The Shape of Code

A very interesting article discussing SpaceX’s dramatically lower launch costs has convinced me that, in a decade or two, it will become economically viable to send people to Mars. Whether lots of people will be willing to go is another matter, but let’s assume that a non-trivial number of people decide to spend many years living in a colony on Mars; what computing hardware and software should they take with them?

Reliability and repairability are crucial. Same-day delivery of replacement parts is not an option; the opportunity for Earth/Mars travel occurs every 2-years (when both planets are on the same side of the Sun), and the journey takes 4-10 months.

Given the much higher radiation levels on Mars (200 mS/year; on Earth background radiation is around 3 mS/year), modern microelectronics will experience frequent bit-flips and have a low survival rate. Miniaturization is great for packing billions of transistors into a device, but increases the likelihood that a high energy particle traveling through the device will create a permanent short-circuit; Moore’s law has a much shorter useful life on Mars, compared to Earth. The lesser high energy particles can flip the current value of one or more bits.

Reliability and repairability of electronics, compared to other compute and control options, dictates minimizing the use of electronics (pneumatics is a viable replacement for many tasks; think World War II submarines), and simple calculation can be made using a slide rule or mechanical calculator (both are reliable, and possible to repair with simple tools). Some of the issues that need to be addressed when electronic devices are a proposed solution include:

• integrated circuits need to be fabricated with feature widths that are large enough such that devices are not unduly affected by background radiation,
• devices need to be built from exchangeable components, so if one breaks the others can be used as spares. Building a device from discrete components is great for exchangeability, but is not practical for building complicated cpus; one solution is to use simple cpus, and integrated circuits come in various sizes.
• use of devices that can be repaired or new ones manufactured on Mars. For instance, core memory might be locally repairable, and eventually locally produced.

There are lots of benefits from using the same cpu for everything, with ARM being the obvious choice. Some might suggest RISC-V, and perhaps this will be a better choice many years from now, when a Mars colony is being seriously planned.

Commercially available electronic storage devices have lifetimes measured in years, with a few passive media having lifetimes measured in decades (e.g., optical media); some early electronic storage devices had lifetimes likely to be measured in decades. Perhaps it is possible to produce hard discs with expected lifetimes measured in decades, research is needed (or computing on Mars will have to function without hard discs).

The media on which the source is held will degrade over time. Engraving important source code on the walls of colony housing is one long term storage technique; rather like the hieroglyphs on ancient Egyptian buildings.

What about displays? Have lots of small, same size, flat-screens, and fit them together for greater surface area. I don’t know much about displays, so won’t say more.

Computers built from discrete components consume lots of power (much lower power consumption is a benefit of fabricating smaller devices). No problem, they can double as heating systems. Switching power supplies can be very reliable.

Radio communications require electronics. The radios on the Voyager spacecraft have been operating for 42 years, which suggests to me that reliable communication equipment can be built (I know very little about radio electronics).

What about the software?

Repairability requires that software be open source, or some kind of Mars-use only source license.

The computer language of choice is obviously C, whose advantages include:

• lots of existing, heavily used, operating systems are written in C (i.e., no need to write, and extensively test, a new one),
• C compilers are much easier to implement than, say, C++ or Java compilers. If the C compiler gets lost, somebody could bootstrap another one (lots of individuals used to write and successfully sell C compilers),
• computer storage will be a premium on Mars based computers, and C supports getting close to the hardware to maximise efficient use of resources.

The operating system of choice may not be Linux. With memory at a premium, operating systems requiring many megabytes are bad news. Computers with 64k of storage (yes, kilobytes) used to be used to do lots of useful work; see the source code of various 1980’s operating systems.

Applications can be written before departure. Maintainability and readability are marketing terms, i.e., we don’t really know how to do this stuff. Extensive testing is a good technique for gaining confidence that software behaves as expected, and the test suite can be shipped with the software.