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|Country of origin||United States|
|Cost per launch||US$40 million |
|Height||16.9 meters (55 ft) (Pegasus)|
17.6 meters (58 ft) (Pegasus XL)
|Diameter||1.27 meters (4.2 ft)|
|Mass||18,500 kilograms (40,800 lb) (Pegasus)|
23,130 kilograms (50,990 lb) (Pegasus XL)
|Payload to LEO||443 kilograms (977 lb) |
(1.18 by 2.13 meters (3.9 ft × 7.0 ft))
|Family||Air launch to orbit|
|Launch sites||Air launch to orbit|
|First flight||5 April 1990 (Pegsat / NavySat)|
|Last flight||11 October 2019 (ICON)|
The Pegasus is an air-launched rocket developed by Orbital Sciences Corporation (now part of Northrop Grumman Innovation Systems after Northrop Grumman acquired Orbital ATK). Capable of carrying small payloads of up to 443 kilograms (977 lb) into low Earth orbit, Pegasus first flew in 1990 and remains active as of 2019[update]. The vehicle consists of three solid propellant stages and an optional monopropellant fourth stage. Pegasus is released from its carrier aircraft at approximately 40,000 ft (12,000 m), and its first stage has a wing and a tail to provide lift and attitude control while in the atmosphere. Notably, the first stage does not have a Thrust Vector Control (TVC) system. 
Pegasus was designed by a team led by Antonio Elias. The Pegasus's three Orion solid motors were developed by Hercules Aerospace (later Alliant Techsystems) specifically for the Pegasus launcher but using advanced carbon fiber, propellant formulation and case insulation technologies originally developed for the terminated USAF Small ICBM program. The wing and fins' structures were designed by Burt Rutan and his company, Scaled Composites, which manufactured them for Orbital.
- Mass: 18,500 kg (Pegasus), 23,130 kg (Pegasus XL):3
- Length: 16.9 m (Pegasus), 17.6 m (Pegasus XL):3
- Diameter: 1.27 m
- Wing span: 6.7 m
- Payload: 443 kg (1.18 m diameter, 2.13 m length)
Started in the spring of 1987, the development project was funded by Orbital Sciences Corporation and Hercules Aerospace, and did not receive any government funding. NASA did provide the use of the B-52 carrier aircraft on a cost-reimbursable basis during the development (captive carry tests) and the first few flights. Two Orbital internal projects, the Orbcomm communications constellation and the OrbView observation satellites, served as anchor customers to help justify the private funding. 
DARPA purchased the first flight and options for five more, but only exercised the first of the five options. The DARPA contract was subsequently transferred to the U.S. Air Force which exercised the remaining four options. In 1993 NASA issued a Request for Proposals for a Small Expendable Launch Vehicles Services (SELVS) requiring a performance slightly higher than the original Pegasus, resulting in Orbital and Hercules developing the XL version to satisfy the NASA SELVS requirement.
There were no Pegasus test launches prior to the first operational launch on April 5, 1990 with NASA test pilot and former astronaut Gordon Fullerton in command of the carrier aircraft. Initially, a NASA-owned B-52 Stratofortress NB-008 served as the carrier aircraft. By 1994, Orbital had transitioned to their "Stargazer" L-1011, a converted airliner which was formerly owned by Air Canada. The name "Stargazer" is an homage to the television series Star Trek: The Next Generation: the character Jean-Luc Picard was captain of a ship named Stargazer prior to the events of the series, and his first officer William Riker once served aboard a ship named Pegasus.
During its 44-launch history, the Pegasus program had three mission failures (STEP-1, STEP-2 and HETI/SAC-B), and two partial failures, (USAF Microsat and STEP-2) followed by 30 consecutive successful flights for a total program success rate of 89%.. The first partial failure on July 17, 1991 caused the 7 USAF microsatellites to be delivered to a lower than planned orbit, significantly reducing the mission lifetime. The last mission failure on November 4, 1996 resulted in the loss of gamma-burst identifying satellite HETE (High Energy Transient Explorer) in 1996..
