Orbital’s Pegasus XL successfully lofts IRIS spacecraft

by William Graham

Orbital Sciences Corporation launched their Pegasus-XL rocket Thursday night, carrying NASA’s Interface Region Imaging Spectrograph (IRIS) satellite. The launch occurred at 19:27 PDT (02:27 UTC on Friday), followed by confirmation of a successful spacecraft separation.

Pegasus Mission:

The Interface Region Imaging Spectrograph, or IRIS, is a small satellite which will be used to observe the sun at ultraviolet wavelengths, in order to study the transfer of energy in the chromosphere and transition region.

IRISDesigned for a two-year mission, IRIS carries an ultraviolet spectrometer attached to a telescope with a diameter of 20 centimeters (8 inches). The satellite will return images and spectra produced every few seconds, allowing material to be tracked as it passes through the chromosphere.

The Sun’s surface, or photosphere, has a temperature of around 6,000 Kelvin (5,726 degrees Celsius); however the outer layer of its atmosphere, the corona, has a temperature far exceeding this. With no fusion occurring in the outer layers of the Sun, scientists would expect the corona to be cooler than other layers, however in the thin transition region, the Sun’s temperature rises dramatically.

It is hoped that by providing high-resolution observations of the photosphere, chromosphere and transition region, IRIS can help physicists to understand the processes involved.

IRISThe IRIS satellite was constructed by Lockheed Martin, who will also operate the spacecraft’s instrument, while NASA’s Ames Research Center will be responsible for operating the satellite itself. The scientific data returned by the spacecraft will be analyzed at Stanford University. The spacecraft has a mass of 183 kilograms (403 lb); 87 kilograms of which is the scientific payload.

The spacecraft can downlink data at a rate of 15 megabits per second by means of a directional x-band transponder. It makes fifteen passes per day over its four ground stations in Alaska, Svalbard, Antarctica and Virginia. The satellite’s peak power consumption is 340 watts, with electrical power being provided by two solar panels measuring 135 by 64 centimeters (53 by 25 inches).

IRIS is the eleventh mission to launch as part of NASA’s Small Explorer (SMEX) program, which is part of the wider Explorer program. Upon reaching orbit successfully, IRIS will be designated Explorer 94; however as these designations have been assigned somewhat inconsistently, it is actually around the 103rd Explorer satellite to be launched, not including hosted payloads on other satellites. In practice the spacecraft will continue to be known as IRIS; Explorer 55 was the last satellite to be known primarily by its Explorer designation.

PegasusThe launch of IRIS was the forty-second flight of the Pegasus rocket, and the thirty-second of the Pegasus-XL. First launched in April 1990, the Pegasus is the only air-launched rocket to have successfully placed spacecraft into orbit, although the US Navy did attempt six launches with the Pilot II rocket in 1958, all of which were unsuccessful.

Pegasus made its maiden flight on 5 April 1990, carrying the Pegsat and SECS satellites for NASA and the US Navy respectively. Early launches were made using NASA’s NB-52B, nicknamed Balls 8, mostly flying from Edwards Air Force Base. Six launches were made using the original Pegasus, two of which used Hydrazine Auxiliary Propulsion System (HAPS) fourth stages to refine the payload’s orbit.

Both of the HAPS launches were partial failures; one due to the first stage being late separating from the rocket, and the other due to the HAPS stage underperforming.

PegasusCam, via L2The sixth Pegasus launch was the first flight of the XL configuration, with extended first and second stages.

(L2 Link to full onboard cam video of a Pegasus launch)

The rocket’s tail was also redesigned, allowing it to be launched by Orbital Sciences’ Lockheed L1011 TriStar aircraft rather than NASA’s B-52.

Originally delivered to Air Canada in March 1974 as C-FTNJ, the aircraft was used for passenger flights until it was bought by Orbital in May 1992. The aircraft carries the registration number N140SC.

Pegsus' Stargazer AircraftThe aircraft is named Stargazer after the USS Stargazer from Star Trek, a ship formerly commanded by Captain Picard.

This was an inside joke, as the ship his first officer had previously served on was named Pegasus. In addition to being used for Pegasus launches, Stargazer is used for high-altitude research flights.

Most Pegasus launches are conducted with Stargazer flying from Vandenberg Air Force Base; however several other sites, including some outside the United States, have been used. Stargazer is one of only 11 TriStars still flying.

In addition to Edwards and Vandenberg, Pegasus launches have been made from the Kennedy Space Center, the Cape Canaveral Air Force Station, NASA’s Wallops Flight Facility, Bucholz Army Airfield at Kwajalein Atoll in the Marshall Islands and Gran Canaria Airport in the Canary Islands.

Pegasus XLThe maiden flight of the Pegasus-XL carried the STEP-1 for the US Space Test Program. The rocket failed to achieve orbit after its wing suffered a structural failure.

The flight following this failure was the final launch of the original Pegasus, and the next mission after that was the debut of the Pegasus-H, which featured the tail modifications of the Pegasus-XL, but without the stretched stages. Four Pegasus-H rockets were launched.

