The first of
NASA's three High Energy Astronomy Observatories,
HEAO 1, launched August 12, 1977 aboard an
Atlas rocket with a
Centaur upper stage, operated until 9 January 1979. During that time, it scanned the
X-ray sky almost three times over 0.2 keV - 10 MeV, provided nearly constant monitoring of X-ray sources near the ecliptic poles, as well as more detailed studies of a number of objects through pointed observations.1978: International Sun-Earth Explorer 3 - USA Interplanetary Monitor launched. The Internation Sun-Earth Explorer was renamed to International Cometary Explorer (ICE). On September 11, 1985 it passed through the plasma tail of comet Giacobini-Zinner.
[h=2]A1: Large-Area Sky Survey instrument[/h] The
A1, or
Large-Area Sky Survey (LASS) instrument, covered the 0.2525 keV energy range, using seven large proportional counters.[SUP]
[1][/SUP] It was designed, operated, and managed at the
Naval Research Laboratory (NRL) under the direction of Principal Investigator Dr. Herbert D. Friedman, and the
prime contractor was
TRW. The HEAO A-1 X-Ray Source Catalog included 842 discrete X-ray sources.[SUP]
[2][/SUP]
[h=2]A2: Cosmic X-ray Experiment[/h] The
A2, or
Cosmic X-ray Experiment (CXE), from the
Goddard Space Flight Center, covered the 2-60 keV energy range with high spatial and spectral resolution. The Principal Investigators were Dr. Elihu A. Boldt and Dr. Gordon P. Garmire.[SUP]
[3][/SUP]
[h=2]A3: Modulation Collimator instrument[/h] Main article:
Modulation collimator
The
A3, or
Modulation Collimator (MC) instrument, provided high-precision positions of X-ray sources, accurate enough to permit follow-up observations to identify optical and radio counterparts. It was provided by the
Center for Astrophysics (
Smithsonian Astrophysical Observatory and the
Harvard College Observatory, SAO/HCO).[SUP]
[4][/SUP] Principal Investigators were Dr. Daniel A. Schwartz of SAO and Dr. Hale V. Bradt of MIT.
[h=2]A4: Hard X-Ray / Low-Energy Gamma-ray experiment[/h] The
A4, or
Hard X-ray / Low Energy Gamma-ray Experiment, used
sodium iodide (NaI)
scintillation counters to cover the energy range from about 20 keV to 10 MeV.[SUP]
[5][/SUP] It consisted of seven clustered modules, of three distinct designs, in a roughly hexagonal array.[SUP]
[6][/SUP] Each detector was actively shielded by surrounding CsI scintillators, in active-anti-coincidence, so that an extraneous particle or gamma-ray event from the side or rear would be vetoed electronically, and rejected. (It was discovered in early balloon flight by experimenters in the 1960s that passive collimators or shields, made of materials such as lead, actually
increase the undesired background rate, due to the intense showers of secondary particles and photons produced by the extremely high energy (GeV) particles characteristic of the space radiation environment.) A plastic anti-coincidence scintillation shield, essentially transparent to gamma-ray photons, protected the detectors from high-energy charged particles entering from the front.
For all seven modules, the unwanted background effects of particles or photons entering from the rear was suppressed by a "phoswich" design, in which the active NaI detecting element was optically coupled to a layer of CsI on its rear surface, which was in turn optically coupled to a single
photomultiplier tube for each of the seven units. Because the NaI has a much faster response time (~0.25 μs) than the CsI (~1 μs), electronic pulse shape discriminators could distinguish good events in the NaI from mixed events accompanied by a simultaneous interaction in the CsI.
The largest, or
High Energy Detector (HED), occupied the central position and covered the upper range from ~120 keV to 10 MeV, with a field-of-view (FOV) collimated to 37°
FWHM. Its NaI detector was 5 inches (13 cm) in diameter by 3 inches (7.6 cm) thick. The extreme penetrating power of photons in this energy range made it necessary to operate the HED in electronic anti-coincidence with the surrounding CsI and also the six other detectors of the hexagon.
Two
Low Energy Detectors (LEDs) were located in positions 180° apart on opposite side of the hexagon. They had thin ~3 mm thick NaI detectors, also 5 inches (13 cm) in diameter, covering the energy range from ~10200 keV. Their FOV was defined to fan-shaped beams of 1.7° x 20° FWHM by passive, parallel slat-plate collimators. The slats of the two LEDs were inclined to ±30° to the nominal HEAO scanning direction, crossing each other at 60°. Thus, working together, they covered a wide field of view, but could localize celestial sources with a precision determined by their 1.7° narrow fields.
The four
Medium Energy Detectors (MEDs), with a nominal energy range of 80 keV 3 MeV, had 3 inches (7.6 cm) dia by 1 inch (2.5 cm) thick NaI detector crystals, and occupied the four remaining positions in the hexagon of modules. They had circular FOVs with a 17° FWHM.
