The myosin head consists of a globular catalytic domain that binds actin and hydrolyzes ATP and a (Fig. 1 Upper). Depending on the magnitude of the angle change, the swinging motion of the -8-nm-long lever arm could produce a displacement of similar size, in keeping with 4-to 10-nm steps directly measured (6-9).Electron microscopy has successfully observed two different shapes of the head in a way that is consistent with this model (10). Physical measurements have also supported this hypothesis by demonstrating a cyclic change in gyratory radius of the head during ATP hydrolysis cycles (11,12). Recently, 3-D helical reconstruction of actin decorated with myosin heads revealed a swinging motion of the neck region associated with release of ADP from the catalytic domain, resulting in 3-toThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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The National Aeronautics and Space Administration's (NASA) Airspace Systems Program is contributing air traffic management research in support of the 2025 Next Generation Air Transportation System (NextGen). Contributions support research and development needs provided by the interagency Joint Planning and Development Office (JPDO). These needs generally call for integrated technical solutions that improve systemlevel performance and work effectively across multiple domains and planning time horizons. In response, the Airspace Systems Program is pursuing an integrated research approach and has adapted systems engineering best practices for application in a research environment. Systems engineering methods aim to enable researchers to methodically compare different technical approaches, consider system-level performance, and develop compatible solutions. Systems engineering activities are performed iteratively as the research matures. Products of this approach include a demand and needs analysis, system-level descriptions focusing on NASA research contributions, system assessment and design studies, and common systemlevel metrics, scenarios, and assumptions. Results from the first systems engineering iteration include a preliminary demand and needs analysis; a functional modeling tool; and initial system-level metrics, scenario characteristics, and assumptions. Demand and needs analysis results suggest that several advanced concepts can mitigate demand/capacity imbalances for NextGen, but fall short of enabling three-times current-day capacity at the nation's busiest airports and airspace. Current activities are focusing on standardizing metrics, scenarios, and assumptions, conducting system-level performance assessments of integrated research solutions, and exploring key system design interfaces.

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