Orbital’s Pegasus successfully launches NASA’s NuSTAR spacecraft

Orbital Sciences Corporation have launched NASA’s NuSTAR spacecraft Thursday. The spacecraft was deployed by a Pegasus-XL carrier rocket dropped from an aircraft flying from the Bucholz Army Airfield in Kwajalein Atoll. Launch came 30 minutes into a four hour window, resulting in a T-0 of 04:00 local time (16:00 UTC on Wednesday).

Pegasus Mission:

NuSTAR, the Nuclear Spectroscopic Telescope Array, is a high-energy x-ray astronomy satellite; intended to be the first mission to produce focussed images of x-ray sources with energies of between 8 and 80 kiloelectronvolts.

The spacecraft was constructed by Orbital Sciences Corporation, around the LEOStar-2 satellite bus. NuSTAR will be the sixth LEOStar-2 to fly, although two of the previous missions, OrbView-4 and OCO, were lost in launch failures.

Stowed inside the Pegasus-XL payload fairing, NuSTAR measures 1.2 by 2.2 metres (3.9 x 7.2 feet). After launch, it will deploy a solar panel, and a 10 metre (33 foot) long mast to separate the telescopes’ optics from their instruments, increasing their focal length so they can detect x-rays.

The mast is derived from the one used for the Shuttle Radar Topography Mission, carried by Space Shuttle Endeavour on STS-99, and was constructed by ATK. The spacecraft has a mass of 350 kilograms (772 pounds), of which 171 kilograms (377 lb) is the scientific instruments.

NuSTAR carries two telescopes with cadmium-zinc telluride detectors, providing it with a 64 by 64 pixel readout. Its mirrors are comprised of thousands of curved glass segments arranged in a cylindrical structure comprised of 133 shells a fifth of a millimetre (8 thou) thick, with graphite separators.

The primary scientific goals of the mission are to discover new supermassive black holes and collapsed stars, to observe supernovae and gamma ray sources, and to map supernova remnants.

The launch of NuSTAR was originally expected to occur in 2008; however in 2006 the mission was cancelled due to budget constraints. In September 2007 it was reinstated, with launch scheduled for 2011. NuSTAR builds on research conducted using the High Energy Focusing Telescope, a high-altitude balloon experiment which demonstrated the technology which the spacecraft would employ.

The principal investigator is Professor Fiona Harrison of the California Institute of Technology. The total cost of the NuSTAR mission is expected to come to $165 million, and the spacecraft is expected to operate for two years, potentially followed by an extended mission of up to three additional years.

The NuSTAR spacecraft is part of NASA’s Small Explorer, or SMEX, programme, and is also designated SMEX-10. The Small Explorer programme is part of the wider Explorer programme; a series of scientific spacecraft which traces back to 1958, and the first American satellite, Explorer 1. NuSTAR will be the ninth Small Explorer to launch, and upon reaching orbit it will be given the designation Explorer 93.

The Explorer programme predates the formation of NASA; it was originally begun by the US Army in the 1950s, and subsequently transferred to NASA upon its formation. The first five missions were only known by Explorer designations, which were assigned before launch and as a result two launch failures, Explorer 2 and Explorer 5, were included in the sequence.

The first five spacecraft all carried magnetospheric experiments, and were built around the Sergeant upper stage of the Juno I carrier rocket. The last was launched on 24 August 1958.

Subsequent missions have carried various scientific payloads, using an array of different spacecraft configurations and launch systems. After Explorer 5, all Explorer spacecraft had at least one alternative name, used before launch, which was replaced with its Explorer designation after launch, unless the spacecraft failed to achieve orbit.

Explorer 55 was the last spacecraft to follow this practise, with subsequent missions being assigned Explorer designations, but not as their primary name, and the numerical designations are now rarely used.

Explorer designations are not assigned to missions flown as secondary instruments on other spacecraft, or flown using non-US spacecraft; for example Japan’s Suzaku spacecraft is considered part of the programme, as are the CINDI instrument on the C/NOFS satellite, and the TWINS instruments on two NRO ‘Improved Trumpet’ ELINT satellites.

The launch of NuSTAR was the first flight of a Pegasus rocket since the 2008 deployment of the IBEX satellite. Overall, it was the forty-first flight of a Pegasus rocket, and the thirty-first flight of a Pegasus-XL. The rocket was designated M48, and was named Dalton after Dalton Watson, a friend of one of the engineers on the mission, who was killed in a road accident last year.

Developed by Orbital Sciences Corporation in the late 1980s, Pegasus is a three-stage air-launched all-solid expendable launch system.

Dropped from a B-52, Pegasus made its first flight in April 1990, successfully delivering the SECS and Pegsat satellites into orbit. Four successful missions followed from the campaigns using the B-52.

The stretched and more powerful Pegasus-XL first flew in June 1994, its maiden flight was intended to orbit the STEP-1 spacecraft, however it failed to achieve orbit.

