If it's solar radiation that flips the bits, why not just stick it in a lead case?

A SSD (flash-based) doesn't need to be powered. Finding a computer that has a SATA/IDE interface to read it back out after 50 years is an interesting problem, however.

[update]

Spansion quotes 20 years minimum. Under optimum conditions (whatever they may be) and with an adequate amount of redundancy, 50 years should be achievable.

Spansion single-bit-per-cell floating-gate flash devices are designed to provide 20 years of data retention after initial programming when exposed to a 125° C environment. Spansion two-bit-per-cell MirrorBit flash devices are designed to provide 20 years of data retention after initial programming when exposed to an 85° C environment. Both MirrorBit flash and floating gate flash are guaranteed to provide 20 years of data retention after initial programming when exposed to a 55° C environment. MirrorBit flash is guaranteed to retain data for up to the minimum guaranteed cycles (10,000). [F]loating gate is guaranteed to retain data after the guaranteed minimum of 100,000 cycles.

http://www.spansion.com/Support/AppNotes/EndureRetentn_AN_A0...

I've never heard of rechargeable batteries with a lifespan of 50 years. Also, the best quality photovoltaic solar cells have a lifespan of 25 years, and their output is down to around 5% of original capacity by the time 20 years rolls around.