the surface? Or from engine shutoff when the last sputter of the engine cause a low velocity blanket of dust to fly out to a much smaller distance than normal? The problem has always been that this blast zone is *too small* to be…
3/ …the area where the dust finally lands, because the dust in rocket exhaust is going far to fast in low gravity to travel only that far. (We think we might know the cause now, but I don’t want to tell here since we will probably write a paper on it.)

So the prior estimate…
Read 10 tweets
Mar 17
Lots of discussion today on space radiation including errors like this one. This has confused water with regolith. Using too thin a layer of *regolith* creates secondaries, increasing the dose. But using water, or PTFE (lots of hydrogen), even very thin, always reduces the dose/1
2/ The thing about using regolith for shielding is that you use it when you are on the surface of a planet, and there’s so much available and you do t carry it on a spaceship so you have no reason to use a thin amount. It entirely solves the problem!

And…
3/ …if you want shielding on your spaceship you aren’t going to use regolith. You’ll use water, food, rocket propellant — mass you need to carry anyhow — and low-density material designed for shielding with lots of hydrogen. And you can travel *faster* to reduce the dose, too.
Read 12 tweets
Feb 25
I could write a 50 page paper answering this :)

A few points in outline form only:

1) The rocket exhaust is expanding into vacuum, so viscosity breaks down, so the gas does not obey the Navier-Stokes equation, which is the basis of CFD (computational fluid dynamics) models. /1
2/ When I was at NASA, one of the things I was doing was writing solicitations to industry to write physics-based code to do CFD without Navier-Stokes. There are many ways to treat the fundamental physics (the Boltzmann Transport Equation) and they all work for different…
3/…approximations, but it is really hard to write a code that will handle the full range of conditions from dense gas inside the rocket nozzle all the way to rarefied gas on the Moon far from the rocket.

2) We don’t understand turbulence when the gas becomes rarefied.
Read 22 tweets
Feb 24
About how the lunar environment makes everything tippier…

1) I’m sure the CLPS contractors know this and designed for it. My point is that the Moon does this to your hardware, so when things go wrong (as they do) then tipping happens more often than on Earth. /1
2/

2) There are different ways you can tip. For static stability, gravity makes no difference. You fall when you are so tilted that the center of gravity (cg) is outside of your footpad. I don’t know where the Nova-C has its cg, but crudely it could handle ~54 degrees tilt. Image
3/

3) But for dynamic stability, gravity does make a difference. Imagine your vehicle is accidentally moving sideways at touchdown with velocity v. The energy of that motion is (1/2)m v^2 where m is the vehicle’s mass. The vehicle will fall over if that energy exceeds…
Read 17 tweets
Feb 3
I finally submitted this paper to Icarus (planetary science journal). I split it into two papers: “Erosion rate of lunar soil under a landing rocket, part 1: identifying the rate-limiting physics” and “…part 2: benchmarking and predictions.” The breakthrough was in part 1.
1/N
2/ It took 8.5 months from the breakthrough while sitting at McDonalds until I got the paper done. 😭 I had to re-do it several times. 💀

I’m not keeping the info secret before publication, so I’ll go ahead and tell a little here.
3/ We tested jets of gas blowing soil in reduced gravity about 13 years ago. I did about 450 parabolas of lunar, Martian, and zero g, plus 2-g pullouts between parabolas where we did additional experiments. So we got 4 gravity levels.
Image
Image
Read 35 tweets
Jan 27
This is a fun and fascinating thread. I’ll add one thought. Latif says that some objects are dynamical and move about but the “regular” planets & moons aren’t that way, but really it’s just a matter of timescales. Everything changes orbits. 1st read Latif’s thread then mine…🙂/1
2/ An example of a moon that changed orbits: Triton. It is currently a moon of Neptune but previously it was a primary planet orbiting the Sun directly (albeit a small planet…a dwarf planet like Pluto). Neptune captured it! Image
3/ Another object that may or may not exist, which *if* it exists then *definitely* changed orbits a lot, is the so-called “planet 9” (terribly misnamed so I’ll call it Planet X or PX). PX is thought by some to exist beyond the Kuiper Belt yet to be the size of Neptune.
Read 19 tweets

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