View source for Space rendezvous

{{Short description|Series of orbital maneuvers}}
{{Use mdy dates|date=November 2011}}
[[File:Range finding from shuttle to ISS.jpg|thumb|[[Astronaut]] [[Christopher Cassidy]] uses a [[Rangefinding telemeter|rangefinder]] to determine distance between the {{OV|105}} and the [[International Space Station]]]]
[[File:Apollo 11 lunar module.jpg|thumb|[[Lunar Module Eagle|Lunar Module ''Eagle'']] ascent stage rendezvous with the [[command module Columbia|command module ''Columbia'']] in lunar orbit after returning from a landing]]
A '''''space rendezvous''''' ({{IPAc-en|ˈ|r|ɒ|n|d|eɪ|v|uː}}) is a set of [[orbital maneuver]]s during which two [[spacecraft]], one of which is often a [[space station]], arrive at the same [[orbit]] and approach to a very close distance (e.g. within visual contact). Rendezvous requires a precise match of the [[Orbital elements|orbital velocities and position vectors]] of the two spacecraft, allowing them to remain at a constant distance through [[orbital station-keeping]]. Rendezvous may or may not be followed by [[docking and berthing of spacecraft|docking or berthing]], procedures which bring the spacecraft into physical contact and create a link between them.

The same rendezvous technique can be used for spacecraft "landing" on natural objects with a weak gravitational field, e.g. landing on one of the [[Moons of Mars|Martian moons]] would require the same matching of orbital velocities, followed by a "descent" that shares some similarities with docking.

== History ==
In its first human spaceflight program [[Vostok programme|Vostok]], the [[Soviet Union]] launched pairs of spacecraft from the same launch pad, one or two days apart ([[Vostok 3|Vostok 3 and 4]] in 1962, and [[Vostok 5|Vostok 5 and 6]] in 1963). In each case, the [[launch vehicle]]s' guidance systems inserted the two craft into nearly identical orbits; however, this was not nearly precise enough to achieve rendezvous, as the Vostok lacked maneuvering thrusters to adjust its orbit to match that of its twin. The initial separation distances were in the range of {{convert|5|to|6.5|km|mi|sp=us}}, and slowly diverged to thousands of kilometers (over a thousand miles) over the course of the missions.<ref name=Gatland>{{Cite book
  | last = Gatland
  | first = Kenneth
  | title = Manned Spacecraft, Second Revision
  | publisher = Macmillan Publishing Co., Inc.
  | date = 1976
  | location = New York
  | pages = 117–118
  | isbn = 0-02-542820-9}}</ref><ref>{{cite book
 |title= The Rocket Men: Vostok & Voskhod, The First Soviet Manned Spaceflights
 |last= Hall
 |first= Rex
 |author2= David J. Shayler
 |date= 2001
 |publisher= [[Springer Science+Business Media|Springer–Praxis Books]]
 |pages= 185–191
 |location= New York
 |isbn= 1-85233-391-X
 |url= https://books.google.com/books?id=zndYLKa26wAC
 |access-date= September 25, 2016
 |archive-date= April 2, 2020
 |archive-url= https://web.archive.org/web/20200402223717/https://books.google.com/books?id=zndYLKa26wAC
 |url-status= live
 }}</ref>

In early 1964 the Soviet Union guided two unmanned satellites designated [[Istrebitel Sputnikov|Polyot 1 and 2]] to within 5km, and the crafts were able to establish radio communication.<ref>{{Cite web |title=The Historic Beginnings Of The Space Arms Race |url=https://www.spacewar.com/reports/The_Historic_Beginnings_Of_The_Space_Arms_Race_999.html |access-date=2024-11-21 |website=www.spacewar.com}}</ref><ref>{{Cite web |last1=RBTH |last2=Novosti |first2=Yury Zaitsev, RIA |date=2008-11-01 |title=The historic beginnings of the space arms race |url=https://www.rbth.com/articles/2008/11/01/311008_space.html |access-date=2024-11-21 |website=Russia Beyond |language=en-US}}</ref><ref>{{Cite web |title=MilsatMagazine |url=http://www.milsatmagazine.com/story.php?number=701833149 |access-date=2024-11-21 |website=www.milsatmagazine.com}}</ref>

In 1963 [[Buzz Aldrin]] submitted his doctoral thesis titled, '' Line-Of-Sight Guidance Techniques For Manned Orbital Rendezvous.''<ref name=buzz>{{cite web |title=Orbital Rendezvous |author=Buzz Aldrin |url=http://buzzaldrin.com/space-vision/rocket_science/orbital-rendezvous/ |access-date=May 4, 2012 |archive-date=October 9, 2011 |archive-url=https://web.archive.org/web/20111009142726/http://buzzaldrin.com/space-vision/rocket_science/orbital-rendezvous/ |url-status=live }}</ref> As a NASA astronaut, Aldrin worked to "translate complex [[orbital mechanics]] into relatively simple flight plans for my colleagues."<ref name=waterkeeper>{{cite web |title=From Earth to Moon to Earth |author=Buzz Aldrin |url=http://www.waterkeeper.org/wp-content/uploads/2013/08/Fall-2005-Hawks-Doves.pdf |url-status=dead |archive-url=https://web.archive.org/web/20140527224845/http://www.waterkeeper.org/wp-content/uploads/2013/08/Fall-2005-Hawks-Doves.pdf |archive-date=May 27, 2014 |df=mdy-all }}</ref>

