Comments of the Week #69: From Moons of Moons to Earth's Twin

“It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn't feel like a giant. I felt very, very small." -Neil Armstrong

This past week was the 46th anniversary of the first Moon landing for humanity, and yet at Starts With A Bang there was so much going on in the Universe we glossed right over it! Here's what you might have missed:

There was also an exciting new article that stirred up a little controversy over at Forbes:

It was an incredible week and you kept it lively over here, so let's dive on in to your Comments of the Week!

From Archibald Tuttle on Moons of Moons: "Apparently we can have quasi-satellites like the asteroid Cruithne, which isn’t a satellite and isn’t a trojan. Is it a quasi-moon? Was or is Pluto a quasi-moon of Neptune? Can a planet or dwarf planet be a quasi-moon? If so can’t it also be moon?"

There are lots of weird things when it comes to orbits. There are the greeks and trojans (as you allude to), there are co-orbiting bodies (which is what Cruithne is), there are orbit-swapping moons (like Janus and Epimetheus, moons of Saturn), and there are orbiting bodies that will decay, becoming impactors.

Can it be called a moon?

Image credit: Wikimedia commons user Jrkenti. Image credit: Wikimedia commons user Jrkenti.

I'm not a fan of arbitrary definitions by any means, but I'm using "moon" a body that orbits (revolves around) a gravitationally dominant body (let's say a planet, to keep it easy) for a reasonably long amount of time... let's say at least a few hundred million years, or ~1+% the total lifetime of our Solar System.

If we adhere to that definition -- because we have to define something -- then that's the type of "moon-of-a-moon" I'm talking about. The kind we haven't found yet, the kind that could be there, but the kind that may not be there at all. And if you want to go a level further -- a moon of a moon of a moon -- you're going to need a much bigger, more well-separated solar system than the one we have!

From PJ on asteroid mining: "That is a lot to think about. How does one ‘ capture ‘ an asteroid in the first place? I would guess tag along with it at its own velocity, settle onto it (in the same fashion as our comet capture), strap on a few retro boosters to slow it down, then start to change its trajectory to head it (fly it) toward the moon & orbit."

There are a lot of potential ways to do it, especially depending on its size. Current plans are for a few-ton asteroid at most, and options are numerous, including:

  • towing it,
  • attaching thrusters and boosting it,
  • setting up an ion drive,

and many others. The optimal solution to be implemented depends on a combination of fuel, asteroid mass and the timescale of orbit change desired.

Is it risky? Of course.

Is it unproven? This application, in particular, is totally unproven.

But this is what it looks like when we try to advance in a big way. Some resources are very rare on Earth but abundant in space, and these rare minerals (and elements, to be more concrete) are some of the more important. It could finally tip the scales towards profitability, directly, for the space sector. But there's a risk to be sure, so much so that no multi-billionaire has stepped up to finance the entire thing themselves.

Image credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski (Arizona State University, Tempe), R. O’Connell (University of Virginia), P. McCarthy (Carnegie Observatories), N. Hathi (University of California, Riverside), R. Ryan (University of California, Davis), H. Yan (Ohio State University), and A. Koekemoer (Space Telescope Science Institute). Image credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski (Arizona State University, Tempe), R. O’Connell (University of Virginia), P. McCarthy (Carnegie Observatories), N. Hathi (University of California, Riverside), R. Ryan (University of California, Davis), H. Yan (Ohio State University), and A. Koekemoer (Space Telescope Science Institute).

From Ragtag Media on Out of the darkness: "We (Humans on Earth) are alone in this Universe and this Russian Billionaire is about to piss away a cool 100 Million to learn the hard way:
Russian billionaire plans to spend $100 million on a search for extraterrestrial life"

I thought this was a nice juxtaposition: spending too little money (but still a lot) on a long-shot that will almost undoubtedly come up empty, rather than spending the right amount to find something out for real. This is how we do things, apparently, and it's something I'm very unhappy about. But when you have money, you can spend it however you damn well please, and while someone like me can tell you my (professional) estimation of what your odds of success/failure and the ROI is likely to be, that logic doesn't really matter when people have their own dreams.

Yuri Milner thinks it's worth $100 million to learn whether $100 million can find alien life. I'm skeptical, but at the end of the day, it's money spent because someone's excited about learning something, and it's really hard to get made about that.

Image credit: CERN / LHC / LHCb collaboration, via Image credit: CERN / LHC / LHCb collaboration, via….

