{{short description|Rocket flight}} {{Use dmy dates|date = May 2015}} {{Infobox spaceflight | name = USA-261 | image = Liftoff (17790124678).jpg | image_caption = Launch of OTV-4 | mission_type = Classified | operator = Air Force Space Command | COSPAR_ID = 2015-025A | SATCAT = 40651 | website = | mission_duration = {{Age in years, months and days|2015|5|20|2017|5|7}} | spacecraft_type = Boeing X-37B | manufacturer = Boeing | launch_mass = {{convert|5400|kg|lb|abbr=on}}<ref name="GSP">{{cite web |url=http://space.skyrocket.de/doc_sdat/x-37.htm |title=X-37B OTV 1, 2, 3 |work=Gunter's Space Page |first=Gunter D. |last=Krebs |access-date=10 July 2013}}</ref> | power = Deployable solar array, batteries<ref name="GSP" /> | launch_date = {{Start date text|20 May 2015|timezone=yes}} | launch_rocket = Atlas V 501 | launch_site = Cape Canaveral SLC-41 | launch_contractor = United Launch Alliance | landing_date = {{End date text|7 May 2017}} | landing_site = Shuttle Landing Facility Runway 15 | orbit_reference = Geocentric | orbit_regime = Low Earth | orbit_periapsis = {{convert|312|km|mi|abbr=on}} | orbit_apoapsis = {{convert|325|km|mi|abbr=on}} | orbit_inclination = 43.50 degrees | orbit_period = 90.93 minutes | apsis = gee | instruments = | insignia = | insignia_caption = | insignia_size = | programme = '''OTV program''' | previous_mission = OTV-3 | next_mission = OTV-5 }} '''USA-261''', also referred to as '''Orbital Test Vehicle 4''' ('''OTV-4''') or '''AFSPC-5''', is the second flight of the second Boeing X-37B, an American unmanned vertical-takeoff, horizontal-landing spaceplane. It was launched to low Earth orbit aboard an Atlas V rocket from Cape Canaveral on May 20, 2015. Its mission designation is part of the USA series.
The spaceplane was operated by the Air Force Space Command, which considers the mission classified and as such has not revealed the objectives. However, the Air Force did reveal that the X-37 would be carrying an experimental hall-effect thruster for testing ahead of potential use in future AEHF satellites.<ref>{{Cite web|title=Hall ion thrusters to fly on X-37B spaceplane|url=https://newatlas.com/us-air-force-x-37b-hall-thruster/37200/|date=2015-04-29|website=New Atlas|language=en-US|access-date=2020-05-16}}</ref> Additionally, NASA announced it was flying a materials science payload similar to MISSE called METIS.<ref name=":0">{{Cite web|title=NASA gives more information on its experiment aboard the X-37B – Spaceflight Now|url=https://spaceflightnow.com/2015/05/06/nasa-gives-more-information-on-its-experiment-aboard-the-x-37b/|last=Ray|first=Justin|language=en-US|access-date=2020-05-16}}</ref>
== Mission == OTV-4 is the second mission for the second X-37B, and the fourth X-37B mission overall. It flew on an Atlas V rocket, with a 5-meter payload fairing and no solid rocket boosters.<ref>{{Cite web|title=Launch story: X-37B embarks on fourth voyage in orbit – Spaceflight Now|url=https://spaceflightnow.com/2015/05/20/recap-story-x-37b-embarks-on-fourth-voyage-in-space/|last=Ray|first=Justin|language=en-US|access-date=2020-05-16}}</ref>
Originally scheduled to launch on May 6, OTV-4 was delayed until May 20 for undisclosed reasons.<ref name=":0" /> OTV-4 launched successfully at 11:05 Eastern Time on May 20, 2015.<ref>{{Cite web|title=Launch story: X-37B embarks on fourth voyage in orbit – Spaceflight Now|url=https://spaceflightnow.com/2015/05/20/recap-story-x-37b-embarks-on-fourth-voyage-in-space/|last=Ray|first=Justin|language=en-US|access-date=2020-05-16}}</ref>
OTV-4 stayed in orbit for a record-setting 718 days, beating OTV-3's record of 674 days.<ref name=":1">{{Cite news|title=X-37B lands after record-setting mission|url=https://spacenews.com/x-37b-lands-after-record-setting-mission/|date=2017-05-07|newspaper=Spacenews|language=en-US|access-date=2020-05-16|last1=Foust |first1=Jeff }}</ref> OTV-4 landed on May 7, 2017, at the Shuttle Landing Facility.<ref name=":1" />
== ELaNa XI == NASA utilized the launch of OTV-4 to fly the eleventh ELaNa CubeSat rideshare mission. Ten cubesats were deployed from a P-POD deployment system affixed to the Centaur upper stage.<ref name=":2">{{Cite web|title=elana xi|url=https://www.nasa.gov/sites/default/files/files/ELaNa-XI-Factsheet-508.pdf}}</ref>
ELaNa XI included LightSail-1, an experimental solar sail.<ref name=":2" />
== See also ==
== References == {{reflist}}{{X-37B flights}}{{Orbital launches in 2015}}
Category:Spacecraft launched in 2015 Category:Satellites of the United States Air Force Category:Spacecraft which reentered in 2017 Category:USA satellites Category:Boeing X-37