===First attempt failed===

NASA's first attempt at rendezvous was made on June 3, 1965, when US astronaut [[Jim McDivitt]] tried to maneuver his [[Gemini 4]] craft to meet its spent [[Titan II GLV|Titan II launch vehicle]]'s upper stage. McDivitt was unable to get close enough to achieve station-keeping, due to depth-perception problems, and stage propellant venting which kept moving it around.<ref name="mcdivittoh">Oral History Transcript / [http://www.jsc.nasa.gov/history/oral_histories/McDivittJA/mcdivittja.pdf James A. McDivitt] {{Webarchive|url=https://web.archive.org/web/20160304130319/http://www.jsc.nasa.gov/history/oral_histories/McDivittJA/mcdivittja.pdf |date=March 4, 2016 }} / Interviewed by Doug Ward / Elk Lake, Michigan – June 29, 1999</ref>
However, the Gemini 4 attempts at rendezvous were unsuccessful largely because NASA astronauts had not yet been taught the [[orbital mechanics]] involved in the process. Simply pointing the active vehicle's nose at the target and thrusting was unsuccessful. If the target is ahead in the orbit and the tracking vehicle increases speed, its altitude also increases, actually moving it away from the target. The higher altitude then increases orbital period due to [[Kepler's laws of planetary motion#Third law|Kepler's third law]], putting the tracker not only above, but also behind the target. The proper technique requires changing the tracking vehicle's orbit to allow the rendezvous target to either catch up or be caught up with, and then at the correct moment changing to the same orbit as the target with no relative motion between the vehicles (for example, putting the tracker into a lower orbit, which has a shorter orbital period allowing it to catch up, then executing a [[Hohmann transfer orbit|Hohmann transfer]] back to the original orbital height).<ref name="gemini-4-ea">{{Cite web |url=http://www.astronautix.com/flights/gemini4.htm |title=Gemini 4 |publisher=Encyclopedia Astronautica |url-status=dead |archive-url=https://web.archive.org/web/20101129073633/http://astronautix.com/flights/gemini4.htm |archive-date=November 29, 2010 |df=mdy-all }}</ref>

{{quote|As [[Project Gemini|GPO]] engineer André Meyer later remarked, "There is a good explanation for what went wrong with rendezvous." The crew, like everyone else at [[Manned Spacecraft Center|MSC]], "just didn't understand or reason out the [[orbital mechanics]] involved. As a result, we all got a whole lot smarter and really perfected rendezvous maneuvers, which [[Apollo program|Apollo]] now uses."|<ref name="gemini-4-ea" />}}

===First successful rendezvous===
[[File:Gemini 7 in orbit - GPN-2006-000035.jpg|right|thumb|Gemini 7 photographed from Gemini 6 in 1965]]
Rendezvous was first successfully accomplished by US astronaut [[Wally Schirra]] on December 15, 1965. Schirra maneuvered the [[Gemini 6A|Gemini 6]] spacecraft within {{convert|1|ft|cm}} of its sister craft [[Gemini 7]]. The spacecraft were not equipped to dock with each other, but maintained station-keeping for more than 20 minutes. Schirra later commented:<ref>{{cite web|url=http://www.hq.nasa.gov/office/pao/History/SP-4203/ch12-7.htm|title=On The Shoulders of Titans - Ch12-7|website=www.hq.nasa.gov|access-date=April 9, 2018|archive-date=April 3, 2020|archive-url=https://web.archive.org/web/20200403013722/http://www.hq.nasa.gov/office/pao/History/SP-4203/ch12-7.htm|url-status=live}}</ref>

{{quote|Somebody said ... when you come to within three miles (5 km), you've rendezvoused. If anybody thinks they've pulled a rendezvous off at three miles (5 km), have fun! This is when we started doing our work. I don't think rendezvous is over until you are stopped – completely stopped – with no relative motion between the two vehicles, at a range of approximately {{convert|120|ft|m}}. That's rendezvous! From there on, it's stationkeeping. That's when you can go back and play the game of driving a car or driving an airplane or pushing a skateboard – it's about that simple.}}

Schirra used another metaphor to describe the difference between the two nations' achievements:<ref name="agle199809">{{Cite magazine |last=Agle |first=D.C. |date=September 1998 |title=Flying the Gusmobile |url=https://www.airspacemag.com/flight-today/flying-the-gusmobile-218187/ |magazine=Air & Space |language=en |access-date=2018-12-15 |archive-date=April 3, 2020 |archive-url=https://web.archive.org/web/20200403013717/https://www.airspacemag.com/flight-today/flying-the-gusmobile-218187/ |url-status=live }}</ref>