The Pegasus XL, introduced in 1994 has lengthened stages to increase payload performance. In the Pegasus XL, the first and second stages are lengthened into the Orion 50SXL and Orion 50XL, respectively. Higher stages are unchanged; flight operations are similar. The wing is strengthened slightly to handle the higher weight. The standard Pegasus has been discontinued; the Pegasus XL is still active as of 2019. Pegasus has flown 44 missions in both configurations, launching 91 satellites as of October 12, 2019.
Dual payloads can be launched, with a canister that encloses the lower spacecraft and mounts the upper spacecraft. The upper spacecraft deploys, the canister opens, then the lower spacecraft separates from the third-stage adapter. Since the fairing is unchanged for cost and aerodynamic reasons, each of the two payloads must be relatively compact. Other multiple-satellite launches involve "self-stacking" configurations, such as the ORBCOMM spacecraft.
For their work in developing the rocket, the Pegasus team led by Antonio Elias was awarded the 1991 National Medal of Technology by U.S. President George H. W. Bush.
The initial launch price offered was US$6 million, without options or a HAPS (Hydrazine Auxiliary Propulsion System) maneuvering stage. With the enlargement to Pegasus XL and the associated improvements to the vehicle, baseline prices increased. In addition, customers usually purchase additional services, such as extra testing, design and analysis, and launch-site support.
As of 2015, the most recent Pegasus XL to be purchased—a planned June 2017 launch of NASA's Ionospheric Connection Explorer (ICON) mission—had a total cost of $56.3 million, which NASA notes includes "firm-fixed launch service costs, spacecraft processing, payload integration, tracking, data and telemetry and other launch support requirements." A series of technical problems delayed this launch, which finally took place on 11 October 2019.
In July 2019, it was announced that Northrop had lost the launch contract of the IXPE satellite to SpaceX. IXPE had been planned to be launched by a Pegasus XL rocket, and had been designed so as to fit within the Pegasus XL rocket constraints. With the IXPE launch removed from the Pegasus XL rocket, there are currently (as of 12 October 2019, after the launch of ICON) no space launch missions announced for the Pegasus XL rocket. The future (under construction as of 2019) NASA Explorer program mission PUNCH was planned to be launched by Pegasus XL; but then NASA decided to merge the launches of PUNCH and another Explorer mission, TRACERS (also under construction as of 2019). These two space missions, consisting of 6 satellites in total, are to be launched by one rocket. It is expected that a larger launcher will be chosen for this dual mission launch. 
Northrop has 2 Pegasus XL's remaining in its inventory (as of 12 October 2019). It is looking for customers for those rockets. Northrop does not plan on retiring the Pegasus XL rocket as of October 2019.
For many small satellites it is desirable to be the primary payload and be placed into the orbit desired, rather than be a secondary payload placed in a compromise orbit. For example, Pegasus launched from equatorial launch sites can put spacecraft in orbits avoiding the South Atlantic Anomaly (a high radiation region over the South Atlantic Ocean) which is desirable for many scientific spacecraft. For some payloads, this may justify the higher cost of Pegasus relative to satellites launched as secondary cargoes on larger launchers.
In a Pegasus launch, the carrier aircraft takes off from a runway with support and checkout facilities. Such locations have included Kennedy Space Center / Cape Canaveral Air Force Station, Florida; Vandenberg Air Force Base and Dryden Flight Research Center, California; Wallops Flight Facility, Virginia; Kwajalein Range in the Pacific Ocean, and the Canary Islands in the Atlantic. Orbital offers launches from Alcantara, Brazil, but no known customers have performed any.
Upon reaching a predetermined staging time, location, and velocity vector the aircraft releases the Pegasus. After five seconds of free-fall, the first stage ignites and the vehicle pitches up. The 45-degree delta wing (of carbon composite construction and double-wedge airfoil) aids pitch-up and provides some lift. The tail fins provide steering for first-stage flight, as the Orion 50S motor does not have a thrust-vectoring nozzle.
