# How long will our artifacts last in moon & space?

Given all the different space probes and equipment that have been either launched into space or lying on the moon. How long will they last before they get decayed into dust or some unrecognizable form?

I read that there's some upper limit to their existence due to evaporation of metal into vacuum. So, I'm curious what folks would think, "What that upper limit would be?" and also, "What other factors would come into play?"

Note: Provided they don't get smashed or broken up by some type of event :-)

I've always been curious how long the Pioneer plaque would last, so I tried doing a very rough order of magnitude estimate - which could be applied similarly to entire spacecraft, or objects on the moon, etc. Note that I'm trying to make the estimate conservative, so I use pessimistic estimates.

Some Numbers:
$v\sim500 \textrm{ km s}^{-1} = 5\times10^{7} \textrm{ cm s}^{-1}$
The velocity of pioneer is about 12 km/s; but the velocity of the solar wind (near earth) is about 500 km/s, which dominates.

$\rho \sim 10^{-23} \textrm{ g cm}^{-3}$
The density of the solar wind near earth

$A_p\sim 340 \textrm{ cm}^2$
$m_p\sim 120 \textrm{ g}$
The surface area, and mass of the plaque

This article says that space dust can create craters up to 10 times the size of the particles, I'm going to use that to estimate that a given impacter can eject 10 times its own mass of material.

We can calculate the mass flux of incident particles as,
$F_m \sim v \cdot \rho \cdot A \sim (5\times10^{7} \textrm{ cm s}^{-1}) \cdot (10^{-23} \textrm{ g cm}^{-3}) \cdot (340 \textrm{ cm}^2)$
$F_m \sim 2\times10^{-13} \textrm{ g s}^{-1}$
Then, with the 10x 'damage factor', the mass flux ejected is,
$F_{e} \sim 2\times10^{-12} \textrm{ g s}^{-1}$.
We can then estimate the lifetime of the object as,
$\tau \sim m_p / F_e \sim 120 \textrm{ g} \div 2\times10^{-12} \textrm{ g s}^{-1}$
$\boxed{\tau \sim 7\times10^{14} \textrm{ s} \sim 20 \textrm{ Myr}}$

For the Pioneer plaque, in particular, one might further estimate that the inscription would become unreadable after one-tenth or half of that time, i.e. 2-10 Myr. At the same time, the plaque is positioned in such a way as to protect its surface from erosion -- but its still a fair ballpark estimate.

The same order of magnitude number should also apply to a plaque on the moon. Note that the amount of damage is proportional to the area of the object, but the lifetime is proportional to the volume of the object. Thus, one might naively expect an objects survival lifetime to scale (very roughly) linearly with size.

This type of estimate would drastically over-estimate the lifetime of objects on mars, which are buffeted by sand-storms.

Finally, I assume that the only statistically significant impactors are gas/dust --- but it is possible that the probability of colliding with larger grains could be non-negligible over 10's of millions of years.