{{quote|[The Russian "rendezvous"] was a passing glance—the equivalent of a male walking down a busy main street with plenty of traffic whizzing by and he spots a cute girl walking on the other side. He's going 'Hey wait' but she's gone. That's a passing glance, not a rendezvous.}}

===First docking===
{{Main|Docking and berthing of spacecraft}}
[[File:S66-25781 PR.jpg|thumb|Gemini 8 Agena target vehicle]]
[[File:Gemini8Docking.gif|thumb|Gemini 8 docking with Agena vehicle]]

The first docking of two spacecraft was achieved on March 16, 1966 when [[Gemini 8]], under the command of [[Neil Armstrong]], rendezvoused and docked with an uncrewed [[Agena Target Vehicle]]. Gemini 6 was to have been the first docking mission, but had to be cancelled when that mission's Agena vehicle was destroyed during launch.<ref>{{cite web|url=http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=GEM|title=NASA - NSSDCA - Spacecraft - Details|website=nssdc.gsfc.nasa.gov|access-date=April 9, 2018|archive-date=April 3, 2020|archive-url=https://web.archive.org/web/20200403231734/https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=GEM|url-status=live}}</ref>

The Soviets carried out the first automated, uncrewed docking between [[Cosmos 186]] and [[Cosmos 188]] on October 30, 1967.<ref>[https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1967-105A NSSDC ID: 1967-105A] {{Webarchive|url=https://web.archive.org/web/20200413185438/https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1967-105A |date=April 13, 2020 }} NASA, NSSDC Master Catalog</ref>

The first Soviet cosmonaut to attempt a manual docking was [[Georgy Beregovoy]] who unsuccessfully tried to dock his [[Soyuz 3]] craft with the uncrewed [[Soyuz 2]] in October 1968. Automated systems brought the craft to within {{convert|200|m|ft|sp=us}}, while Beregovoy brought this closer with manual control.<ref>{{Cite web |title=Part 1 - Soyuz |url=https://historycollection.jsc.nasa.gov/history/shuttle-mir/references/documents/mirhh-part1.pdf |url-status=live |archive-url=https://web.archive.org/web/20221007084620/https://historycollection.jsc.nasa.gov/history/shuttle-mir/references/documents/mirhh-part1.pdf |archive-date=2022-10-07 |website=History Collection - Johnson Space Center - NASA |page=11}}</ref>

The first successful crewed docking<ref name="MAAS Collection">{{cite web | title=Model of a Soyuz-4-5 spacecraft | website=MAAS Collection | url=https://collection.maas.museum/object/157010 | access-date=Oct 22, 2021}}</ref> occurred on January 16, 1969 when [[Soyuz 4]] and [[Soyuz 5]] docked, collecting the two crew members of Soyuz 5, which had to perform an [[extravehicular activity]] to reach Soyuz 4.<ref name="NASA">{{cite web | title=NSSDCA - Spacecraft - Details | website=NASA | url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1969-004A | language=no | access-date=Oct 22, 2021}}</ref>

In March 1969 [[Apollo 9]] achieved the first internal transfer of crew members between two docked spacecraft.

The first rendezvous of two spacecraft from different countries took place in 1975, when an Apollo spacecraft docked with a Soyuz spacecraft as part of the [[Apollo–Soyuz]] mission.<ref>{{cite encyclopedia|url=https://books.google.com/books?id=K751AwAAQBAJ&pg=PT747|title=Encyclopedia of United States National Security|isbn=978-0-7619-2927-7|publisher=[[SAGE Publications]]|editor-first=Richard J.|editor-last=Samuels|editor-link=Richard J. Samuels|edition=1st|date=December 21, 2005|page=669|quote=Most observers felt that the U.S. moon landing ended the space race with a decisive American victory. […] The formal end of the space race occurred with the 1975 joint Apollo–Soyuz mission, in which U.S. and Soviet spacecraft docked, or joined, in orbit while their crews visited one another's craft and performed joint scientific experiments.|access-date=September 20, 2020|archive-date=July 26, 2020|archive-url=https://web.archive.org/web/20200726071509/https://books.google.com/books?id=K751AwAAQBAJ&pg=PT747|url-status=live}}</ref>

A multiple space docking took place when [[Soyuz 26]] and [[Soyuz 27]] were both docked to the [[Salyut 6]] space station during January 1978. This was the first time a second crew visited a space station with another crew already present.<ref>{{cite web | title=Spaceflight mission report: Soyuz 27 | url=https://www.spacefacts.de/mission/english/soyuz-27.htm }}</ref>

