The DIRT ROCKET

ToughSF pointed Thursday the 27th to 2007's DOI 10.1063/1.2722266. No link b/c X. 50V for 50-100 I_sp.

I'd consider it for docking a smol robot with some regolith of low G (like you-know-where), then moving on. Meh. Hope it's cheap anyway.

Really we should be making colonies from the dust on this rubble. The colonies refine the dust into oxygen, and into something that can react with oxygen. That offers a lot more impulse and, also, thrust. They say this might work even on our Moon. As for Bennu et al. I'd just scoop it all and move it somewhere less dangerous.

Warp drive roundup

Poring through a random month of this 'ere blog, I ran across Erik Lentz then Alexey Bobrick and Gianni Martire. Most of this got trashed by Santiago, Schuster, and Visser (SSV) the following year. So, five years on - last May - Caroline Delbert ran a pulse-check. Seems there's been some activity in what has become, uggh, a "community".

We are here interested primarily in the warp-bubble, where the prospective colonists don't age as the colony ship travels at a sedate subrelativistic pace. Interstellar space operatics may refer to José Rodal last year for Alcubierre's negative-energy (see below). The bubble remains attractive if only because relativistic risks all what we saw in Passengers (this went by red giant Arcturus 33.78 ly, which implies something like .76C). Such theories also claim not to require negative energy.

Last February, Hamed Barzegar, Thomas Buchert, and Quentin Vigneron got on all warp cases (BBV). SSV had a few errors, but not such as vindicated the warp core Doors.

The BBV classification under which Bobrick-Martire falls is the R-Warp model, definition III.3. These satisfy the "restrictions" R1+R2+R3b+R4(+R6); developing Barzegar and Buchert last year. R4 is new here: asymptotic flatness. The Natário metric is a subset; here to assure a globally hyperbolic spacetime. That had been an assumption of Alcubierre 1994; so a rule had to be thought up where this could be assured, done in Choquet-Bruhat–Cotsakis 2002 BBV's "theorem IV.3". BBV have their own theorem IV.7 such that superluminal R-warp models can never be hyperbolic.

But hey. I remind my readers I wasn't asking for tachyon beams. We just wanted a bubble that got to Arcturus in however-many-external-decades without the hibernation-poddery. José Natário has been saying all year his bubble - hyperbolic - is subluminal. (The BBV paper reports he was arguing differently in earlier years, but maybe those weren't R-Warp.)

Nah. SSV's big stinkbomb to the R-Warp, even the slow one, was that it violates the null energy condition. BBV can only affirm SSV here, as "theorem IV.34". As asymptotically flat (R4), theorem IV.20 says they violate the dominant energy condition too; that is, that the energy can't be negative.

BBV point out that some of these R's in the R-Warp models may be lifted, as long as we are in Gedankenexperimentland; but engineers, not such Landsers, will find difficult to explain how they can. Same problem has afflicted Alcubierre when he said "just use negative energy bro".

An T. Le follows up, providing the service of explaining the four energy conditions, not just "null" and "dominant". These are pointwise inequalities on the stress-energy tensor contracted with timelike or null vectors. Any vio represents an exotic fuel (or, per BBV - "fantastical") like the negative energy - which we don't have. He also submits the WARPAX, eigenvalue tests on the various tensors. Sure, per SSV the fuels all suck; but how bad do they suck? Oddly Rodal's drive seems to suck least, considering it remains superluminal. But that might be because Rodal was willing to clarify his model to Le, in personal communications.

Since May, An Le has gone on to evaluate several fuels' "source consistency"; all subluminal [and] positive-energy. They all suck in newly found ways, not so much interior as on the boundary with our nonfantastical universe.

Maybe call it "the Lewinsky Drive".

Barnard's Rossby

Barnard's classical habitable-zone estimation relies, I think, on this 2013 paper. But a couple years ago, we were looking into Alfvén systems. This is before we even had planets around Barnard's star, Gliese's #699. It turns out this star - as "GJ 699" - had convection calculations in 2018. For its colour V − Kₛ ≈ 4.99 (thank you copilot), logτ=2.07. Convective turnover should be 124 days based just on their own calc of 130.4/1.05.

We do have to correct the Rossby, given rotation period has since 2018 been updated from 130 to 140 days. That pushes Rossby to 1.13.

Plugging into Equation 5, 17.46 * (1.13^0.41) = 18.36 Sol radius maximum; 17.46 * (1.13^0.03) = 17.5 Sol radius minimum. In terms of semimajor, 0.08-0.085 AU. Barnard e - outer, 0.0382 AU - rolls well inside the minimum.

Like Barnard's inner worlds didn't have enough problems. A ten-day "HZ" planet would still be fried. 22.6-to-42-day on the other hand standeth a chance out to 0.13 AU.

Barnard's inclination

Copilot is telling me we don't have Barnard's inclination. It rotates too slowly and it doesn't offer sunspots. What we do have are constraints on the inner four planets. They are resonant and seem to have been there for - well, ever, as far as Earthlings can tell. This suggests they formed there.

From Table 3 in that paper I see minimum sini≈0.4 across the board. That is 0.4115 rad; or 23.6° up to not quite 90 (90 is edge-on which means transits as we don't see).

Going to the Basant paper which confirmed the four, the Msini limit on a HZ (then-calc'ed 10-42 days) planet is a paltry 0.57 M_⊕. But. sini of 0.4 would allow up to 1.425 M_⊕ out there.

