5 seconds in its reference frame or in Earth's reference frame?

@Yev Earth's. (Assuming the Earth is in an inertial reference frame at the time. If it's not, I think we'll all have bigger problems.)

@IsaacKing I'm assuming the reference frame is not rotating with the earth?

(for the purposes of speed calculation)

@Yev Correct. The reference point is the center of mass of the Earth.

Fastest so far is the Parker Solar Probe at 0.064% of light speed. (That's relative to the Sun, not the Earth, but the Earth's orbital speed is only about 0.01% of light speed, so adding it on doesn't help much.)

~7 million miles per hour.

~18,026,000,000 furlongs per fortnight.

@IsaacKing Tipped