== Uses ==
[[File:Portrait of ASTP crews - restoration.jpg|thumb|Most rendezvous are for docking, as in this photo of the crews and spaceship models of the historic first time Soviet and US spacecraft [[Apollo–Soyuz]] docking in 1975 of the concluding [[Space Race]]]]
[[File:Mir collision damage STS086-720-091.JPG|right|thumb|Damaged solar arrays on ''Mir'''s ''[[Spektr]]'' module following a collision with an uncrewed [[Progress spacecraft]] in September 1997 as part of [[Shuttle-Mir]]. The Progress spacecraft were used for re-supplying the station. In this space rendezvous gone wrong, the Progress collided with Mir, beginning a depressurization that was halted by closing the hatch to ''Spektr''.|alt=A gold-coloured solar array, bent and twisted out of shape and with several holes. The edge of a module can be seen to the right of the image, and Earth is visible in the background.]]
A rendezvous takes place each time a spacecraft brings crew members or supplies to an orbiting space station. The first spacecraft to do this was [[Soyuz 11]], which successfully docked with the [[Salyut 1]] station on June 7, 1971.<ref name="soyuz-11-ea">{{Cite web |url=http://www.astronautix.com/flights/soyuz11.htm |title=Soyuz 11 |publisher=Encyclopedia Astronautica |author=Mark Wade |url-status=dead |archive-url=https://web.archive.org/web/20071030215242/http://www.astronautix.com/flights/soyuz11.htm |archive-date=October 30, 2007 |df=mdy-all }}</ref> [[Human spaceflight]] missions have successfully made rendezvous with six [[Salyut]] stations, with [[Skylab]], with ''[[Mir]]'' and with the [[International Space Station]] (ISS). Currently [[Soyuz spacecraft]] are used at approximately six month intervals to transport crew members to and from ISS. With the introduction of NASA's Commercial Crew Program, the US is able to use their own launch vehicle along with the Soyuz, an updated version of SpaceX's Cargo Dragon; Crew Dragon.<ref>{{cite web |title= Space Station Launch Delays Will Have Little Impact on Overall Operations |url= https://spacepolicyonline.com/news/sufferdini-space-station-launch-delays-will-have-little-impact-on-overall-operations-correction/?Spacepolicyonline_(SpacePolicyOnline_News) |author= Marcia S. Smith |date= 3 February 2012 |publisher= spacepolicyonline.com |access-date= June 13, 2020 |archive-date= June 13, 2020 |archive-url= https://web.archive.org/web/20200613032707/https://spacepolicyonline.com/news/sufferdini-space-station-launch-delays-will-have-little-impact-on-overall-operations-correction/?Spacepolicyonline_(SpacePolicyOnline_News) |url-status= live }}</ref>

<!-- Need paragraph here discussing rendezvous as part of on-orbit assembly of modular stations. -->
[[Robotic spacecraft]] are also used to rendezvous with and resupply space stations. [[Soyuz spacecraft|Soyuz]] and [[Progress spacecraft]] have automatically docked with both ''Mir''<ref>Bryan Burrough, Dragonfly: NASA and the crisis aboard Mir, (1998, {{ISBN|0-88730-783-3}}) 2000, {{ISBN|0-06-093269-4}}, page 65, "Since 1985 all Russian spacecraft had used the Kurs computers to dock automatically with the Mir station" ... "All the Russian commanders had to do was sit by and watch."</ref> and the ISS using the [[Kurs (docking system)|Kurs docking system]], Europe's [[Automated Transfer Vehicle]] also used this system to dock with the Russian segment of the ISS. Several uncrewed spacecraft use NASA's [[common berthing mechanism|berthing mechanism]] rather than a [[Androgynous Peripheral Attach System#APAS-95|docking port]]. The Japanese [[H-II Transfer Vehicle]] (HTV), [[SpaceX Dragon]], and [[Cygnus (spacecraft)|Orbital Sciences' Cygnus]] spacecraft all maneuver to a close rendezvous and maintain station-keeping, allowing the ISS [[Canadarm2]] to grapple and move the spacecraft to a berthing port on the US segment. However the updated version of Cargo Dragon will no longer need to berth but instead will autonomously dock directly to the space station. The Russian segment only uses docking ports so it is not possible for HTV, Dragon and Cygnus to find a berth there.<ref>{{cite web |url=https://www.nasa.gov/content/station-crew-captures-japanese-cargo-craft |title=Japanese Cargo Craft Captured, Berthed to Station |author=Jerry Wright |date=30 July 2015 |publisher=nasa.gov |access-date=May 15, 2017 |archive-date=May 19, 2017 |archive-url=https://web.archive.org/web/20170519202311/https://www.nasa.gov/content/station-crew-captures-japanese-cargo-craft/ |url-status=live }}</ref>

Space rendezvous has been used for a variety of other purposes, including recent service missions to the [[Hubble Space Telescope]]. Historically, for the missions of [[Project Apollo]] that landed astronauts on the [[Moon]], the ascent stage of the [[Apollo Lunar Module]] would rendezvous and dock with the [[Apollo Command/Service Module]] in [[lunar orbit rendezvous]] maneuvers. Also, the [[STS-49]] crew rendezvoused with and attached a rocket motor to the [[Intelsat VI]] F-3 [[communications satellite]] to allow it to make an [[orbital maneuver]].{{citation needed|date=August 2012}}

