# Space rendezvous

Gemini 7 photographed from Gemini 6 in 1965

A space rendezvous is an orbital maneuver 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 velocities 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 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 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, the Soviet Union launched pairs of spacecraft from the same launch pad, one or two days apart (Vostok 3 and 4 in 1962, and Vostok 5 and 6 in 1963). In each case, the launch vehicles' 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 5 kilometers (3.1 mi) to 6.5 kilometers (4.0 mi), and slowly diverged to thousands of kilometres (over a thousand miles) over the course of the missions.[citation needed]

In 1963 Buzz Aldrin submitted his doctoral thesis titled, Line-Of-Sight Guidance Techniques For Manned Orbital Rendezvous.[1] As a NASA astronaut, Aldrin worked to "translate complex orbital mechanics into relatively simple flight plans for my colleagues." [2]

### First attempt failed

The first attempt at rendezvous was made on June 3, 1965, when US astronaut Jim McDivitt tried to maneuver his Gemini 4 craft to meet back up with its spent 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.[3] Mostly however, the Gemini 4 attempts at rendezvous were unsuccessful largely because NASA engineers had yet to learn the orbital mechanics involved in the process. Simply pointing the active vehicle's nose at the target and thrusting won't do. 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 decreases velocity, putting the tracker above and 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 change to the same orbit as the target with no relative motion between the vehicles.[4]

As GPO engineer André Meyer later remarked, "There is a good explanation for what went wrong with rendezvous." The crew, like everyone else at 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 now uses."

[4]

### First successful rendezvous

Rendezvous was first successfully accomplished by US astronaut Wally Schirra on December 15, 1965. Schirra maneuvered the Gemini 6 spacecraft within 1 foot (30 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:

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 120 feet (37 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.

[5]

### First docking

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 unmanned 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.[3]

The Soviets carried out the first automated, unmanned docking between Cosmos 186 and Cosmos 188 on October 30, 1967.[6]

The first Soviet cosmonaut to attempt a manual docking was Georgi Beregovoi who unsuccessfully tried to dock his Soyuz 3 craft with the unmanned Soyuz 2 in October 1968. He was able to bring his craft from 200 meters (660 ft) to as close as 1 foot (0.30 m), but was unable to dock before exhausting his maneuvering fuel.[citation needed]

The Soviet's first successful manned docking occurred on January 16, 1969 when Soyuz 4 and Soyuz 5 docked and exchanged two crew members.[citation needed]

The first rendezvous of two spacecraft from different countries took place on June 17, 1975, when an Apollo spacecraft docked with a Soyuz spacecraft as part of the Apollo-Soyuz Test Project.[citation needed]

The first multiple space docking took place when both Soyuz 26 and Soyuz 27 were docked to the Salyut 6 space station during January 1978.[citation needed]

 Gemini 8 Agena target vehicle Gemini 8 docking with the Agena in March 1966

## Uses

Soyuz TMA-03M docking with MRM-1 Rassvet in Earth orbit, bringing three crew to the International Space Station in December 2011

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 the ill-fated Soyuz 11, which successfully docked with the Salyut 1 station on June 7, 1971.[7] 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.[8]

Robotic spacecraft are also used to rendezvous with and resupply space stations. Soyuz and Progress spacecraft have automatically docked with both Mir[9] and the ISS using the Kurs docking system, the Automated Transfer Vehicle also uses this system. The robotic H-II Transfer Vehicle flies to a close rendezvous and maintains station-keeping without docking, allowing the ISS Canadarm2 to grapple it and berth it to the station.[citation needed]

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]

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 thrusters. [4]

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]
• 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 manned, LM at this stage unmanned:[citation needed]
• 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

## Phases and methods

The standard technique for rendezvous and docking is to dock an active vehicle with a passive target. This technique has been used successfully for the Gemini, Apollo, Apollo/Soyuz, Salyut, Skylab, Mir, ISS, and Tiāngōng programs.[citation needed]

For this to be possible, both spacecraft must be in the same orbital plane, and the phase of the orbit must be matched. This can be achieved by one spacecraft being in a higher orbit than the other, and the lower orbit will gain on the higher orbit over time.[citation needed]

The active vehicle is then put on an intercept course with the target. The closure rate is then reduced by use of the active vehicle's reaction control system. Docking typically occurs at a rate of 0.1 ft/s (0.030 m/s) to 0.2 ft/s (0.061 m/s).[10]

