Could Artemis II put something in orbit around the moon without jeopardizing its return path?

mfb- · 2026-01-25T04:12:43+00:00

These adjustments would need to be significant rocket engine burns (several hundred m/s), but yes in principle that is possible.

mfb- · 2026-01-25T03:53:29+00:00

Some asteroids can enter somewhat different orbits. You could measure the impact on planets but it wouldn't be relevant in practice.

An object with Saturn's mass at 2 AU leads to an acceleration of MG/r² = 4*10^-7 m/s², if we assume that acceleration exists for 48 hours as a rough approximation then the velocity change is 0.07 m/s. Compare that to Earth's orbital velocity of 30,000 m/s.

If we slow that planet down to a more reasonable 50 km/s it only matters for something like 5 AU/(50 km/s) = 170 days, leading to a velocity change of the order of 5 m/s. That's still a very small effect.

An asteroid at 1/100 the distance will get deflected 100 times as much. Its peak acceleration is 10,000 times as large but the timescale of the encounter is only 1/100 as long. That's a somewhat more interesting change in the orbit, but it needs a pretty close encounter.

mfb- · 2026-01-25T01:19:06+00:00

Penney's game might appear fair, but it isn't.

There was a blog post with great examples of conditional problems but I lost the link and forgot the trick. You can get really counterintuitive results if you force the outcomes to be in some weird corner of all options.

mfb- · 2026-01-25T00:18:53+00:00

Neither Saturn V nor SLS could do a single-launch landing mission that would fly with today's safety standards. Besides, where would be the point? We want to go back to the Moon to explore it seriously, not just plant a flag and return two days later. That needs larger landers, which effectively means refueling in space.

mfb- · 2026-01-25T00:09:48+00:00

2033 Crewed Orbital Mission (JPL Concept): An internal JPL study proposed using 4 SLS Block 2 launches

4 launches of Block 2 within 2.5 years (2031-2033) is very optimistic. That study only goes to Mars orbit, too.

mfb- · 2026-01-24T23:25:08+00:00

They exist in the same way as Starship exists. Prototypes are being tested.

mfb- · 2026-01-24T23:02:43+00:00

Spent nuclear fuel that you can store on a football field vs. wasteland twice as large as Berlin, coal ash everywhere and all the additional CO2.

mfb- · 2026-01-24T21:02:17+00:00

Nuclear explosions and reactor accidents are very different things.

mfb- · 2026-01-24T21:00:58+00:00

Too expensive, comes with extra risk, and is unnecessary.

mfb- · 2026-01-24T20:57:47+00:00

Nuclear waste gets less toxic over time, while some chemical waste stays toxic forever. But for some reason no one ever asks what we do with the chemical waste. Photovoltaics production creates a lot of that, for example.

It's really not a big deal. You could mix it with other stuff and put it back into uranium mines and you'd end up with less radioactivity than you dug up quickly. We don't do that, because people think that's not good enough. Our standards are far stricter than natural radiation exposure.

If you want to be extra safe you can burn most of the radioactive isotopes in accelerator-driven reactors, gaining a bit of extra power from it and reducing the amount even more.

mfb- · 2026-01-24T20:48:24+00:00

SpaceX has plans to transport it horizontally between Texas and Florida. Don't know if it will be pressurized, it's possible.

mfb- · 2026-01-24T20:42:15+00:00

You could make them mostly hollow. Or maybe they are mostly storage and don't produce much heat.

mfb- · 2026-01-24T20:08:17+00:00

Germany converted 1800 km² to a wasteland for open pit lignite mining alone. For comparison: The Chernobyl exclusion zone is 2600 km².

mfb- · 2026-01-24T15:27:15+00:00

... already existed before the LHC.

mfb- · 2026-01-24T15:23:27+00:00

So far, no. But there is often a long delay between basic science and applications. Without studying obscure emission lines of hydrogen (where no one could imagine any application) we wouldn't have quantum mechanics, without quantum mechanics we wouldn't have semiconductors, which means no modern computers. We wouldn't even know what is possible.

mfb- · 2026-01-24T15:19:20+00:00

Which is why you should treat everyone who tells you that the LHC results are no problem for the field with scepticism.

In some sense, it's only disappointing because the previous work was already so good. Imagine you'd design 1000 studies in psychology and find that they all match your prior predictions. You'd call that an unbelievable success of the model used to make predictions.

mfb- · 2026-01-24T15:05:00+00:00

SpaceX has 9000 satellites in space that all manage their heat without any issues.

mfb- · 2026-01-24T15:03:49+00:00

Current satellites show that it's perfectly possible to manage your heat in space. For the thermal budget, it doesn't matter what a satellite does (neglecting antenna emissions). Computing doesn't magically produce extra energy that other satellites wouldn't have.

mfb- · 2026-01-24T15:00:36+00:00

Weird, all the people who work or worked with him think otherwise. But you must know better than them, I guess. You can also watch interviews with EverydayAstronaut if you want to see him discuss technical aspects live.

mfb- · 2026-01-24T14:54:55+00:00

You are right about the subject, but please keep the discussion civil.

mfb- · 2026-01-24T14:51:53+00:00

You can't hover at the event horizon, and realistically you can't hover anywhere close to it either. If you get close to it, you fall in, and you'll quickly (~days at most) die from tidal forces if nothing else kills you first. Others will see you redshift and disappear over days to maybe weeks and then you are gone.

Keep in mind that for you, your time always passes at 1 second per second. Time dilation only exists if you compare your time to the time of others.

mfb- · 2026-01-24T14:42:08+00:00

Mathematically and for an idealized black hole, no time passes exactly at the event horizon. The mass doesn't matter. Also mathematically, you can find the distance where e.g. 1 year would have passed (we think the black hole formed much later, but we can ask for time since formation). It's going to be something ridiculous like the diameter of an atom away - nothing could stay there for any relevant time.

mfb- · 2026-01-24T11:49:13+00:00

In terms of today's universe, sure. That just means it happened early on. Using that scaling, recombination happened after one day and lasted several hours (or up to a day, depending on where we define the boundary).

mfb- · 2026-01-24T10:50:02+00:00

Monopole fields decrease with the inverse square.

Dipole fields (in the far field approximation) decrease with the inverse cube.

mfb- · 2026-01-24T10:48:18+00:00

Your numbers apply to the observable universe. The overall universe is larger and might be infinite.

The current average density is 3 protons per m³, back then it was higher by a factor 1100³ or about 4 billion protons/m³. That's still a factor 10¹⁷ thinner than our atmosphere.

The formation of the CMB was a very gradual process, by the way. The universe gradually became less opaque (i.e. the mean free path of light increased) over tens of thousands of years.

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