Possible future rendezvous may be made by a yet to be developed automated Hubble Robotic Vehicle (HRV), and by the [[CX-OLEV]], which is being developed for rendezvous with a [[geosynchronous satellite]] that has run out of fuel. The CX-OLEV would take over [[orbital stationkeeping]] and/or finally bring the satellite to a graveyard orbit, after which the CX-OLEV can possibly be reused for another satellite. Gradual transfer from the [[geostationary transfer orbit]] to the [[geosynchronous orbit]] will take a number of months, using [[Hall effect thruster]]s.<ref>{{cite web|url=http://www.orbitalrecovery.com/news15.html|title=orbitalrecovery.com|website=www.orbitalrecovery.com|access-date=April 9, 2018|archive-date=February 10, 2010|archive-url=https://web.archive.org/web/20100210072513/http://www.orbitalrecovery.com/news15.html|url-status=live}}</ref>

Alternatively the two spacecraft are already together, and just undock and dock in a different way:
*Soyuz spacecraft from one docking point to another on the ISS or Salyut{{citation needed|date=August 2012}}
*In the [[Apollo spacecraft]], a maneuver known as [[transposition, docking, and extraction]]  was performed an hour or so after [[Trans Lunar Injection]] of the sequence third stage of the [[Saturn V]] rocket / LM inside LM adapter / CSM (in order from bottom to top at launch, also the order from back to front with respect to the current motion), with CSM crewed, LM at this stage uncrewed:{{citation needed|date=August 2012}}
**the CSM separated, while the four upper panels of the LM adapter were disposed of
**the CSM turned 180 degrees (from engine backward, toward LM, to forward)
**the CSM connected to the LM while that was still connected to the third stage
**the CSM/LM combination then separated from the third stage

NASA sometimes refers to "Rendezvous, [[Proximity operations|Proximity-Operations]], [[Docking and berthing of spacecraft|Docking, and Undocking]]" (RPODU) for the set of all spaceflight procedures that are typically needed around spacecraft operations where two spacecraft work in proximity to one another with intent to connect to one another.<ref name=OE-LL>{{Cite web |url=https://www.nasa.gov/externalflash/dart/Resources/Rendezvous%20Proximity%20Operations%20Docking%20and%20Undocking%20Lessons%20Learned.pdf |title=A Summary of the Rendezvous, Proximity Operations, Docking, and Undocking (RPODU) Lessons Learned from the Defense Advanced Research Project Agency (DARPA) Orbital Express (OE) Demonstration System Mission
|access-date=May 16, 2020 |archive-date=August 7, 2020 |archive-url=https://web.archive.org/web/20200807104848/https://www.nasa.gov/externalflash/dart/Resources/Rendezvous%20Proximity%20Operations%20Docking%20and%20Undocking%20Lessons%20Learned.pdf |url-status=live }}</ref>

== Phases and methods ==
[[File:Apollo 10 command module.jpg|thumb|[[Apollo command and service module|Command and service module]] ''Charlie Brown'' as seen from [[Apollo Lunar Module|Lunar Module]] ''Snoopy'']]
[[File:ATVtoISS.png|thumb|Orbital rendezvous. 1/ Both spacecraft must be in the same orbital plane. ISS flies in a higher orbit (lower speed), ATV flies in a lower orbit and catches up with ISS. 2/At the moment when the ATV and the ISS make an alpha angle (about 2°), the ATV crosses the elliptical orbit to the ISS.<ref name="Arrival of the ATV to the ISS">Arrival of the ATV to the ISS, {{cite web |title=ATV: a very special delivery - Lesson notes |url=https://www.esa.int/Education/Space_In_Bytes/ATV_a_very_special_delivery_-_Lesson_notes |publisher=ESA |access-date=April 29, 2021 |archive-date=April 29, 2021 |archive-url=https://web.archive.org/web/20210429203106/https://www.esa.int/Education/Space_In_Bytes/ATV_a_very_special_delivery_-_Lesson_notes |url-status=live }}</ref>]]
{{Multiple issues|section=yes|
{{Cleanup-Jargon|section|date=April 2010}}
{{More citations needed|section|date=August 2020}}
}}
The standard technique for rendezvous and docking is to dock an active vehicle, the "chaser", with a passive "target". This technique has been used successfully for the Gemini, Apollo, Apollo/Soyuz, Salyut, Skylab, Mir, ISS, and Tiangong programs.{{citation needed|date=August 2012}}

To properly understand spacecraft rendezvous it is essential to understand the relation between spacecraft velocity and orbit. A spacecraft in a certain orbit cannot arbitrarily alter its velocity. Each orbit correlates to a certain orbital velocity. If the spacecraft fires thrusters and increases (or decreases) its velocity it will obtain a different orbit, one with a higher or lower altitude. In circular orbits, higher orbits have a lower orbital velocity. Lower orbits have a higher orbital velocity.