Approximately 1 minute and 17 seconds later, the Orion 50S motor burns out. The vehicle is at over 200,000 feet (61 km) in altitude and hypersonic speed. The first stage falls away, taking the wing and tail surfaces, and the second stage ignites. The Orion 50 burns for approximately 1 minute and 18 seconds. Attitude control is by thrust vectoring the Orion 50 motor around two axes, pitch and yaw; roll control is provided by nitrogen thrusters on the third stage.
Midway through second-stage flight, the launcher has reached a near-vacuum altitude. The fairing splits and falls away, uncovering the payload and third stage. Upon burnout of the second-stage motor, the stack coasts until reaching a suitable point in its trajectory, depending on mission. Then the Orion 50 is discarded, and the third stage's Orion 38 motor ignites. It too has a thrust-vectoring nozzle, assisted by the nitrogen thrusters for roll. After approximately 64 seconds, the third stage burns out.
A fourth stage is sometimes added for a higher altitude, finer altitude accuracy, or more complex maneuvers. The HAPS (Hydrazine Auxiliary Propulsion System) is powered by three restartable, monopropellant hydrazine thrusters. As with dual launches, the HAPS cuts into the fixed volume available for payload. In at least one instance, the spacecraft was built around the HAPS.
Guidance is via a 32-bit computer and an IMU. A GPS receiver gives additional information. Due to the air launch and wing lift, the first-stage flight algorithm is custom-designed. The second- and third-stage trajectories are ballistic, and their guidance is derived from a Space Shuttle algorithm.
The carrier aircraft (initially a NASA B-52, now an L-1011 owned by Orbital) serves as a booster to increase payloads at reduced cost. 40,000 feet (12,000 m) is only about 4% of a low earth orbital altitude, and the subsonic aircraft reaches only about 3% of orbital velocity, yet by delivering the launch vehicle to this speed and altitude, the reusable aircraft replaces a costly first-stage booster.
The single biggest cause of traditional launch delays is weather. Carriage to 40,000 feet takes the Pegasus above the troposphere, into the stratosphere. Conventional weather is limited to the troposphere, and crosswinds are much gentler at 40,000 feet. Thus the Pegasus is largely immune to weather-induced delays and their associated costs, once at altitude. (Bad weather is still a factor during takeoff, ascent, and the transit to the staging point).
Air launching reduces range costs. No blastproof pad, blockhouse, or associated equipment are needed. This permits takeoff from a wide variety of sites, generally limited by the support and preparation requirements of the payload. The travel range of the aircraft allows launches at the equator, which increases performance and is a requirement for some mission orbits. Launching over oceans also reduces insurance costs, which are often large for a vehicle filled with volatile fuel and oxidizer.
Launch at altitude allows a larger, more efficient, yet cheaper first-stage nozzle. Its expansion ratio can be designed for low ambient air pressures, without risking flow separation and flight instability during low-altitude flight. The extra diameter of the high-altitude nozzle would be difficult to gimbal. But with reduced crosswinds, the fins can provide sufficient first-stage steering. This allows a fixed nozzle, which saves cost and weight versus a hot joint.
A single-impulse launch results in an elliptical orbit, with a high apogee and low perigee. The use of three stages, plus the coast period between second- and third-stage firings, help to circularize the orbit, ensuring the perigee clears the Earth's atmosphere. If the Pegasus launch had begun at low altitude, the coast period or thrust profile of the stages would have to be modified to prevent skimming of the atmosphere after one pass.
For launches which do not originate from Vandenberg Air Force Base, the carrier aircraft is also used to ferry the assembled launch vehicle to the launch site. For such missions, the payload can either be installed at the base and ferried by the launch vehicle or be installed at the launch site.