The XL’s second flight – - the ninth Pegasus mission overall –  carrying STEP-3, also failed. However in March 1996 the Pegasus-XL successfully placed REX-2 into orbit. In addition to stretched first and second stages, the Pegasus-XL introduced redesigned tail fins, allowing it to be launched from an L-1011 aircraft instead of a B-52. The revised fins were incorporated into the original Pegasus as the Pegasus-Hybrid, or Pegasus-H, which made four flights between 1995 and 2000.

The aircraft used to launch Pegasus-XL rockets is a Lockheed L-1011-1 TriStar named ‘Stargazer’, which bears the aircraft registration N140SC, and the serial number 1067.

It made its first flight on 22 February 1974, and was delivered to Air Canada on 9 March of the same year for passenger service, with the registration C-FTNJ.

In February 1982 it was briefly leased to Air Lanka of Sri Lanka under the registration 4R-TNJ, returning to Air Canada and resuming its previous registration number at the end of the month. It was sold to Orbital Sciences in May 1992 to replace the NASA-operated NB-52B ‘Balls 8′ used for early Pegasus flights.

The first launch from Stargazer was the maiden flight of the Pegasus-XL. The first successful launch from the aircraft came in April 1995, when a Pegasus-H was used to orbit Orbview-1, and two Orbcomm satellites.

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

Launching from an aircraft means that the rocket does not need to travel through as much of the Earth’s atmosphere before reaching space, and also allows a greater flexibility in terms of launch sites.

The launch of NuSTAR was the thirty-fifth from Stargazer, and the fourth from Kwajalein. In addition, twenty launches have been made with Stargazer flying from Vandenberg Air Force Base in California, six from the Wallops Flight Facility in Virginia, three from Cape Canaveral Air Force Station in Florida, and one from Gran Canaria Airport in the Canary Islands.

Most of the NB-52 launches took place from Edwards Air Force Base, except for one which used the Kennedy Space Center. Kwajalein has also served as a launch site for SpaceX’s Falcon 1 rocket, which made five launches from a pad on Omelek Island in the atoll.

Stargazer’s crew boarded the aircraft about 140 minutes before launch, and begin preparations for flight. The aircraft’s three Rolls-Royce RB211-524B engines were started between 130 and 80 minutes before launch, with the aircraft taxiing for takeoff approximately 75 minutes before the scheduled launch time. The aircraft took off approximately an hour before launch, flying out over the Pacific Ocean and climbing to an altitude of 11.9 kilometres (39,000 feet).

Once Stargazer reacheed the designated drop zone, and the countdown reached zero, the rocket was dropped from the aircraft. The release of the vehicle was recorded as the time of launch. Five seconds later, its first stage ignited and the vehicle pitched up to begin its ascent to orbit.

The first stage of the Pegasus-XL is an ATK Orion-50SXL, which burns hydroxyl-terminated polybutadiene (HTPB) solid propellant, and also houses the wings and tail fins used to control the vehicle during atmospheric flight.

The first stage burned for approximately 71 seconds, before separating fifteen seconds after burnout.

Around 92.05 seconds into powered flight, the second stage, an Orion-50XL, ignited. Like the first stage, the second stage burns HTPB, and it fires for 72.75 seconds.

During the second stage burn, 128.3 seconds after launch, the payload fairing separated from around the NuSTAR satellite. When the second stage burned out, the mission entered a coast phase lasting approximately six minutes, 21 seconds.

During this coast phase, the second stage separated, with third stage ignition coming at the end, nine minutes and six seconds into the mission.

The third stage of the Pegasus-XL is an Orion-38, which also burns HTPB. It burned for sixty-eight seconds, inserting itself and NuSTAR into low Earth orbit, with powered flight ending ten minutes and 14 seconds after the mission began.

The Pegasus placed NuSTAR into a circular orbit at an altitude of 600 kilometres, with six degrees of inclination.

Following spacecraft separation, which happened shortly after third stage burnout, the spent stage will perform a series of collision avoidance manoeuvres; two 90 degree turns five minutes apart to attain a retrograde orientation, followed by firing its aft thrusters to depletion, lowering its orbit. The collision avoidance manoeuvres will begin three seconds after NuSTAR has been deployed.

NuSTAR, meanwhile, will stabilise itself, deploy its solar array, and make first contact with the ground via the Tracking and Data Relay Satellite System (TDRSS). The mission will then enter a checkout and calibration phase; testing instruments and deploying its mast.

Checkout and calibration is expected to last 23 days. Finally, a known astronomical object will be used to align and calibrate the telescopes. The Crab nebula or quasar 3C273 will be used for this purpose.

The launch of NuSTAR was the only Pegasus launch in 2012. The next launch of a Pegasus is expected in January 2013, when another Pegasus-XL will launch from Vandenberg, carrying the IRIS satellite; the next scheduled Explorer mission.

The NuSTAR launch is also the first orbital launch to be conducted by Orbital Sciences this year, their next launch is expected to be the maiden flight of the Antares rocket in August, carrying three PhoneSat spacecraft.

(Images via Orbital, NASA and L2)

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