[85] The realm of test gases is limited only by the ingenuity of the experimenter. Helium, for instance, [86] liquefies at just a few degrees above absolute zero, thereby permitting expansion of room- temperature helium to Mach numbers approaching 30 without liquefaction. Unfortunately, helium differs appreciably from air in molecular weight and thermodynamic characteristics. Helium tunnels play an important role in studying basic fluid mechanics, but their results require careful interpretation for application to space vehicle design. Hypersonic tests in highmolecular-weight gases (such as Freon C₂F₄ and C₂F₆) provide excellent simulation of the density ratio across the shock and the real-gas effects experienced by blunt bodies during reentry. Some facilities employ the actual constituents of planetary atmospheres (combinations of helium, hydrogen, nitrogen, etc.) to study the entry problem into neighboring planets.