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We explore the effects of lower thermospheric water vapor deposited by launch vehicle plumes on polar mesospheric cloud (PMC) frequencies at 80°N. We use July‐averaged PMC frequencies from 2007 to 2022 from the Cloud Imaging and Particle Size (CIPS) instrument on NASA's Aeronomy of Ice in the Mesosphere (AIM) satellite. Launch sites worldwide are typically located near northern mid‐latitudes. Using the orbital launch record for the same time period, we find that the number of launches correlates with PMC frequencies with a coefficient of r = 0.60, which increases to r = 0.75 when only selecting launches from 2.5 to 21.5 local time (LT), indicating a weak LT dependence on global‐scale transport to 80°N. To support our findings, we use meridional winds from the Michelson Interferometer for Global High‐resolution Imaging experiment on NASA's Ionospheric Connection Explorer satellite and winds from the Horizontal Wind Model climatology to interpret the northward motion of air parcels at 105 km. We find the launch LT window that maximizes the correlation coefficient to be consistent with the expected maximum northward motion from the diurnal variation of mid‐latitude meridional winds. Comparisons with Microwave Limb Sounder satellite observations of upper mesospheric temperature and water vapor reveal a strong dependence of cloud frequency on water vapor (r = 0.86) but not on temperature (r = −0.26), indicating that water vapor is the primary source of PMC variability for the bright PMCs at 80°N. We therefore find that launch vehicle plumes originating primarily from northern mid‐latitudes modulate PMC frequency at 80°N in July. Plain Language Summary: Formation of clouds in the mesosphere in the polar summer depends heavily on atmospheric water vapor. Main engine exhaust plumes from space shuttles and other launch vehicles contain many tons of water vapor which can be transported poleward at an altitude of ∼105 km. Over several days this water vapor descends to ∼80–85 km and aids in the formation of summertime polar mesospheric clouds (PMCs). This work shows how the number of launches per year correlates with the mean interannual PMC frequency averaged over July at 80°N latitude. The correlation maximizes for vehicles launched from 2.5 to 21.5 local time. To provide additional important evidence for the poleward transport of plumes launched at these local times, we use wind data to quantify the maximum northward motion of an exhaust plume over the diurnal cycle near 105 km altitude. We also use satellite temperature and water vapor data at PMC altitudes to show that bright PMCs vary more with the water vapor content than with the air temperature. We conclude that the total water vapor deposited by summertime space traffic worldwide modulates the PMC frequency at 80°N. Key Points: The correlation between orbital rocket launches and polar mesospheric clouds at 80°N in July from 2007 to 2022 is quantifiedMeridional wind, temperature, and water vapor data are used to support causal interpretation of the correlation coefficientsEvidence indicates that water vapor in space traffic exhaust plumes modulates frequencies of polar mesospheric clouds at 80°N [ABSTRACT FROM AUTHOR]