For orbital rendezvous to occur, both spacecraft must be in the same [[Orbital plane (astronomy)|orbital plane]], and the [[Orbit phasing|phase of the orbit]] (the position of the spacecraft in the orbit) must be matched.<ref name="Arrival of the ATV to the ISS" /> For docking, the speed of the two vehicles must also be matched. The "chaser" is placed in a slightly lower orbit than the target. The lower the orbit, the higher the orbital velocity. The difference in orbital velocities of chaser and target is therefore such that the chaser is faster than the target, and catches up with it.{{citation needed|date=October 2011}}

Once the two spacecraft are sufficiently close, the chaser's orbit is synchronized with the target's orbit. That is, the chaser will be accelerated. This increase in velocity carries the chaser to a higher orbit. The increase in velocity is chosen such that the chaser approximately assumes the orbit of the target. Stepwise, the chaser closes in on the target, until proximity operations (see below) can be started.
In the very final phase, the closure rate is reduced by use of the active vehicle's [[reaction control system]].
Docking typically occurs at a rate of {{convert|0.1|ft/s|m/s|abbr=on}} to {{convert|0.2|ft/s|m/s|abbr=on}}.<ref>{{cite web | url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870015906_1987015906.pdf | title = TRACK AND CAPTURE OF THE ORBITER WITH THE SPACE STATION REMOTE MANIPULATOR SYSTEM | publisher = NASA | access-date = July 7, 2017 | archive-date = August 7, 2020 | archive-url = https://web.archive.org/web/20200807191602/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19870015906_1987015906.pdf | url-status = live }}</ref>

===Rendezvous phases===
Space rendezvous of an active, or "chaser", spacecraft with an (assumed) passive spacecraft may be divided into several phases, and typically starts with the two spacecraft in separate orbits, typically separated by more than {{convert|10000|km|sp=us}}:<ref name=wertz2003/>

{| class="wikitable"
|-
! Phase
! Separation distance
! Typical phase duration 
|-
| '''Drift Orbit A'''<br>(out of sight, out of contact)
| >2 λ<sub>max</sub>{{efn-num|λ<sub>max</sub> is the [[Angular diameter|angular radius]] of the spacecraft's true [[horizon]] as seen from the center of the planet; for [[low Earth orbit|LEO]], it is the maximum Earth central angle from the altitude of the spacecraft.}}
| 1 to 20 days
|-
| '''Drift Orbit B'''<br>(in sight, in contact)
| 2 λ<sub>max</sub> to {{convert|1|km|ft|sp=us}}
| 1 to 5 days
|-
| '''Proximity Operations A'''
| {{convert|1000|-|100|m|ft|sp=us}}
| 1 to 5 orbits
|-
| '''Proximity Operations B'''
| {{convert|100|-|10|m|ft|sp=us}}
| 45 – 90 minutes
|-
| '''Docking'''
| <{{convert|10|m|ft|sp=us}}
| <5 minutes
|}

A variety of techniques may be used to effect the [[Translation (physics)|translational]] and [[rotation]]al [[Orbital maneuver|maneuvers]] necessary for proximity operations and docking.<ref name=lee2010>
{{cite journal |last=Lee |first=Daero |author2=Pernicka, Henry |title=Optimal Control for Proximity Operations and Docking |journal= International Journal of Aeronautical and Space Sciences|date=2010 |volume=11 |issue=3 |pages=206–220 |doi=10.5139/IJASS.2010.11.3.206 |bibcode=2010IJASS..11..206L |df=mdy-all |doi-access=free }}</ref>

===Methods of approach===

The two most common methods of approach for [[proximity operations]] are in-line with the flight path of the spacecraft (called V-bar, as it is along the velocity vector of the target) and [[orthogonal|perpendicular]] to the flight path along the line of the radius of the orbit (called R-bar, as it is along the radial vector, with respect to Earth, of the target).<ref name=wertz2003>
{{cite journal |last=Wertz|first=James R. |author2=Bell, Robert  |editor-first1=Peter |editor-first2=James |editor-last1=Tchoryk, Jr. |editor-last2=Shoemaker |title=Autonomous Rendezvous and Docking Technologies – Status and Prospects |journal=SPIE AeroSense Symposium |date=2003 |series=Space Systems Technology and Operations Conference, Orlando Florida, April 21–25, 2003 |volume=5088 |page=20 |doi=10.1117/12.498121 |bibcode=2003SPIE.5088...20W |s2cid=64002452 |id=Paper 5088-3 |url=http://microcosminc.com/analysis/spie03.pdf|archive-url=https://web.archive.org/web/20120425122952/http://microcosminc.com/analysis/spie03.pdf|url-status=dead|archive-date=2012-04-25|access-date=August 3, 2019}}</ref>
The chosen method of approach depends on safety, spacecraft / thruster design, mission timeline, and, especially for docking with the ISS, on the location of the assigned docking port.