In October 2016, Orbital ATK announced a partnership with Stratolaunch Systems to launch Pegasus-XL rockets from the giant Scaled Composites Stratolaunch, which could launch up to three Pegasus-XL rockets on a single flight.
Pegasus components have also been the basis of other OSC launchers. The ground-launched Taurus rocket places the Pegasus stages and a larger fairing atop a Castor 120 first stage, derived from the first stage of the MX Peacekeeper missile. Initial launches used refurbished MX first stages.
The Minotaur I, also ground-launched, is a combination of stages from Taurus launchers and Minuteman missiles, hence the name. The first two stages are from a Minuteman II; the upper stages are Orion 50XL and 38. Due to the use of surplus military rocket motors, it is only used for US Government and government-sponsored payloads.
A third vehicle is dubbed Minotaur IV despite containing no Minuteman stages. It consists of a refurbished MX with an Orion 38 added as a fourth stage.
The NASA X-43A hypersonic test vehicles were boosted by Pegasus first stages. The upper stages were replaced by exposed models of a scramjet-powered vehicle. The Orion stages boosted the X-43 to its ignition speed and altitude, and were discarded. After firing the scramjet and gathering flight data, the test vehicles also fell into the Pacific.
The most numerous derivative of Pegasus is the booster for the Ground-based Midcourse Defense (GBMD) interceptor, basically a vertical (silo) launched Pegasus minus wing and fins, and with the first stage modified by addition of a Thrust Vector Control (TVC) system.
- Edwards AFB
- Cape Canaveral
- Base Aerea de Gando
- Wallops Flight Facility
- Kwajalein Atoll
- Partial failure
Pegasus has flown 44 missions between 1990 and 2019.
|Flight №||Date / time (UTC)||Rocket,
|Launch site||Payload||Payload mass||Orbit||Customer||Launch |
|1||April 5, 1990
|Standard (B-52)||Edwards AFB||Pegsat, NavySat||Success|
|2||July 17, 1991
|Standard w/ HAPS (B-52)||Edwards AFB||Microsats (7 satellites)||Partial failure|
|Orbit too low, spacecraft reentered after 6 months instead of planned 3-years lifetime|
|3||February 9, 1993
|Standard (B-52)||Cape Canaveral||SCD-1||Success|
|4||April 25, 1993
|Standard (B-52)||Edwards AFB||ALEXIS – Array of Low Energy X-ray Imaging Sensors||Success|
|5||May 19, 1994
|Standard w/ HAPS (B-52)||Edwards AFB||STEP-2 (Space Test Experiments Platform/Mission 2/SIDEX)||Partial failure|
|Orbit slightly low|
|6||June 27, 1994
|XL (L-1011)||Vandenberg AFB||STEP-1 (Space Test Experiments Platform/Mission 1)||Failure|
|Loss of vehicle control 35s into flight, flight terminated|
|7||August 3, 1994
|Standard (B-52)||Edwards AFB||APEX||Success|
|8||April 3, 1995
|Hybrid (L-1011)||Vandenberg AFB||Orbcomm (2 satellites), OrbView-1||Success|
|9||June 22, 1995
|XL (L-1011)||Vandenberg AFB||STEP-3 (Space Test Experiments Platform/Mission 3)||Failure|
|Destroyed during second-stage flight|
|10||March 9, 1996
|XL (L-1011)||Vandenberg AFB||REX II||Success|
|11||May 17, 1996
|Hybrid (L-1011)||Vandenberg AFB||MSTI-3||Success|
|12||July 2, 1996
