Long-duration spaceflight (~6 months) exposes astronauts to sustained microgravity, producing a cephalad fluid shift, progressive hypovolemia, and cardiovascular deconditioning that impairs mission readiness and post-flight orthostatic tolerance. Seismocardiography (SCG) offers a non-invasive method to monitor systolic time intervals (STIs) as indices of cardiac mechanical function. This study aimed to characterize changes in SCG-derived STIs in astronauts before and after long-duration International Space Station (ISS) missions. As part of the CARDIOBREATH study, funded by the Canadian Space Agency, ECG and SCG signals were collected from five astronauts (n=10 planned) using the CardioClin device at pre-flight baseline (L-100 to L-60; BDC), and at early (R+6 to R+8; POST1) and late (R+13 to R+15; POST2) post-flight recovery. Each session included six minutes of supine rest, a six-minute orthostatic stand, and a 25-minute cycling exercise, followed by another six-minute orthostatic stand. STIs were derived from annotation of 20-cycle averaged SCG waveforms during the supine rest period. Outcome measures included the pre-ejection period (PEP; a validated index of cardiac sympathetic contractility), left ventricular ejection time (LVET; reflecting mechanical efficiency), their ratio (PEP/LVET), electromechanical delay (EMD), and total systolic time (QS2). Preliminary analysis of five astronauts revealed a consistent post-flight pattern of increased PEP (133 to 150 ms), decreased LVET (365 to 354 ms), and elevated PEP/LVET ratio (0.38 to 0.44) relative to pre-flight baseline, alongside a prolongation of total systolic time QS2 (498 to 504 ms). This pattern is consistent with a post-flight shift toward reduced contractile efficiency and shortened ejection duration, reflecting the hemodynamic consequences of microgravity-induced cardiovascular deconditioning. These findings extend prior wearable SCG evidence of spaceflight-induced STI alterations and, given that microgravity serves as an accelerated model of cardiovascular aging, support the use of supine SCG for non-invasive cardiac monitoring in older adults and patients undergoing prolonged bedrest.