==== V-bar approach====
The V-bar approach is an approach of the "chaser" horizontally along the passive spacecraft's velocity vector. That is, from behind or from ahead, and in the same direction as the orbital motion of the passive target. The motion is [[wikt:parallel|parallel]] to the target's orbital velocity.<ref name=wertz2003/><ref name="pearson1989">{{cite web | last=Pearson|first=Don J.|date=November 1989 |title=Shuttle Rendezvous and Proximity Operations|url=http://home.comcast.net/~djpearson/rndz/rndzpaper.html|access-date=November 26, 2011|work=originally presented at COLLOQUE: MECANIQUE SPATIALE (SPACE DYNAMICS) TOULOUSE, FRANCE NOVEMBER 1989 |publisher=NASA|archive-date=July 27, 2013|archive-url=https://web.archive.org/web/20130727004238/http://home.comcast.net/~djpearson/rndz/rndzpaper.html|url-status=live}}</ref><!-- ref 'pearson1989' provides examples of V-bar and R-bar approaches, without definitions -->
In the V-bar approach from behind, the chaser fires small thrusters to increase its velocity in the direction of the target. This, of course, also drives the chaser to a higher orbit. To keep the chaser on the V-vector, other thrusters are fired in the radial direction. If this is omitted (for example due to a thruster failure), the chaser will be carried to a higher orbit, which is associated with an orbital velocity lower than the target's. Consequently, the target moves faster than the chaser and the distance between them increases. This is called a ''natural braking effect'', and is a natural safeguard in case of a thruster failure.{{citation needed|date=June 2014}}

[[STS-104]] was the third [[Space Shuttle]] mission to conduct a V-bar arrival at the [[International Space Station]].<ref>
{{cite web | url = http://spaceflight.nasa.gov/shuttle/archives/sts-104/crew/inthobaugh.html | archive-url = https://web.archive.org/web/20020203145933/http://spaceflight.nasa.gov/shuttle/archives/sts-104/crew/inthobaugh.html | url-status = dead | archive-date = 2002-02-03 | title = STS-104 Crew Interviews with Charles Hobaugh, Pilot | publisher = NASA}}</ref>  The V-bar, or [[Orbital state vectors|velocity vector]], extends along a line directly ahead of the station. Shuttles approach the ISS along the V-bar when docking at the [[Pressurized Mating Adapter|PMA-2]] docking port.<ref name=harwood>{{cite web |url=http://spaceflightnow.com/station/stage5a1/010309fd2/ |title=Shuttle Discovery nears rendezvous with station |publisher=SPACEFLIGHT NOW |author=WILLIAM HARWOOD |date=March 9, 2001 |access-date=March 17, 2009 |archive-date=December 2, 2008 |archive-url=https://web.archive.org/web/20081202151744/http://spaceflightnow.com/station/stage5a1/010309fd2/ |url-status=live }}</ref>

==== R-bar approach====
The R-bar approach consists of the chaser moving below or above the target spacecraft, along its radial vector. The motion is [[orthogonal]] to the orbital velocity of the passive spacecraft.<ref name=wertz2003/><ref name=pearson1989/><!-- ref 'pearson1989' provides examples of V-bar and R-bar approaches, without definitions -->
When below the target the chaser fires radial thrusters to close in on the target. By this it increases its altitude. However, the orbital velocity of the chaser remains unchanged (thruster firings in the radial direction have no effect on the orbital velocity). Now in a slightly higher position, but with an orbital velocity that does not correspond to the local circular velocity, the chaser slightly falls behind the target. Small rocket pulses in the orbital velocity direction are necessary to keep the chaser along the radial vector of the target. If these rocket pulses are not executed (for example due to a thruster failure), the chaser will move away from the target. This is a ''natural braking effect''. For the R-bar approach, this effect is stronger than for the V-bar approach, making the R-bar approach the safer one of the two.{{citation needed|date=June 2014}}
Generally, the R-bar approach from below is preferable, as the chaser is in a lower (faster) orbit than the target, and thus "catches up" with it. For the R-bar approach from above, the chaser is in a higher (slower) orbit than the target, and thus has to wait for the target to approach it.{{citation needed|date=June 2014}}

[[Astrotech Corporation|Astrotech]] proposed meeting ISS cargo needs with a vehicle which would approach the station, "using a traditional nadir R-bar approach."<ref>
{{cite conference
 |last        = Johnson
 |first       = Michael D.
 |author2     = Fitts, Richard
 |author3     = Howe, Brock
 |author4     = Hall, Baron
 |author5     = Kutter, Bernard
 |author6     = Zegler, Frank
 |author7     = Foster
 |author8     = Mark
 |title       = Astrotech Research & Conventional Technology Utilization Spacecraft (ARCTUS)
 |book-title  = AIAA SPACE 2007 Conference & Exposition
 |page        = 7
 |place       = Long Beach, California
 |date        = September 18, 2007
 |url         = http://pdf.aiaa.org/preview/CDReadyMSPACE07_1808/PV2007_6130.pdf
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20080227050750/http://pdf.aiaa.org/preview/CDReadyMSPACE07_1808/PV2007_6130.pdf
 |archive-date = February 27, 2008
 |df          = mdy-all
}}</ref> The nadir R-bar approach is also used for flights to the ISS of [[H-II Transfer Vehicle]]s, and of [[SpaceX Dragon]] vehicles.<ref>''Rendezvous Strategy of the Japanese Logistics Support Vehicle to the International Space Station,'' [http://adsabs.harvard.edu/full/1997ESASP.381..103Y] {{Webarchive|url=https://web.archive.org/web/20210505175034/http://adsabs.harvard.edu/full/1997ESASP.381..103Y|date=May 5, 2021}}</ref><ref>''Success! Space station snags SpaceX Dragon capsule'' [http://news.cnet.com/8301-11386_3-57441570-76/success-space-station-snags-spacex-dragon-capsule/] {{Webarchive|url=https://web.archive.org/web/20120525171424/http://news.cnet.com/8301-11386_3-57441570-76/success-space-station-snags-spacex-dragon-capsule/|date=May 25, 2012}}</ref>