|XL (L-1011)||Vandenberg AFB||TOMS-EP||Success|
|13||August 21, 1996
|XL (L-1011)||Vandenberg AFB||FAST (Fast Auroral Snapshot Explorer)||Success|
|14||November 4, 1996
|XL (L-1011)||Wallops Flight Facility||HETE, SAC-B||Failure|
|Satellites not ejected from third stage|
|15||April 21, 1997
|XL (L-1011)||Base Aerea de Gando, Gran Canaria, Spain||MiniSat, Celestis space burial||Success|
|16||August 1, 1997
|XL (L-1011)||Vandenberg AFB||OrbView-2||Success|
|On the line with partial success|
|17||August 29, 1997
|XL (L-1011)||Vandenberg AFB||FORTE||Success|
|18||October 22, 1997
|XL (L-1011)||Wallops Flight Facility||STEP-4 (Space Test Experiments Platform/Mission 4)||Success|
|19||December 23, 1997
|XL w/ HAPS (L-1011)||Wallops Flight Facility||Orbcomm (8 satellites)||Success|
|20||February 26, 1998
|XL (L-1011)||Vandenberg AFB||SNOE, BATSAT||Success|
|21||April 2, 1998
|XL (L-1011)||Vandenberg AFB||TRACE||Success|
|22||August 2, 1998
|XL w/ HAPS (L-1011)||Wallops Flight Facility||Orbcomm (8 satellites)||Success|
|23||September 23, 1998
|XL w/ HAPS (L-1011)||Wallops Flight Facility||Orbcomm (8 satellites)||Success|
|24||October 22, 1998
|Hybrid (L-1011)||Cape Canaveral||SCD-2||Success|
|25||December 6, 1998
|XL (L-1011)||Vandenberg AFB||SWAS||Success|
|26||March 5, 1999
|XL (L-1011)||Vandenberg AFB||WIRE – Wide Field Infrared Explorer||Success|
|27||May 18, 1999
|XL w/ HAPS (L-1011)||Vandenberg AFB||Terriers, MUBLCOM||Success|
|28||December 4, 1999
|XL w/ HAPS (L-1011)||Wallops Flight Facility||Orbcomm (7 satellites)||Success|
|29||June 7, 2000
|XL (L-1011)||Vandenberg AFB||TSX-5 (Tri-Services Experiments Platform/Mission 5)||Success|
|30||October 9, 2000
|Hybrid (L-1011)||Kwajalein Atoll||HETE 2||Success|
|31||February 5, 2002
|XL (L-1011)||Cape Canaveral||RHESSI||Success|
|32||January 25, 2003
|XL (L-1011)||Cape Canaveral||SORCE||Success|
|33||April 28, 2003
|XL (L-1011)||Cape Canaveral||GALEX – Galaxy Evolution Explorer||Success|
|34||June 26, 2003
|XL (L-1011)||Vandenberg AFB||OrbView-3||Success|
|35||August 13, 2003
|XL (L-1011)||Vandenberg AFB||SCISAT-1||Success|
|36||April 15, 2005
|XL w/ HAPS (L-1011)||Vandenberg AFB||DART||Success|
|37||March 22, 2006
|XL (L-1011)||Vandenberg AFB||ST-5 – Space Technology 5 (3 satellites)||Success|
|38||April 25, 2007
|XL (L-1011)||Vandenberg AFB||AIM – Aeronomy of Ice in the Mesosphere||Success|
|39||April 15, 2008
|XL (L-1011)||Kwajalein Atoll||C/NOFS||Success|
|40||October 19, 2008
|XL (L-1011)||Kwajalein Atoll||IBEX – Interstellar Boundary Explorer||Success|
|41||June 13, 2012
|XL (L-1011)||Kwajalein Atoll||NuSTAR – Nuclear Spectroscopic Telescope Array||Success |
|42||June 28, 2013
|XL (L-1011)||Vandenberg AFB||IRIS – Interface Region Imaging Spectrograph SMEX||Success |
|43||December 15, 2016
|XL (L-1011)||Cape Canaveral||Cyclone Global Navigation Satellite System (CYGNSS) ||Success|
|44||October 11, 2019
|XL (L-1011)||Cape Canaveral||Ionospheric Connection Explorer (ICON)||Success|
As of 12 October 2019 (after the launch of ICON) there are no space launch missions declared for the Pegasus XL rocket.
