Fluorescent gas of a strange isotope
Argon has three nonradioactive isotopes. Ar-40 has a natural abundance of 99.6% while Ar-36 is 0.3365%. Dump a large quantity of Ar-36 beyond the orbit of pluto and then stimulate the expanding argon cloud to fluoresce at a frequency revealing the strangeness of the isotope (use a nuclear pumped laser?). Modulate the intensity of the fluorescence to produce the first 16 prime numbers and then repeat the pattern until the gas density gets too low. Then dump another load of argon and repeat the process until you run out of argon.
This is pure science fiction because no one would pay for it.
Gary