The primary data from A4 consisted of "event-by-event" telemetry, listing each good (i.e., un-vetoed) event in the NaI detectors. The experiment had the flexibility to tag each event with its pulse height (proportional to its energy), and a one or two byte time tag, allowing precision timing of objects such as
gamma-ray bursts and
pulsars.
Results of the experiment included a catalog of the positions and intensities of hard X-ray (10200 keV) sources,[SUP]
[7][/SUP] a strong observational basis for extremely strong magnetic fields (of order 10[SUP]13[/SUP] G) on the rotating neutron stars associated with Her X-1[SUP]
[8][/SUP][SUP]
[9][/SUP] and 4U 0115+634, a definitive diffuse component spectrum between 13 and 200 keV, discovery of the power-law shape of the
Cygnus X-1 power density spectrum, and discovery of slow intensity cycles in the X-Ray sources SMC X-1 and LMC X-4, resulting in approximately 15 Ph.D theses and ~100 scientific publications.
The A4 instrument was provided and managed by the University of California at San Diego, under the direction of Prof.
Laurence E. Peterson, in collaboration with the X-ray group at
MIT, where the initial A4 data reduction was performed under the direction of Prof.
Walter H. G. Lewin.
Space shuttle Enterprise makes 1st atmospheric flight
[FONT=arial,helvetica]Enterprise was the first Space Shuttle built for NASA. Although officially classified as an Orbiter and given the designation OV-101 for Orbiter Vehicle 101, Enterprise was not actually capable of traveling in space. Enterprise had no rocket engines or thermal protection tiles and was equipped with a pitot tube on the nose that limited it to low speed atmospheric flights only. Enterprise was instead a prototype used to test the Orbiter's flight characteristics during landing. It performed a similar function as Russia's
Analog Buran did in collecting information on how well the vehicle performed during approach and landing.
Roll-out ceremony for Enterprise attended by the cast of Star Trek The Space Shuttle program was first approved in 1972 and construction of OV-101 began at Rockwell International in Palmdale, California, in 1974. Enterprise was completed and rolled out of the assembly plant in 1976. The vehicle was originally supposed to be called Constitution in celebration of America's bicentennial, but a write-in campaign by fans of the television show
Star Trek encouraged NASA to name OV-101 as Enterprise instead.
Enterprise was used for several ground vibration tests before being moved 36 miles (56 km) over land to NASA's Dryden Flight Research Center located at Edwards Air Force Base north of Palmdale. From February through October of 1977, Enterprise conducted a series of taxi and flight tests to demonstrate that the Orbiter design could operate from a conventional runway. Known as the Approach and Landing Test, or ALT, the program began with a series of three taxi tests while Enterprise was attached to the modified
Boeing 747 Shuttle Carrier Aircraft (SCA). The taxi tests measured structural loads and responses to determine how well the mated vehicles could be handled and controlled on the ground up to speeds of 157 mph (253 km/h). The SCA's braking and steering systems were also tested.
Enterprise mated to the 747 Shuttle Carrier Aircraft during a captive flight Enterprise became airborne for the first time on 18 February 1977 as the second stage of testing began. These five flights were called captive-inert and had Enterprise remain mated to the SCA with no crew aboard. A tailcone was placed on the back of Enterprise to reduce turbulence on the tail control surfaces of the 747 while the two planes were attached. These flights reached altitudes up to 30,000 ft (9,145 m) and a maximum speed of 474 mph (763 km/h). The longest captive flight lasted over three hours, and this phase of testing allowed NASA to determine the structural integrity and handling qualities of the mated vehicles in flight.
Next came a series of three additional captive flights with two crewmen aboard Enterprise while it remained attached to the SCA. These flights allowed the crew to operate the systems and flight controls aboard Enterprise while it was in the air and verify that the vehicle functioned properly. This phase also included flutter tests of the mated craft at low and high speed, a separation trajectory test, and a practice for the first free flight.
Enterprise separating from the SCA during free flights At last on 12 August 1977, Enterprise was released from the SCA for the first time to fly and make a gliding landing on its own under astronaut control. A total of five free flights were conducted, the last two without the tailcone attached. Mockups of the three space shuttle main engines and two orbital maneuvering system engines were exposed for these two flights to simulate the Orbiter's aerodynamic configuration during a return from space. The longest free flight lasted nearly 5½ minutes after Enterprise was released from an altitude of 26,000 ft (7,925 m). The free flights were performed at minimum weight and various
center of gravity locations as well as several release altitudes to test the Orbiter's flight characteristics under different conditions.
The first four gliding flights landed on the dry lakebed at Edwards AFB while the final test touched down on the main concrete runway to simulate a return from space. The free flights demonstrated the vehicle's approach and landing capabilities while under pilot control, an autoland approach mode, the airworthiness of the Orbiter design in subsonic flight, and the operation of several key systems in preparation for the first manned orbital flight. The final flight also revealed a problem in the control system that could allow Pilot-Induced Oscillation (PIO), a phenomenon where the pilot's control inputs result in the vehicle becoming unstable. Because this issue was identified so early, engineers were able to develop a correction for the control system years before the first orbital flight.
