Unexpected Finger propelled herself by burning iron into photons, which flew out her exhaust pipe.
How much iron?
As Sir Newton captured so beautifully in his famous Third Sonnet, parting is always sweet conservation of momentum. Which means Finger's momentum changed by an amount equal to the momentum of her departing exhaust, but in the opposite direction.
The momentum of an exhaust photon is given by
Oh screw that. Finger is just an ideal photon rocket. Her mathematics were worked out in the 1950's. Her maximum velocity is v = c (Mi2‑Mf2) / (Mi2+Mf2) where c is the speed of light, and Mi, Mf are the ship’s initial and final mass. (Fuel consumed = Mi ‑ Mf.)
This equation even accounts for all that wacky Einstein stuff, which is minor at 0.2c.
Let's plug in some real numbers. And when I say real, I mean fictional, even though Gaia's Wasp was based on actual historical events that could have happened.
The Finger's initial mass was 4000 metric tons, and I will guess that her fuel consumption was, say, 730 tons, leaving her a final (arrival) mass of 4000‑730=3270 tons. Plugging those numbers into the formula gives her velocity v = c(40002‑32702) / (40002+32702) = c(5307100 / 26692900) = 0.2c.
Wow, v = 0.2c, just like in the book. I got her fuel consumption right on my first guess. What are the odds?
So to decelerate from 20% of the speed of light, Finger had to burn 730/4000 = 18% of her initial mass, which was 22% of her final mass, or a non-coincidental 20% of her average mass.
How was her mileage? Finger decelerated for a period of 210 days, which is 1.8×107 seconds. Thus her burn rate was fuel / time = (7.3×105kg) / (1.8×107s), or 0.04 kg/s. In commoner units, Finger's fuel consumption was 1.4 ounces per second.
Wait, 1.4 ounces of mass converts into almost a megaton of energy. Finger decelerated by firing a megaton per second out her tailpipe?
A megaton per second raises a question, which I will try to phrase delicately.
Did the deceleration squish the crew?