Explorer 17 (also known as Atmosphere Explorer-A (AE-A) and S6) was a

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...

, launched at

Cape Canaveral , image = cape canaveral.jpg , image_size = 300 , caption = View of Cape Canaveral from space in 1991 , map = Florida#USA , map_width = 300 , type =Cape , map_caption = Location in Florida , location ...

from

LC-17B Space Launch Complex 17 (SLC-17), previously designated Launch Complex 17 (LC-17), was a launch site at Cape Canaveral Air Force Station (CCAFS), Florida used for Thor and Delta launch vehicles launches between 1958 and 2011. It was built in 1 ...

on a Delta B

launch vehicle A launch vehicle or carrier rocket is a rocket designed to carry a payload (spacecraft or satellites) from the Earth's surface to outer space. Most launch vehicles operate from a launch pad, launch pads, supported by a missile launch contro ...

, on 3 April 1963, at 02:00:02

GMT Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...

, to study the Earth's upper atmosphere. It was the first satellite of five "Atmosphere Explorer".

Mission

The successful launch and operating of Explorer 17 allowed scientists for the first time to obtain instantaneous atmospheric density measurements using several independent measuring systems, to measure the atmosphere during a single day under nearly constant local time conditions and geomagnetic activity, and to compare direct measurements of density with those inferred from measurements of perturbations in the satellite period orbit.

Spacecraft

Explorer 17 was a spin-stabilized sphere in diameter. The spacecraft was

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...

sealed in order to prevent contamination of the local atmosphere.

Instruments

Explorer 17 carried four

pressure gauges Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and ...

for the measurement of total neutral particle density, two mass

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

s for the measurement of certain neutral particle concentrations, and two electrostatic probes for

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

concentration and

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

temperature measurements. Battery power failed on 10 July 1963. Three of the four pressure gauges and both electrostatic probes operated normally. One spectrometer malfunctioned, and the other operated intermittently.

Experiments

Langmuir Probes

The Explorer 17 experiment payload included two independent

Langmuir probe A Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...

systems. One of the sensors was used to provide measurements of the positive

density, and the other measured

temperature. Each system used a two-element sensor consisting of an outer cylindrical guard electrode long which was concentric with an inner collector electrode in diameter and long. The potentials of the electrodes were varied with respect to the satellite shell. The electron temperature probe was swept at a rate of 10 sweeps per second over two different voltage intervals, 0 to 0.75 V and 0 to 1.5 V. The ion density probe was swept from -3 to +2

Volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defi ...

s in 2 seconds. The currents to the collectors were measured and telemetered. The ion concentration and electron temperature could be determined from the current versus voltage information. The experiment operated normally from launch until 10 July 1963, when the spacecraft batteries failed.

Mass Spectrometers

Two identical double-focusing magnetic mass spectrometers were used to measure the concentrations of the major neutral particle constituents of the

upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes: * The mesosphere, which on Earth lies between the ...

, namely, atomic and molecular

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

, atomic and molecular

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

, and

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

. These neutral particles were ionized by electron bombardment. Measurements of the six different ion currents and the total current were made sequentially for 4 seconds in high sensitivity and 4 seconds in low sensitivity. A period of 64 seconds was required for the entire measurement cycle. Included in the cycle was an operation to correct any DC drift of the zero voltage level in the output signal. One spectrometer produced useless data due to a malfunction. The other detector system experienced intermittent degeneration of the amplifier output, and, consequently, the data were good only during certain periods. This degeneration was not a result of instrument malfunction but of an unexpected spacecraft attitude which oriented the sensor toward the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

and caused it to overheat.

Pressure Gauges

Two Redhead ( cold cathode) and two Bayard-Alpert (hot filament) ionization vacuum gauges were used to measure the neutral particle density and ambient pressure of the upper atmosphere between and . The pressure gauges were operated for 4-minutes periods when the satellite was within range of a ground telemetry station. The neutral particles were ionized by electron bombardment, and the resulting ion currents were detected and converted to voltages suitable for telemetry. These two types of sensors together were capable of measuring over the pressure range 10.E-4

torr The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one "millimeter of mercury ...

(10.E12 molecules/cc) to 10.E-11 torr. One Bayard-Alpert gauge suffered a loss in sensitivity, and no useful data were obtained from it. The remaining three gauges operated normally.

Orbital decay

The spacecraft decayed from orbit after 1,325 days on 24 November 1966.

References

External links

NASA's Explorer Missions
* Gunter's Space Page - information o

{{Orbital launches in 1963 Spacecraft launched in 1963 Satellites formerly orbiting Earth Explorers Program