Editing Cosmos 1 (section)

== Physics ==
{{main|Solar sail}}

The craft would have been gradually accelerating during each orbit as a result of the [[radiation pressure]] of photons colliding with the sails. As photons reflected from the surface of the sails, they would transfer [[momentum]] to them. As there would be no air resistance to oppose the velocity of the spacecraft, [[acceleration]] would be proportional to the number of photons colliding with it per unit time. Sunlight amounts to a tiny {{val|5|e=-4|u=m/s2}} acceleration in the vicinity of the Earth. Over one day, the spacecraft's speed would reach {{cvt|45|m/s}}; in 100 days its speed would be {{cvt|4500|m/s}}, in 2.74 years {{cvt|45000|m/s}}.

At that speed, a craft would reach [[Pluto]], a very distant dwarf planet in the [[Solar System]], in less than 5 years,<ref name=ambitious-experiment-BBC-report>{{cite journal|title=''Cosmos 1'': Sailing on sunlight|journal=BBC News Science/Nature|date=22 June 2005|url=http://news.bbc.co.uk/1/hi/sci/tech/4104282.stm|access-date=2007-09-04}}</ref> although in practice the acceleration of a sail drops dramatically as the spacecraft gets farther from the [[Sun]]. However, in the vicinity of Earth, a solar sail's acceleration is larger than that of some other propulsion techniques; for example, the [[ion thruster]]-propelled [[SMART-1]] spacecraft has a maximum acceleration of {{val|2|e=-4|u=m/s2}}, which allowed SMART-1 to achieve lunar orbit in November 2004 after launch in September 2003.