We cannot yet rule out an Earthlike in the HZ. Well, as earthlike as an airless tidally-locked rock can be.

Skyhooks of silica

I suspect these two goofs felt comfy with CNTs only because they already knew all this stuff was (literally) not going to fly in the 2000s. In these the 2020s, Jensen instead has figured upon anhydrous silica with no "Griffith flaws" (per Copilot). This is a cable we should be able to spin from Lunar regolith and ship up to space for cheaper. Let's see what this buys us.

Jensen was targeting 13.8 GPa of pure laboratory SiO₂ so ρ = 2.2. The deltaV equation √(13.8*2/2.2) = 3.54 km/s. That'll get you to Hohmann for any porkchop of Mars - or for Venus (2.8 km/s); but the next-good Byrnes-Longuski cycler to Mars is "6S9" which is terrible.

If we could stationkeep a tether-hook at STL1, this would work for a faster cycler to/from Venus: the 2L4. Venus herself could not use a 13.8 GPa cable. Both of us can however run the 3-0-2-9 cycler - in the 3.4+ km/s range on either side - and then we can keep both tethers in high-orbit.

The Nuclear-Inertial skyhook

On topic of ideas from the Shuttle era, ToughSF links the Nuclear Inertial rotational-tether. doi 10.2514/6.2000-3611 by Allan Goff and (Dr.) Joel Siegel. I don't think many picked up on this as opposed to other tether plans.

What disassociates this from the others: the counterweight is a moving crawler that also doubles an an electric thruster for reboosting the tether, all kept powered by a nuclear reactor.

They recommended it for yeeting cargo to... Mars. No seriously. From here the bestcase (Hohmann) injection burn requires 2.95 km/s; another 2.65 km/s is required to push from Hohmann to match Mars'. To take the strain, they moot *sigh* Carbon Naynoe Toobz of 20 gigapascals ("GP") breaking-stress over that ρ = 1.3 g/cm. Equation #6 told them up to 5.5 km/s of deltaV out of this setup, on the Earth side. 8.8 GPa for 50 GPa.

That's a lot to play with. Say we have already injected something into the 100 Mm orbit. We have a chart for several cycler orbits in Longuski-Byrnes 2002 (remember that?), with their outgoing v_∞. For Aldrin's 6.54 km/s, which is a Martian flyby to return our chassis, Copilot recommends 5.2 km/s. The bespoke "S1L1" is also in reach.

The mission profile at the end, even for the Earth side, relies on a relay of two skyhooks: Earth supplies the lower 'hook from a "spaceplane" (in 2000, considered more workable than a reusable rocket), which LEO hook flings cargo up to the HEO 'hook. We'll wholly ignore the Mars side for now (likely forever).

They thought the cargo could go up to 100 "tons" at a time, implicitly metric. Equations #9-11 say more tonnes means increasing the thickness (in cm₂) of the cable whatever-it-is, which tapers.

For this poast we'll leave aside the CNT hardcore. We assume such GPa as can do Mars' or Venus' Hohmann - only; like Jensen's SiO₂. Which is itself a reach.

Here we'll talk about those nuclear reactors. Per 'hook, such is to transfer momentum between payload and tether. HEO as in, past GEO: maybe; LEO, I doubt. Nuclear is chosen over solar for a few (good) reasons: brute force, and infrequency of resupply. The paper assures that that each pull to another orbit is minor. This may allow some enthalpic solution as doesn't have to stay in LEO for long. But that would have to be supplied.

Barnard's necropolis

So last year, and the year before that, we were talking Proxima Ophiuchi aka Barnard. It has four planets, all with minimum masses double-to-triple Mars'. Xander Byrne and crew are pondering What It All Means.

Besides irradiated Marslikes. We already knew their sunsides were too baked. Also the UV on any of them would be too low for "abiogenesis" even for inner "d" which is the most-roasted. The latter doesn't account for seeding from any other systems this ancient star had passed-through; but if I'm a spore-shooting alien fungus, I'd not bother with Barnard.

The star is ancient, so doesn't flare much, which helps in telling that these planets were for real in the first place (this had been a problem). It is enriched in magnesium by contrast with silicon and thorium. Byrne's paper extrapolates that the planets should be ferropericlase. I don't frankly know what that mineral is, but Petrov says that this doesn't hold water well.

The four are resonant like Trappist-1... and the Jovian system. The inner planets d,b,c might be in a 4:3 MMR chain but since we have less data than what we have for - say - Trappist-1; the paper whiffs on this one.

The paper is able to use the resonance to impose upper mass limits - which is great. Even if the planets aren't great; they're all subEarths. Given the resonance they likely formed about where we see 'em. That is (well) within the soot line let alone the snow line.

So these are all low-mass. And: with more space than Jupiter's moons have. So they don't pull on each other as much. Add to that, their internal radioactive elements (which are halflived well beyond Earth's): they are internally "cool". And the outermost planet e is tidally locked which people aren't anymore saying about Trappist-1.

Knock-on effects are that the planets won't have seeped out volatiles after initial formation. Barnard is quiet... now; but these stars - M dwarf - tend to be flare stars when younger.

The paper also brings "gap complexity". Barnard's four are simple, 0.131. The paper argues that this implies no giant outer planets, but we must counterargue that our system has a lower 0.126 inward of 1.5 AU and... we have four giant outers. Of course we've been looking at this one since van de Kamp so... yea, we ain't seen nuffin'.