==== Z-bar approach====
An approach of the active, or "chaser", spacecraft horizontally from the side and orthogonal to the [[Orbital elements|orbital plane]] of the passive spacecraft—that is, from the side and out-of-plane of the orbit of the passive spacecraft—is called a Z-bar approach.<ref name=bessel1993>
{{cite journal |last=Bessel|first=James A. |author2=Ceney, James M.  |author3=Crean, David M.  |author4=Ingham, Edward A.  |author5= Pabst, David J.  |title=Prototype Space Fabrication Platform |journal=Air Force Institute of Technology, Wright-Patterson AFB, Ohio – School of Engineering |date=December 1993 |series=Accession number ADA273904 |page=9 |bibcode=1993MsT..........9B |url=http://handle.dtic.mil/100.2/ADA273904 |archive-url=https://web.archive.org/web/20120531101530/http://handle.dtic.mil/100.2/ADA273904 |url-status=dead |archive-date=May 31, 2012 |access-date=November 3, 2011}}</ref><!-- Note: a drawing exists in this US Federal Government publication https://web.archive.org/web/20120531101530/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA273904&Location=U2&doc=GetTRDoc.pdf that graphically illustrates the R-Bar, V-Bar, and Z-Bar approaches. This could probably be cleaned up and modified to make an illustration for this article. -->

==Surface rendezvous==
[[File:AS12-48-7133 (21470506269).jpg|thumb|[[Apollo 12]] astronaut Conrad with [[Surveyor 3]] and Apollo 2 lander in the background, in a first ever visit of an independent mission beyond [[Low Earth Orbit]]]]
[[Apollo 12]], the second crewed [[lunar landing]], performed the first ever rendezvous outside of [[Low Earth Orbit]] by landing close to [[Surveyor 3]] and taking parts of it back to Earth.

==See also==
{{Portal|Spaceflight}}
* [[Androgynous Peripheral Attach System]]
* [[Clohessy-Wiltshire equations]] for co-orbit analysis
* [[Common Berthing Mechanism]]
* [[Deliberate crash landings on extraterrestrial bodies]]
* [[Flyby (spaceflight)]]
* [[Lunar orbit rendezvous]]
* [[Mars orbit rendezvous]]
* [[Nodal precession]] of orbits around the Earth's axis
* [[Path-constrained rendezvous]] – the process of moving an orbiting object from its current position to a desired position, in such a way that no orbiting obstacles are contacted along the way
* [[Soyuz Kontakt]]

== Notes ==
{{notelist-num}}

== References ==
{{reflist |2}}

==External links==
{{Commons category}}
*[http://issfd.org/ISSFD_2012/ISSFD23_FF1_4_abstract.pdf Analysis of a New Nonlinear Solution of Relative Orbital Motion by T. Alan Lovell]
*[http://www.hq.nasa.gov/office/pao/History/SP-4203/ch12-7.htm The Visitors (rendezvous)] {{Webarchive|url=https://web.archive.org/web/20200403013722/http://www.hq.nasa.gov/office/pao/History/SP-4203/ch12-7.htm |date=April 3, 2020 }}
*{{cite web | url = http://www.nasa.gov/centers/langley/news/factsheets/Rendezvous.html | title = Lunar Orbit Rendezvous and the Apollo Program | publisher = NASA}}
*[https://www.cambridge.org/core/books/automated-rendezvous-and-docking-of-spacecraft/73C7A1056AF42242D7B5549E8BB1FE17 Automated Rendezvous and Docking of Spacecraft] by [[Wigbert Fehse]]
*[http://spaceflightnow.com/news/n1010/20dockingsystem/ Docking system agreement key to global space policy] – October 20, 2010

{{Orbits}}
{{Spacecraft Docking Systems}}

{{Authority control}}

{{DEFAULTSORT:Space Rendezvous}}
[[Category:Space rendezvous| ]]
[[Category:Astrodynamics]]
[[Category:Orbital maneuvers]]
[[Category:1965 introductions]]
[[Category:Projects established in 1965]]