One of the recurrent Internet memes since the successful landing of the Curiosity Mars rover has been that the iPhone has more processing power than the computer onboard the rover. It's true -- but some of the numbers touted in the rush of blog posts have been flawed. Let's take a closer look at Curiosity's computing power versus the iPhone 4S.
First of all, just about every post I've seen so far neglects to point out that Curiosity has two identical computers, each called a Rover Compute Element or RCE. Of course, it only uses one at a time; the other is configured as a live backup that will take over the tasks of navigation, control, and communications if the other has problems. Does your iPhone 4S have a backup computer? I didn't think so.
Each RCE uses a RAD750 CPU (based on the IBM PowerPC 750), a radiation-hardened single-board computer made by BAE Systems Electronic Solutions. That computer has 256 KB of EEPROM, 256 MB of DRAM, and 2 GB of flash memory. The CPU runs at 200 MHz and is capable of 400 million instructions per second (MIPS).
The iPhone 4S uses an Apple A5 system-on-a-chip containing an 800 MHz dual-core ARM Cortex-A9 CPU. The A5 also includes a PowerVR SGX543MP2 GPU that adds even more processing power. The iPhone is equipped with 512 MB of DRAM and either 16 GB, 32 GB or 64 GB of flash memory. The CPU runs at 800 MHz.
So, why not just use a couple of iPhones to run the next Mars rover? Well, in the first place, the device would probably fail rather quickly in interplanetary space and in the harsh conditions of the Martian surface, where a thin atmosphere fails to shield rovers from high radiation levels. That RAD750 CPU is capable of handling a radiation dose of up to 1,000 gray -- a 5 gray dose is enough to kill a human being within 14 days.
Putting it all into perspective, the iPhone 4S does have a much more powerful brain than the Curiosity rover, but it's a moot point. Engineers design products, whether they're mass market smartphones or one-off Mars rovers, to handle the conditions that they'll see in everyday usage. To quote Elton John, "Mars ain't the kind of place to raise your kids." That delicate flower of an iPhone wouldn't last a minute on Mars.