Lava tubes: summary

So, to summarise what I've been writing over the last week:

1. Lava tubes are natural tunnels formed by flowing lava.
2. In some places, they have collapsed to form rilles.
3. On the Moon they are roughly an order of magnitude larger than their equivalents on Earth.
4. They offer considerable protection for a manned base from a variety of environmental dangers.
5. Lava tubes would provide an ideal location for a the base for a mission lasting longer than a couple of days.
6. Satellite photography can be used to look for intact lava tubes indirectly by looking for discontinuities in rilles.
7. GPR can see lava tubes directly, but has serious problems associated.
8. Geophones can generate precise data on lava tubes, but require a network of sensors and a number of shock waves in different places.
9. "EMP thumpers" combine the advantages of geophones and GPR, but is still under development as a technique.

Tomorrow I will begin to discuss problems (and their possible solutions) associated with staying on the Moon for any length of time.

By the way, a disclaimer: writing these articles is part of a learning process for me. I don't suggest that they're even factually correct (although I do my best). If you find them useful, then great! And if you find any problems with them please let me know.

Lava tubes, seismic survey and EMR

Yesterday I outlined the ground-penetrating radar technique for finding lava tubes on the Moon. Another two methods have been suggested [1], and I will briefly describe them.

Firstly, seismic survey. This involves deploying a network of seismic sensors (geophones) in the area you wish to search for lava tubes; essentially, microphones embedded in the ground. Then you send shock waves through the ground, like a miniature earthquake. You can generate the vibrations either by explosive charges, or by impacts (i.e. crashing something into the ground from orbit).

Although seismic survey provides excellent resolution and good penetration of the surface, there are problems: you have to non-destructively land your sensors and then embed them. That limits you to a small area. If you're using explosives, you need to set them up as well, or if you're chucking stuff from orbit you need to have some sort of accurate delivery system. To generate good data, you need to have lots of sensors and lots of shock waves from different places. I think that this technique would be good for mapping an area you've already identified as interesting.

The final method is still theoretical, and combines the ideas of GPR and seismic survey. You lob devices from orbit - devices that, when they hit the Moon's surface, behave a bit like EMP bombs, converting their kinetic energy into a very powerful radar pulse. The radar pulse bounces off features in the Moon - such as lava tubes - and the reflections are detected by a very long baseline array of receivers, and converted into 3D geographical data.

References:

[1] D. Stephenson et al., 'lunar resources/lava tubes', sci.space.policy, 22-01-1996

Our Father

From bash.org: This was much too hilarious not to put up on here!

<Firefly> Time for my prayers:
<Firefly> Our Father, who 0wnz heaven, j00 r0ck!
<Firefly> May all 0ur base someday be belong to you!
<Firefly> May j00 0wn earth just like j00 0wn heaven.
<Firefly> Give us this day our warez, mp3z, and pr0n through a phat pipe.
<Firefly> And cut us some slack when we act like n00b lamerz, just as we teach n00bz when they act lame on us.
<Firefly> Please don't give us root access on some poor d00d'z box when we're too pissed off to think about what's right and wrong, and if you could keep the fbi off our backs, we'd appreciate it.
<Firefly> For j00 0wn r00t on all our b0x3s 4ever and ever, 4m3n.

Lava tubes and surface-penetrating radar

Yesterday I discussed satellite photography. As I mentioned, there are problems with this technique, but there are others that might do the job just as well.

Consider a very simplified radar: you send out a pulse of radio waves, and you wait for reflections to come back. The longer the reflection takes, the farther away the thing that's reflecting.

During the Cold War, the US satellite reconnaisance researchers developed radar into a way to find concealed underground missile silos, and then as a demonstration published images of the network of underground rivers beneath the Nile delta, if I remember correctly. This worked by getting reflections back off features progressively deeper into the ground.

So this could work for looking for lava tubes. Mount a big radar on a Moon-orbiting satellite, and then set it looking at the surface of the Moon. Most of the time it'll get one reflection back, from the surface, but when it encounters a lava tube it'll get another reflection back from the tube roof, and then a third from the tube floor. This proposed mission would in fact provide two useful sets of data: a three-dimensional map of the Moons surface, which could be useful in mission-planning or geological studies, and then lava tube positions as a by-product.