- Flight F-6, June 27, 1994: The vehicle lost control 35 seconds into flight, telemetry downlink lost 38 seconds into flight, range safety commanded flight termination 39 seconds into flight. The likely reason for loss of control was improper aerodynamic modelling of the longer (XL) version of which this was the first flight. The Pegasus carried the DoD Space Test Program's satellite - Space Test Experiments Platform, Mission 1 (STEP-1).
- Flight F-9, June 22, 1995: The interstage ring between the 1st and 2nd stages did not separate, constraining movement of the 2nd-stage nozzle. As a result, the rocket deviated from its intended trajectory and was ultimately destroyed by range safety. The Pegasus carried the DoD Space Test Program's satellite - Space Test Experiments Platform, Mission 3 (STEP-3).
- Flight F-14, November 4, 1996: Failed to separate payloads because of a discharged battery intended to start separation pyros. Battery damage during launch was the likely reason.
- Flight F-2, July 17, 1991: A faulty pyrotechnic system caused the rocket to veer off course during 1st-stage separation, resulting erratic maneuvers that prevented the rocket reaching the correct orbit, and the mission life, planned for 3 years, was reduced to 6 months
- Flight F-5, May 19, 1994: A software navigation error caused the HAPS upper stage to shut down early, resulting in a lower than planned orbit. The Pegasus carried the DoD Space Test Program's satellite - Space Test Experiments Platform, Mission 2 (STEP-2).
- Air launch to orbit
- Comparison of orbital launchers families
- Comparison of orbital launch systems
- Pegasus II (rocket)
- "Pegasus User's Guide" (PDF). orbitalatk.com. October 2015. Archived from the original (PDF) on 13 January 2016.
- Brown, Stuart (May 1989), "Winging it Into Space", Popular Science, Bonnier Corporation, p. 128, ISSN 0161-7370, retrieved 27 June 2013
- Thompson, David (2007), An Adventure Begins - Orbital's First 25 Years, Orbital Sciences Corporation
- Mosier, Marty; Harris, Gary; Richards, Bob; Rovner, Dan; Carroll, Brent (1990). "Pegasus First Mission Flight Results". Proceedings of the 4th AIAA/USU Conference on Small Satellites.
- Rebecca Hackler (3 June 2013). "NASA Johnson Space Center Oral History Project Commercial Crew & Cargo Program Office Edited Oral History Transcript". NASA.
- "startrek.com". startrek.com.
- NASA reviews space access after second Pegasus failure
- Pegasus clings to its satellite cargo.
- "Pegasus Mission History" (PDF). Northrop Grumman.
- "NASA Awards Launch Services Contract for Ionospheric Connection Explorer". NASA.
- Clark, Stephen (8 July 2019). "SpaceX wins NASA contract to launch X-ray telescope on reused rocket". Spaceflight Now. Retrieved 10 July 2019.
- Foust, Jeff (October 6, 2016). "Stratolaunch to launch Pegasus rockets". SpaceNews. Retrieved June 7, 2018.
- Barron Beneski (6 December 2011). "Letter: 'Qualified Success' Sells Pegasus Short". Space News.
- "NuSTA" (PDF). December 2010. Archived from the original (PDF) on 2011-07-17.
- "NASA's Consolidated Launch Schedule". NASA. 2013-05-14.
- "IRIS Launch Coverage". NASA. June 27, 2013.
- "NASA Awards Launch for Orbital's Pegasus Rocket". Orbital press release. April 1, 2014.
- Graham, William (15 December 2016). "Pegasus launches CYGNSS constellation following Stargazer release". NASASpaceFlight. Retrieved 16 December 2016.
- Gebhardt, Chris (11 October 2019). "NASA's ICON mission launches on Northrop Grumman Pegasus XL rocket". NASASpaceFlight. Retrieved 11 October 2019.
- International reference guide to space launch systems, Fourth edition, page 290, ISBN 1-56347-591-X
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