Following the first Pegasus-H launch, the Pegasus-XL made its second launch. Like its first, this failed to achieve orbit, this time because of a problem with the second stage. That launch would have deployed the STEP-3 satellite – the third STP satellite to be involved in a Pegasus launch failure as STEP-2 was payload for the second of the partially unsuccessful Pegasus/HAPS launches.

The third Pegasus-XL launch, in March 1996, successfully deployed the STP’s REX-2 satellite. Another failure occurred a few flights later, when in November 1996 the sixth Pegasus-XL launched from Wallops Island with NASA’s HETE gamma-ray astronomy satellite, and Argentina’s SAC-B.

Despite reaching the planned orbit, both payloads failed to separate from the rocket, running out of power a few days after launch as they were unable to orient their solar panels towards the sun. Since that failure, Pegasus has made 27 consecutive successful launches.

Pegasus has formed the basis for several other rockets. The Taurus rocket, which has made nine launches with three failures, consists of a wingless Pegasus or Pegasus-XL boosted by a Castor 120 solid rocket motor, or the first stage of a retired Peacekeeper missile.

The Orbital Boost Vehicle, developed for the US military’s Ground Based Interceptor program, uses the upper stages of the Taurus, making it essentially a silo-launched Pegasus with no wings.

Pegasus II via L2Pegasus II – as nicknamed – is also being designed for use with the Stratolaunch system.

The HXLV was an air-launched rocket, using the first stage of the Pegasus dropped from the NB-52, to boost a hypersonic flight experiment as part of NASA’s Hyper-X program.

Following the launch of IRIS, Pegasus faces an uncertain future. While NASA has baselined it’s CYGNSS and TESS satellites for launch on Pegasus in 2017, no launch contracts have been signed and these missions could easily go to other rockets.

The GEMS satellite was scheduled to launch on a Pegasus-XL next year, however it was cancelled after going over budget, leaving Pegasus with an empty launch manifest. It has been suggested that the rocket may be retired if it has not found a payload by 2015.

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The original Pegasus and Pegasus-H were three-stage all-solid rockets, with an Orion-50S first stage fitted with wings and fins, an Orion-50 second stage, and an Orion-38 third stage. The Pegasus-XL uses an Orion-50SXL, an Orion-50XL and an Orion-38. Pegasus-XL F42 will be used for this launch.

IRIS and Pegasus during processingIRIS arrived at Vandenberg on 16 April, and was attached to its carrier rocket on 29 May.

Following a final flight simulation two days later, it was encapsulated in the payload fairing between 10-12 June, and the rocket was attached to the belly of Stargazer on 19 June.

Stargazer departed runway 12/30 at Vandenberg at 01:27 UTC (18:27 local time), an hour before launch.

The Pegasus was dropped over the Pacific Ocean at coordinates 36.0 north, 123.0 west. At deployment, Stargazer was at an altitude of 11.89 kilometers (39,000 ft) and flying at a velocity of 243 meters per second (797 ft/sec).

Launch ProfileOnce Stargazer entered its designated drop zone, and the countdown reached zero, the rocket was released. Five seconds later, the first stage motor ignited to begin the rocket’s ascent to orbit.

The first stage burn lasted 72.9 seconds, which was followed by a 15.5 seconds coast before the stage separated, with second stage ignition occurred one second later.

Around halfway through the second stage burn, 36.8 seconds after ignition, the payload fairing was separated from around the satellite. Another 36.4 seconds later the stage completed its 73.2 second burn, beginning a 370.3 second coast. The depleted second stage remained attached for most of the coast phase, finally being jettisoned eleven seconds before the end of the coast.

Third stage ignition occurred eight minutes and 57.9 seconds into the mission. The 68.4 second burn concluded Pegasus F42’s powered flight.

Pegasus heading uphillOnce that was complete, the third stage maneuvered to the required attitude for spacecraft separation. IRIS separated from the third stage three minutes later, 13 minutes and 6.3 seconds after launch.

About a minute after the satellite separated, the third stage deactivated its telemetry downlink, before performing a collision avoidance maneuver five minutes after spacecraft separation. This maneuver lasted four minutes and forty seconds, with the stage then safing itself by venting its remaining supply of nitrogen.

The target orbit for spacecraft separation was a perigee of 620 kilometers and an apogee of 670 kilometers (385 by 416 miles), giving a semi-major axis of 7,023 kilometers (4,364 miles), 97.89 degrees inclination and a mean local time of the ascending node of 06:02:30.

IRIS will be operated in this orbit, as does not carry any maneuvering thrusters or propellant to change its orbit.

Thursday’s launch was the second for Orbital Sciences this year, following April’s test flight of the Antares rocket. Orbital’s next launch is scheduled for 6 September, when the Minotaur V will make its maiden flight, carrying NASA’s LADEE spacecraft bound for the Moon.

America’s next launch is expected to be of an Atlas V on 19 July, with a MUOS communications satellite for the US military.

(Images via Orbital, NASA and L2)

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