Pegasus was an air-launched, small-lift launch vehicle developed by Orbital Sciences Corporation and later built and launched by Northrop Grumman. Pegasus was the world's first privately developed orbital launch vehicle. Capable of carrying small payloads of up to into low Earth orbit, Pegasus first flew in 1990 and last flew before its retirement in 2026. It had primarily launched scientific satellites for NASA.
The vehicle consisted of three solid-propellant stages and an optional monopropellant fourth stage. Pegasus used to be released from a carrier aircraft at about over open ocean, where it free-falls for five seconds before igniting its first-stage motor. The first stage did not have a thrust vectoring control system, instead using a delta wing and fins for lift and attitude control within the atmosphere. Pegasus initially used the NASA-owned B-52 Stratofortress named "Balls 8" as its carrier aircraft. By 1994, launches had transitioned to a modified L-1011 TriStar named "Stargazer" purchased by Orbital.
History Pegasus was designed by a team led by Antonio Elias (born 1949/1950). 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.
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. Government funding was received to support operational testing. 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.
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 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 percent. The first partial failure on July 17, 1991, caused the seven 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-1 (High Energy Transient Explorer).
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 US$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 October 11, 2019.
In July 2019, it was announced that Northrop Grumman had lost the launch contract of the Imaging X-ray Polarimetry Explorer (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 October 12, 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 Polarimeter to Unify the Corona and Heliosphere (PUNCH) was planned to be launched by Pegasus XL; but then NASA decided to merge the launches of PUNCH and another Explorer mission, Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) (also under construction as of 2019). These two space missions, consisting of 6 satellites in total, are to be launched by one launch vehicle. It is expected that a larger launcher will be chosen for this dual mission launch. In August 2022 NASA announced that the 4 microsatellites of the PUNCH constellation will be launched as rideshare payloads together with SPHEREx in April 2025 on a SpaceX Falcon 9 rocket.
Demand for the Pegasus XL declined sharply by the late 2010s and early 2020s, as most small satellites shifted to lower-cost rideshare opportunities on larger launch vehicles. Between 2016 and 2026, Pegasus XL flew only three times: 2019, 2021, and 2026.
In November 2025, Katalyst Space Technologies selected Pegasus XL to launch a rescue mission for NASA's Neil Gehrels Swift Observatory to prevent an uncontrolled re-entry. The mission required insertion into a specific low-inclination orbit not served by common rideshare launches, and smaller dedicated rockets lacked the necessary performance. The flight was the last Pegasus XL in Northrop's inventory, likely sold at a discount.
Launch profile
In a Pegasus launch, the carrier aircraft takes off from a runway with support and checkout facilities. Such locations had 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 Ocean. Orbital offered launches from Alcantara, Brazil, but no known customers had performed any.
Upon reaching a predetermined staging time, location, and velocity the aircraft released the Pegasus. After five seconds of free-fall, the first stage ignited and the vehicle pitched up. The 45-degree delta wing (of carbon composite construction and double-wedge airfoil) aided pitch-up and provided some lift. The tail fins provided 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 would be 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 burnt for approximately 1 minute and 18 seconds. Attitude control was by thrust vectoring the Orion 50 motor around two axes, pitch and yaw; roll control was 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 would be discarded, and the third stage's Orion 38 motor ignites. It too had a thrust-vectoring nozzle, assisted by the nitrogen thrusters for roll. After approximately 64 seconds, the third stage burns out.
A fourth stage was sometimes added for a higher altitude, finer altitude accuracy, or more complex maneuvers. The HAPS (Hydrazine Auxiliary Propulsion System) was 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 was via a 32-bit computer and an IMU. A GPS receiver gave additional information. Due to the air launch and wing lift, the first-stage flight algorithm was custom-designed. The second- and third-stage trajectories were ballistic, and their guidance was derived from a Space Shuttle algorithm.
Carrier aircraft
The carrier aircraft (initially a NASA B-52, now an L-1011 owned by Northrop Grumman) served as a booster to increase payloads at reduced cost. is only about 4% of a low Earth orbital altitude, and the subsonic aircraft reached only about 3% of orbital velocity, yet by delivering the launch vehicle to this speed and altitude, the reusable aircraft replaced a costly first-stage booster. Notably, the atmosphere at 39,000 feet is roughly one quarter as dense as the atmosphere at sea level, eliminating much of the drag a ground-launched rocket would experience climbing through thick air.
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.
Related projects
Pegasus components had also been the basis of other Orbital Sciences Corporation 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 U.S. Government and government-sponsored payloads. This is because U.S. law forbids selling government property (like ICBM parts) for non institutional purpose.
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.