A penny has a thickness of approximately $1.0 \mathrm{~mm} .$ If you stacked Avogadro's number of pennies one on top of the other on Earth's surface, how far would the stack extend (in $\mathrm{km}$ )? [For comparison, the sun is about 150 million $\mathrm{km}$ from Earth, and the nearest star (Proxima Centauri) is about 40 trillion $\mathrm{km}$ from Earth.]