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It is well-known that when we are in space for a long time, our bodies lose some mass. My question is how much time would be needed for a person to lose 10% of their body mass?
The tortoise inside the Zond 5 spacecraft seems to have lost the 10% in just a lunar flyby. Is it the same for humans, or is it just because tortoises are smaller than us?
It's important to note that the tortoises carried on the Zond flight were probably not given any food or water, and I suspect they were selected because they could survive that for a long period. So weight loss would be inevitable. (We know that Apollo 17 carried pocket mice in the Command Module to study radiation exposure, and that the species was chosen for their ability to survive on very small amounts of water.)
For people it's completely different as of course they are well supplied.
Muscle mass through atrophy is common if exercise is not taken. There is a good summary on the Wikipedia page Effect of spaceflight on the human body, and the headline claim there is:
without regular exercise astronauts can lose up to 20% of their muscle mass in just 5 to 11 days
Also see a NASA factsheet on muscle atrophy which is the source of this figure.
We should remember the "up to 20%" figure is for muscle loss, so the overall body mass loss will be much less and anyway will depend on the calorie intake...
Not only were they starved, they were starved for a lot longer than the 6-day duration of the mission. According to this translation of Gaidamakin et al. (1969), the "turtles" (steppe tortoises, Testudo horsfieldi Gray) were actually starved for 39 days:
The turtles were put on board the "Zond-5" on 2 September 1968. From that time on they ceased to received food ... lift-off was on 15 September ... splashed down on 21 September ... returned to Moscow on 7 October ... Patho-morphological investigations [i.e. dissection!] on the turtles were carried out on 11 October.
As is seen from the cited sequence ... the test animals were subjected to 39 days of starvation, flight factors lasting 7 days, the effect of tropical climate and conditions associated with a stay in the ocean after splashdown, and with transportation via ship and aircraft.
...
Weight loss of the animals that flew in the probe was about 10%. Weight loss of the control animals was only 5%.
Gaidamakin, N.A., G. P. Parfenov, V. G Petrukhin, V. V. Antipov, P. P. Saksonov, and A. V. Smirnova. “Patho-Morphological and Histochemical Changes in the Organs of Turtles on Board the ‘Zond-5’ Probe.” edited by George R. Zug and James A. Peters, translated by Morris D. Friedman. La Plata, Argentina: Smithsonian Herpetological Information Services, 1969. https://repository.si.edu/bitstream/handle/10088/32682/1970.SHIS24.pdf.