There are naturally a couple of problems to overcome with this technique: on a basic level, resolution, penetration, and signal to noise ratio [1]. Resolution is very important, as to find the sorts of lava tubes we're interested in we need a 10 m resolution, or even finer. Penetration is also important, as we want to find lava tubes even quite deep down, and a low signal to noise ration is vital if we want to extract any useful data at all! To get a high resolution, you need a high frequency, but increasing the frequency of the radar signal reduced the penetrating power. It's a thorny issue. Additionally, to get a 10 m resolution you'd need to know the position of the spacecraft to within 10 m.

Even with those problems, this seems to me to be the most economical way of getting the sort of data that would be useful for finding the sort of lava tubes suitable for locating a Moon base in.

So what are the last two methods?

References:

[1] T.L. Billings, 'Re: lunar resources/lava tubes', sci.space.tech, 25-01-1996

Lava tubes and satellite photography

So, after yesterday's interlude, I will address the problem of how to find lava tubes on the Moon.

There are several techniques available to use. The first one I will consider is satellite photography.

Initially this may seem more than a little silly: how can (visible spectrum) photographs show lava tubes, when they're underground? Fortunately, we can use indirect methods to find lava tubes. If you recall, last Thursday I explained that rima are collapsed lava tubes, and this fact can be used to find intact lava tubes: all you need to do is look for gaps, or more precisely discontinuities, in a visible rille.

A detailed study was done by Coombs and Hawke [1] of imagery from the Apollo missions and the Lunar Orbiter spacecraft, which turned up more than ninety possible lava tubes. An analysis of the data from the 1994 Clementine mission has yet to be carried out [2]. Which brings us neatly to another problem with visible spectrum photography.

The easiest way to pick out rima is by the shadows of their walls - nice dark lines on a light background. Unfortunately, the Clementine mission was geared toward providing good data for geochemical data, and so they arranged to have all its photos taken at local noon - when the shadows are at their least significant. This is a pretty annoying obstacle to using Clementine data to find lava tubes. That and the rather low resolution.

But there are of course ways to look for lava tubes other than satellite photos.

References:

[1] Coombs, C.R., and B.R. Hawke, 'A Search for Intact Lava Tubes on the Moon: Possible Lunar Base Habitats', in The Second Conference on Lunar Bases and Space Activities of the 21st Century (W.W.Mendell, Ed.), NASA CP-3166, Vol. I, p. 219, 1992.

[2] T.L. Billings, 'Lunar Lava Tubes via Clementine', sci.space.tech, 17-02-2004

Thanks:

T.L. Billings, Oregon L-5 Society Portland Chapter

Lava tube sizes

Yesterday I brought you to the conclusion that lava tubes look like good locations for manned lunar bases. But before I tackle the thorny problem of finding lava tubes in the first place, I feel I ought to give some idea of scale of these lava tubes. I said that you could put a village down inside the largest ones. This was probably an exaggeration, although there's at least one rille 3 km wide. Here's some example data[1]:

A small lava tube might be 20 m under ground, 20 m across and 15 m high.

A large tube might be 500 m under ground, 300 m across and 100 m high. So pretty large.

A good rule of thumb is that lunar lava tubes grow to about 10 times the size of their Terran equivalents.

Another aside: in Latin, the plural of rille is rima, and lava tubes are ducta. So if I start babbling about rima and ducta, you know what I'm on about.

Anyway, back to the point: how do we go about finding intact lava tubes?

References:

[1] Artemis Project: Lunar Lava Tube Dimensions http://www.asi.org/adb/m/04/02/01/02/lava-tube-size.html

Other lava tube advantages

Yesterday we saw how lava tubes would act as a radiation shelter for astronauts on the moon. There are, however, several other potential advantages in putting a base in a lava tube[1].

