Editing International Space Station (section)

====Freefall====
[[File:ISS-20 Robert Thirsk at the Minus Eighty Degree Laboratory Freezer.jpg|thumb|ISS crew member storing samples]]
[[File:Space Fire.jpg|thumb|A comparison between the combustion of a candle on [[Earth]] (left) and in a free fall environment, such as that found on the ISS (right)]]

Gravity at the altitude of the ISS is approximately 90% as strong as at Earth's surface, but objects in orbit are in a continuous state of [[Free fall|freefall]], resulting in an apparent state of [[weightlessness]].<ref name="nasa-whatismicrogravity">{{Cite web|last=May|first=Sandra|date=15 February 2012|title=What Is Microgravity?|url=https://www.nasa.gov/learning-resources/for-kids-and-students/what-is-microgravity-grades-5-8/|url-status=live|archive-url=https://web.archive.org/web/20231107174300/https://www.nasa.gov/learning-resources/for-kids-and-students/what-is-microgravity-grades-5-8/|archive-date=7 November 2023|access-date=3 September 2018|series=NASA Knows! (Grades 5–8)|publisher=[[NASA]]}} {{PD-notice}}</ref> This perceived weightlessness is disturbed by five effects:<ref name="gravity">{{cite web|date=6 December 2005|title=European Users Guide to Low Gravity Platforms|url=http://www.esa.int/Our_Activities/Human_Spaceflight/Human_Spaceflight_Research/European_User_Guide_to_Low-Gravity_Platforms|url-status=dead|archive-url=https://web.archive.org/web/20130402225556/http://www.esa.int/Our_Activities/Human_Spaceflight/Human_Spaceflight_Research/European_User_Guide_to_Low-Gravity_Platforms|archive-date=2 April 2013|access-date=22 March 2013|publisher=European Space Agency}}</ref>
* Drag from the residual atmosphere.
* Vibration from the movements of mechanical systems and the crew.
* Actuation of the on-board attitude [[control moment gyroscope]]s.
* [[Rocket engine|Thruster]] firings for attitude or orbital changes.
* [[Gravity-gradient stabilization|Gravity-gradient effects]], also known as [[tidal force|tidal]] effects. Items at different locations within the ISS would, if not attached to the station, follow slightly different orbits. Being mechanically connected, these items experience small forces that keep the station moving as a [[rigid body]].

Researchers are investigating the effect of the station's near-weightless environment on the evolution, development, growth and internal processes of plants and animals. In response to some of the data, NASA wants to investigate [[microgravity]]'s effects on the growth of three-dimensional, human-like tissues and the unusual [[protein crystal]]s that can be formed in space.<ref name="NASA Fields of Research" />

Investigating the physics of fluids in microgravity will provide better models of the behaviour of fluids. Because fluids can be almost completely combined in microgravity, physicists investigate fluids that do not mix well on Earth. Examining reactions that are slowed by low gravity and low temperatures will improve our understanding of [[superconductivity]].<ref name="NASA Fields of Research" />

The study of [[materials science]] is an important ISS research activity, with the objective of reaping economic benefits through the improvement of techniques used on Earth.<ref name="nasa-materials-science-101">{{Cite web|url=https://science.nasa.gov/newhome/headlines/msad15sep99_1.htm|title=Materials Science 101|date=15 September 1999|publisher=[[NASA]]|access-date=18 June 2009|url-status=dead|archive-url=https://web.archive.org/web/20090614033947/http://science.nasa.gov/newhome/headlines/msad15sep99_1.htm|archive-date=14 June 2009}} {{PD-notice}}</ref> Other areas of interest include the effect of low gravity on combustion, through the study of the efficiency of burning and control of emissions and pollutants. These findings may improve knowledge about energy production and lead to economic and environmental benefits.<ref name="NASA Fields of Research" />