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<h2 class="hd hd-2 unit-title">Overview</h2>
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<h2>5.1.1 Overview of Unit 5</h2>
<p><img style="display: block; margin: 20px auto 20px auto;" src="/assets/courseware/v1/aeab21ce7ffe1de421c94832a9a93e8a/asset-v1:MITx+16.00x+2T2019+type@asset+block/McCandless-Floating-600.jpg" alt="On the STS-41B mission in 1984, NASA astronaut Bruce McCandless II performed the first untethered spacewalk using the Manned Maneuvering Unit (MMU). McCandless reached a distance of about 100 meters from the Space Shuttle Challenger and is seen free-floating in the microgravity environment of space" type="saveimage" target="[object Object]" /></p>
<p style="text-align: center;"><em><span style="font-family: 'Open Sans', Verdana, Arial, Helvetica, sans-serif;">On the STS-41B mission in 1984, NASA astronaut Bruce McCandless II performed the first untethered spacewalk using the Manned Maneuvering Unit (MMU). McCandless reached a distance of about 100 meters from the Space Shuttle Challenger and is seen free-floating in the microgravity environment of space (Image credit: NASA)</span></em></p>
<p>Among the many differences between life on Earth and in orbit, perhaps the most prominent is the sensation of weightlessness in space. Often we hear the term "zero-gravity" used to describe this phenomenon. However, this is a misnomer: although the force of gravity is reduced, it is never actually zero, even in deep space. In fact, without gravity, it would not be possible to keep spacecraft and satellites in orbit. Instead, a more accurate term is "microgravity", which is equal to \(10^{-6} g\).</p>
<p>In Unit 5, we will examine the effects of microgravity on the human body and the consequent implications for human spaceflight. We begin our lesson with a discussion about weightlessness --- what causes weight, what kinds of experiments can be performed in the absence of weight, and how humans move in the microgravity environment. Then, we delve into three areas of human physiology in space: the circulatory system, the musculoskeletal system, and the neurophysiological system. Finally, we conclude Unit 5 with an in-depth look at the issue of space radiation.</p>
<p>Upon completing Unit 5, students should be able to:</p>
<ul>
<li>MO 5.1: Define "microgravity" and explain why gravity on its own does not produce weight</li>
<li>MO 5.2: Identify the different types of research that are enabled by the weightless environment of space</li>
<li>MO 5.3: Understand the threat of syncope (fainting) when astronauts return from space</li>
<li>MO 5.4: Describe the effects of weightlessness on the body's ability to regulate blood pressure</li>
<li>MO 5.5: Describe the fluid shift in the body that occurs when transitioning between Earth gravity, the weightlessness of space, and back again</li>
<li>MO 5.6: Explain the "anemia" associated with spaceflight</li>
<li>MO 5.7: Describe the effects of weightlessness on muscles</li>
<li>MO 5.8: Describe the effects of weightlessness on bones and the relationship to osteoporosis (bone calcium loss)</li>
<li>MO 5.9: Discuss the three ways in which humans perceive their orientation and motion and how each of these ways is affected by weightlessness</li>
<li>MO 5.10: Explain the sensory conflicts in space that may contribute to "space motion sickness"</li>
<li>MO 5.11: Discuss the countermeasures that have been developed to mitigate the adverse effects of weightlessness on the circulatory system, the musculoskeletal system, and the neurophysiological system</li>
<li>MO 5.12: Identify the two types of space radiation and explain why each type poses risks to human spaceflight</li>
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