Firstly, protection from meteoroids. The moon is constantly pounded by meteors weighing tiny fractions of a gram (we call them micrometeors), as well as meteor showers of larger meteors now and again. Although the micrometeorites are an unfortunate annoyance and can be compensated for by building equipment slightly bulkier than ideal, the larger meteor showers would be a serious danger to an exposed base. Placing a base inside a lava tube would protect it from meteor showers, thus making anti-meteor armour less of a necessity, and the tube would also provide a shelter for delicate equipment that would be easily damaged by meteors.

Second are thermal considerations. The lunar surface varies from ≅100 K in the nighttime to ≅400 K in the day, which means that thermal regulation of a base in order to keep it comfortable for humans could be a bit of a nightmare. If a base is placed in a lava tube, protection from heat is no longer a problem - all you need to do would is try your utmost to keep heat in, and we have experience of dealing with that kind of problem from polar research bases.

Finally is the problem of lunar dust. Formed from lunar rock by millions of years of bombardment by meteors, lunar dust is a serious headache. It's a very fine powder that gets into everything, clogging up motors, covering solar panels and shorting out electronics. But because lava tube interiors have been protected from meteors, we expect them to be relative dust free. Of course, by the time we've been walking and driving in and out for a couple of weeks it won't be.

So we've established that putting a lunar base inside a lava tube would be a good idea. So how do we go about finding a good site?

References:

[1] Moon Miners' Manifesto: 12 Questions About Lunar Lava Tubes http://www.asi.org/adb/06/09/03/02/100/12-questions.html

All singing, all dancing, totally electric

(I think) I have managed to iron out all the remaining issues with my website. That includes a brand new design for my weblog! It's quite a bit more lightweight, and has the additional advantage that it won't need to be updated every time my main website's design is updated.

The NEWTS website is now an exercise in geekery. I run NEWTS (a PHP application) over the source tree. It invokes Ant (a Java application), which in turn invokes Docutils (a Python application). But it does what it's meant to do just about perfectly, which is to check out the NEWTS sources from CVS, build a CVS snapshot, build the documentation, and build all that into the website.

If you see any problems with my website (before you point it out, yes, I know MSIE has a broken CSS implementation) scream. I will be.

Can of worms

Yep, I've unwittingly broken my website again. I'll see if I can fix it later today.

Lava tubes and radiation shielding

Yesterday I described lava tubes, and the current evidence for their existence. But why are lava tubes so interesting for engineers investigating the possibility of a manned, long-term lunar base?

The reason is safety - principally with regard to radiation exposure, but other factors come into it as well.

Hazardous radiation in space in the inner solar system divides into two main categories: galactic cosmic rays (GCR) and solar particle events, but for our purposes we can summarize them as high energy particles and low energy particles. On Earth we are protected from the radiation by the Van Allen belts (part of the Earths magnetic field), which trap most of the radiation before it gets to us. We see the interaction between the magnetic field and the particles as the Northern Lights. That's why the Van Allen belts are particularly extensive when there's a solar flare, for instance. But the Moon has no magnetic field, and not even an atmosphere to shield astronauts from radiation. If there was a solar flare that hit the Earth-Moon system, any astronauts on the Moons surface would most likely receive a lethal radiation dose.

Using data from samples brought back from the Moon on the Apollo missions, detailed studies have shown that placing a base in a lava tube with 6 metres of lunar rock and regolith above it would filter out almost all the high energy particles. Less than a metre's depth would shield the base from the effects of a solar flare (De Angelis et al. [1])

So putting the base inside a lava tube would mean that the lunar base would be cheaper to set up, because it would not be necessary to take heavy and bulky radiation shielding to the moon. It would also mean that astronauts on the Moon would be able to continue to work during a solar flare, instead of cowering inside a hardened radiation shelter (although they couldn't go outside, of course).

So what secondary reasons are there for putting a base inside a lava tube?

References:

[1] De Angelis, J.W.Wilson, M.S.Clowdsley, J.E.Nealy, D.Humes and J.M.Clem, 'Lunar Lava Tube Radiation Safety Analysis' http://lowdose.tricity.wsu.edu/2001mtg/abstracts/deangelis2.htm