Context: The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its absolute pressure, but a complete characterization requires further parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (~6997100000000000000♠1×10−3 Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes. The Crookes radiometer turns when the MFP is larger than the size of the vanes.

Question: What indicated the quality of a vacuum?
Answer: amount of matter remaining in the system

Question: How is vacuum generally measured?
Answer: its absolute pressure

Question: What does the MFP of residual gases show?
Answer: average distance that molecules will travel between collisions with each other.

Question: What is particle gas dynamics?
Answer: study of fluid flows

Question: What indicates the quality of residual gases?
Answer: Unanswerable

Question: How much matter is left in residual gases?
Answer: Unanswerable

Question: How are residual gases measured?
Answer: Unanswerable

Question: What other factors are necessary to measure residual gases?
Answer: Unanswerable

Question: What is the study of residual gases in a spacecraft called?
Answer: Unanswerable