### 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 10,000 kilometres (6,200 mi):[11]

Phase Separation distance Typical phase duration
Drift Orbit A
(out of sight, out of contact)
>2 λmax[12] 1 to 20 days
Drift Orbit B
(in sight, in contact)
2 λmax to 1 kilometer (3,300 ft) 1 to 5 days
Proximity Operations A 1,000–100 metres (3,280–330 ft) 1 to 5 orbits
Proximity Operations B 100–10 metres (328–33 ft) 45 – 90 minutes
Docking <10 meters (33 ft) <5 minutes

A variety of spacecraft control techniques may be used to effect the translational and rotational maneuvers necessary for proximity operations and docking.[13]

### 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) and perpendicular to the flight path along the line of the radius of the orbit (called R-bar).[11]

V-bar approach

An approach of the active, or "chaser," spacecraft horizontally along the passive spacecraft velocity vector—that is, from behind and in the same direction as the orbital velocity of the passive spacecraft—is called a V-bar approach.[11][14]

STS-104 was the third Space Shuttle mission to conduct a V-bar arrival at the International Space Station.[15] The V-bar, or velocity vector, extends along a line directly ahead of the station. Shuttles approach the ISS along the V-bar when docking at the PMA-2 docking port.[16]

R-bar approach

An approach of the active, or "chaser," spacecraft vertically along the passive spacecraft radial vector—that is, from below and orthogonal to the orbital velocity of the passive spacecraft—is called an R-bar approach.[11][14]

Astrotech proposed meeting ISS cargo needs with a vehicle which would approach the station, "using a traditional nadir R-bar approach."[17] The nadir R-bar approach is also used for flights to the ISS of H-II Transfer Vehicles, and of SpaceX Dragon vehicles.[18][19]

Z-bar approach

An approach of the active, or "chaser," spacecraft horizontally from the side and orthogonal to the 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.[20]

## References

1. ^ Buzz Aldrin. "Orbital Rendezvous".
2. ^ Buzz Aldrin. "From Earth to Moon to Earth".
3. ^ Oral History Transcript / James A. McDivitt / Interviewed by Doug Ward / Elk Lake, Michigan – June 29, 1999
4. ^ a b "Gemini 4". Encyclopedia Astronautica.
5. ^ On The Shoulders of Titans – Ch12-7
6. ^ NSSDC ID: 1967-105A NASA, NSSDC Master Catalog
7. ^ Mark Wade. "Soyuz 11". Encyclopedia Astronautica.
8. ^ Marcia S. Smith (3 Feb 2012). "Space Station Launch Delays Will Have Little Impact on Overall Operations". spacepolicyonline.com.
9. ^ 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."
10. ^
11. ^ a b c d Wertz, James R.; Bell, Robert (2003). "Autonomous Rendezvous and Docking Technologies – Status and Prospects". SPIE AeroSense Symposium. Space Systems Technology and Operations Conference, Orlando Florida, April 21–25, 2003: Paper No. 5088–3. Retrieved October 31, 2011.
12. ^ λmax is the angular radius of the spacecraft’s true horizon as seen from the center of the planet; for LEO, it is the maximum Earth central angle from the altitude of the spacecraft.
13. ^ Lee, Daero; Pernicka, Henry (2010). "Optimal Control for Proximity Operations and Docking". Int’l J. of Aeronautical & Space Science 11 (3): 206–220. Bibcode:2010IJASS..11..206L. doi:10.5139/IJASS.2010.11.3.206. Retrieved November 3, 2011.
14. ^ a b Pearson, Don J. (November 1989). "Shuttle Rendezvous and Proximity Operations". originally presented at COLLOQUE: MECANIQUE SPATIALE (SPACE DYNAMICS) TOULOUSE, FRANCE NOVEMBER 1989. NASA. Retrieved 2011-11-26.
15. ^
16. ^ WILLIAM HARWOOD (March 9, 2001). "Shuttle Discovery nears rendezvous with station". SPACEFLIGHT NOW.
17. ^ Johnson, Michael D.; Fitts, Richard; Howe, Brock; Hall, Baron; Kutter, Bernard; Zegler, Frank; Foster; Mark (September 18, 2007). "Astrotech Research & Conventional Technology Utilization Spacecraft (ARCTUS)" (PDF). AIAA SPACE 2007 Conference & Exposition. Long Beach, California. p. 7.
18. ^ Rendezvous Strategy of the Japanese Logistics Support Vehicle to the International Space Station, [1]
19. ^ Success! Space station snags SpaceX Dragon capsule [2]
20. ^ Bessel, James A.; Ceney, James M. ; Crean, David M. ; Ingham, Edward A. ; Pabst, David J. (December 1993). "Prototype Space Fabrication Platform". Air Force Institute Of Technology, Wright-Patterson AFB, Ohio – School Of Engineering. Accession number ADA273904. Retrieved November 3, 2011.