Program: GS-2004A-Q-68

In recent months during 2003, $\eta$ Carinae experienced a dramatic ``spectroscopic event'', when high-excitation lines in its UV, optical, and IR spectrum disappeared, and its hard X-ray and radio continuum flux crashed. This behavior is part of a 5.5 year cycle, the periodicity has been attributed to a binary system, but the changes also resemble a shell ejection. UV spectra obtained during the event with HST suggest that {\it a shell ejection did indeed occur}, so we expect dust to form 6-12 months afterward, and mid-IR images and spectra with T-ReCS are needed to measure the changing mass of dust and the current bolometric luminosity. Near-IR emission lines trace related changes in the post-event wind, and can test specific models for the cause of $\eta$ Car's variability as it recovers from its recent ``event''. High resolution near-IR spectra with PHOENIX/Gemini will surpass even HST for investigating the complex kinematic structure of $\eta$ Car's ejecta, which hold important clues to its mass ejection history. This most recent event was studied with an intensive observing campaign from X-rays to radio and with HST. Now is a critical time to obtain unique IR measurements of the ejecta as $\eta$ Carinae recovers from this event and the shell expands, cools, and forms dust grains. This will provide the first direct estimate of the mass ejected.