Views of Enterprise gliding to a landing at Edwards AFB Two different astronaut crews flew aboard Enterprise during the captive and free flights. Five of the eight missions were flown by Fred Haise (of Apollo 13 fame) and Gordon Fullerton while the remaining three included Joe Engle and Richard Truly. The same crew of four was aboard the 747 SCA during all 16 taxi and flight tests. This crew included pilots Fitzhugh Fulton and Thomas McMurtry as well as flight engineers Louis Guidry and Victor Horton.
Following its final free flight on 26 October 1977, Enterprise was transferred to several other NASA facilities for additional tests preparing the way for the first Space Shuttle launch. Modifications were first made to the SCA and Enterprise for a series of four ferry flights to various NASA facilities. OV-101 was then ferried to NASA Marshall Space Flight Center in Huntsville, Alabama, in 1978. At Marshall, Enterprise was mated to an External Tank (ET) and two Solid Rocket Boosters (SRBs) so the assembled stack could undergo vertical ground vibration tests. The tests measured structural dynamic response modes of the mated vehicle that were used to refine the design of connection interfaces. Enterprise was next ferried to NASA Kennedy Space Center where it was again mated to an ET and SRBs. The combined stack was transported on the mobile launcher platform to Launch Complex 39-A for fit checks and to practice launch and handling procedures. Finally, Enterprise was brought back to NASA Dryden in 1979 where the Orbiter was again moved over land to the Rockwell International factory. Here, several components were removed and refurbished for installation on the spaceworthy Orbiters under construction.
Enterprise as part of a complete Space Shuttle stack at Kennedy Space Center The first orbital mission of the Space Shuttle was launched in April 1981 when Columbia (OV-102) made the maiden journey into space. Originally, it was planned that Enterprise would be modified with the same systems as Columbia and converted into a spaceworthy Orbiter. Although Enterprise resembled the other Orbiters externally, it had not been built with working engines, orbital maneuvering thrusters, or thermal protecting tiles. These systems were to be refitted so that Enterprise would be the second Orbiter to be launched into space. However, the Orbiter design was revised as Columbia was being built to save weight. Many changes would have to be made to the wing and fuselage structure of Enterprise to make it compatible with the rest of the Shuttle fleet.
NASA decided this conversion was too expensive. A cheaper alternative was to modify another test vehicle called Static Test Article 099 (STA-099) into a spaceworthy orbiter. Once converted, STA-099 became OV-099 and was named Challenger. Thus, Challenger became NASA's second spaceworthy Orbiter and was joined by Discovery (OV-103) and Atlantis (OV-104) to create a fleet of four Shuttles. Enterprise, meanwhile, was essentially retired. The Orbiter was ferried around the world in 1983 to make appearances in France, Germany, Italy, the United Kingdom, and Canada. Enterprise was also transported by barge from Mobile, Alabama, to New Orleans, Louisiana, for the 1984 World's Fair. One of the vehicle's final official uses came in 1985 when Enterprise was ferried to Vandenburg Air Force Base in California. The US Air Force had originally planned to launch Space Shuttle flights from this west coast site to achieve
polar orbits. Enterprise was attached to an ET and SRBs and rolled out to the SLC-6 launch pad for fit checks and other tests. Plans to launch the Shuttle from this site were later abandoned and no other Orbiter ever visited SLC-6.
Enterprise during its first free flight without the tailcone Enterprise was officially retired on 18 November 1985 when the vehicle was ferried to Washington, DC, and donated to the Smithsonian Institution National Air & Space Museum. After the
Challenger disaster in 1986, NASA again explored converting Enterprise into a spaceworthy replacement, but it was decided to instead build a new Orbiter from spare structural parts that had been purchased with Discovery and Atlantis. These components were used to construct OV-105 that was given the name Endeavour.
Enterprise remained in storage at the Smithsonian's Dulles Airport facility throughout the 1990s but parts of it were removed for use in the investigation that followed the loss of
Columbia in 2003. Fiberglass panels along Enterprise's wing leading edge simulated the reinforced carbon-carbon (RCC) thermal protection panels of Columbia. Panels taken from Enterprise were used in a series of tests to evaluate the damage a
foam insulation impact could do to the thermal protection system. These tests as well as tests on actual RCC panels removed from Discovery demonstrated that a foam impact could indeed create cracks or holes in the RCC panels, which was the most likely cause of the Columbia disaster.
Enterprise displayed at the National Air & Space Museum Enterprise remains part of the Smithsonian collection today and is now the featured attraction of the space collection displayed at the
National Air and Space Museum's Steven F. Udvar-Hazy Center near Dulles International Airport.
- answer by Molly Swanson, 7 January 2007
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