From Frank Jansen on Pentaquarks (and more): "I must have missed something, but why is a tetraquark not just two mesons?
And a pentaquark a meson and a barion?
And barionium a barion and an antibarion?"

This is exactly the point of understanding "bound states" in physics. Above is a single, bound state of a pentaquark. Below is the other thing you're talking about: an unbound meson and a baryon.

Image credit: CERN / LHC / LHCb collaboration, via Image credit: CERN / LHC / LHCb collaboration, via….

You produce a known particle, that particle decays into other known particles, and you look at what happened in the interim. If you have no other new particles that participate in that decay, you get a smooth distribution in the event rate as a function of energy.

But if there is a new particle that participates -- like, say, an intermediate, pentaquark state -- you'll see a peak in the energy distribution.

Image credit: © CERN / LHCb Collaboration. Image credit: © CERN / LHCb Collaboration.

That pentaquark state at 4450 MeV is most definitely there, and it's likely (but not 100% certain) that a second pentaquark state (at 4380 MeV) is present also. But the one is definitely needed, and a composite of two particles wouldn't exhibit that same signature.

Experimental particle physics isn't my strongest suit, but I'm pretty sure that's the right, solid explanation. I hope if it isn't, someone corrects me!

Image credit: SDSS, via, possibly lifted from Wikipedia. Image credit: SDSS, via, possibly lifted from Wikipedia.

From Denier on stellar evolution: "Just before the star goes into the red giant phase and burns up the planets, the graph shows the stars spend half a billion years cooling slightly. What is happening there? What is it that creates a pause in the otherwise inexorable rise in temperature over a star’s lifetime?"

The graph I pasted in, above, shows how a Sun-like star evolves over time. Notice how, on this "luminosity/temperature" diagram, the Sun gets cooler as it evolves... but also brighter and more luminous? The energy output is increasing, so this will work to increasingly fry the planet, despite the lower temperatures.

Image credit: Mark Garlick / HELAS. Image credit: Mark Garlick / HELAS.

Remember the difference between temperature and heat: you can have a more intense heat source at a lower temperature. The Sun will increase its energy output, causing it to expand rapidly, and that rapid expansion is what causes the temperature drop. It becomes a subgiant at first on its way to the helium-burning phase, expanding and cooling but always increasing its energy output. Over ~10 million years, the Sun increases its energy output fivefold... you're not going to like it!

The point is that the habitable zone moves outwards, not inwards, during this time.

Image credit: NASA/JPL-CalTech/R. Hurt. Image credit: NASA/JPL-CalTech/R. Hurt.

And finally, from G on other possibly habitable worlds: "Now heading off into wild speculation territory: Re. what you said about planets in the habitable zone of M-class red dwarfs being exposed to solar flares and the like: How would an intelligent civilization protect a planet’s ecosystems & habitability from any such events?"

So there are pros and cons to Earth-sized planets orbiting in the habitable zones of M-stars. The pros are easy: M-stars are more numerous, they're less likely to have giant worlds in there, they're longer lived that the Sun, they're more stable in luminosity over time than the Sun, they give off less ionizing radiation, and their planets are in closer orbit and thus better protected from chance encounters originating from interplanetary or interstellar space.

But the cons are rough:

  • more frequent and nastier solar flares,
  • less energy available from starlight/sunlight for spurring life processes,
  • and tidal locking is a much greater danger at such close distances.
Image credit: PHL @ UPR Arecibo, via Image credit: PHL @ UPR Arecibo, via

From my perspective, only the tidal locking is a real danger, and even that may not matter all that much if the atmosphere circulates well. The lower energy likely means that life will evolve to take advantage of the energy that's available to it, just as we (where "we" is all living things) have around our Sun here on Earth. And as for solar flares?

So long as you have either a thick atmosphere (like Venus) or a magnetic field (like Earth), those flares aren't going to do anything to life or intelligent life; the worst they'll do is induce currents in electrical wires. If a civilization has made it that far, they're doing ok!

Image credit: E. Siegel and World Scientific... preliminary. Image credit: E. Siegel and World Scientific... preliminary.

Thanks for a great week of comments, and if you haven't yet, this is the time to get in on the Patreon, and to do it at the $3 level and up. You'll get a free advance book chapter of my upcoming book, Beyond The Galaxy, and you'll get to vote on which (of the 11) chapters you get!

Enjoy, and see you next week for more.


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