The initial elliptical orbit was altered many times in the first year of life by means of an onboard propulsion system employing a 1.6 kg (3.5 lb) thruster.

[6] The miniature electrostatic analyzer (MESA) obtained data on the neutral density of the atmosphere in the altitude range of 120 to 400 km (75 to 249 mi) from the measurements of satellite deceleration due to aerodynamic drag.

Laboratory and inflight determination of spectrometer efficiency and mass discrimination permitted direct conversion of measured ion currents to ambient concentrations.

The experiment's four primary mechanical components were the guard ring and ion-analyzer tube, collector and preamplifier assembly, vent, and main electronics housing.

A three-stage Bennett tube with 7 to 5-cycle drift spaces was flown and was modified to permit ion concentration measurements to be obtained at low altitudes.

Primary analog instrument output was a compressed ion current spectrum which displayed the full dynamic range of the amplifier system on a single telemetry channel.

The mass-spectrometer sensor had a gold-plated stainless steel thermalizing chamber and ion source, a hyperbolic-rod quadrupole analyzer, and an off-axis electron multiplier.

When operating in the "normal format", the analyzer measured all masses in the range 1 to 44 with emphasis on hydrogen, helium, oxygen, nitrogen and argon.

In orbit, the pre-sealed spectrometer was opened, and the atmospheric constituents passed through a knife-edged orifice into the thermalization chamber and ion source.

Selected ions left the quadrupole analyzer through a weak focusing lens and were accelerated into a 14-stage electron multiplier, where they were turned 90° to strike the first dynode.

[10] The cold cathode ion gauge flown on Explorer 51 (AE-C) was primarily an engineering experiment to provide data on spacecraft operation.

The cylindrically shaped sensor package consisted of a wedge-shaped orifice, a cathode near ground potential, an anode operating at about 1300 VDC, and a permanent magnetic field of about 1600 gauss.

Each instrument was a retarding potential Langmuir probe device that produced a current-voltage (I-V) curve for a known voltage pattern placed on the collector.

Most modes involved an automatic or fixed adjustment of collector voltage limits (and/or electrometer output) such that the region of interest on the I-V profile provided high resolution.

Each detector consisted of a cylindrical electrostatic analyzer for species and energy selection and a Spiraltron electron multiplier for particle detection.

Detector look angles were chosen to give optimum magnetic pitch-angle coverage when the spacecraft was moving either poleward or equatorward.

It was mounted on the satellite equator normal to the spin axis, and the entrance aperture faced forward when the spacecraft was in the despun mode.

An option existed in the locked mode to continuously measure any set of mass numbers in the ratio 1 to 4 to 16 to give high spatial resolution.

Analysis of the measured molecular nitrogen density variation over a spin cycle with a knowledge of the satellite's motion and orientation led to a determination of the ambient temperature, independent of scale height.

An alternate measurement of neutral temperature was also undertaken, using a baffle inserted in front of the orifice to intercept a portion of the gas particle stream entering the chamber.

From the source, this ionized nitrogen beam was directed from a quadrupole analyzer, tuned to pass those particles whose mass-to-charge ratio (M/Q) is 28, on to an electron multiplier.

Those ambient particles that did not strike the ion source retained their incoming energy of several eV after ionization and escaped into the accelerating region of the analyzer.

The other 12 monochromators operated at fixed wavelengths with fields of view smaller than the full solar disk to aid in the atmospheric absorption analysis.

[21] This Ultraviolet Nitric-Oxide Experiment (UVNO) consisted of a two-channel fixed-grating Ebert-Fastie spectrometer which measured the airglow in the (1, 0) gamma-band in a 15-A region centered at 2149 A.

As the spacecraft spun, the spectrometer, which looked outward through the rim of the satellite, repeatedly had its field of view carried down through the atmosphere onto the Earth's limb, and altitude profiles of the emitted airglow intensity were obtained.

The sensor's spherical fused-quartz telescope mirror had a 125-mm focal length, and focused incident light on the entrance slit of the spectrometer.

For the Explorer 51 (AE-C) mission, the following six specific lines and bands were chosen for study since they play an important role in the photochemical energy balance of the atmosphere (expressed in Angstroms): 3371, 4278, 5200, 5577, 6300, and 7319.

The wide-angle high-sensitivity system (designated channel 2) had a field of view of 3° half-angle and was used to measure the nightglow, dayglow above the satellite, and other weak emission features.

The less sensitive system (designated channel 1) had a field of view of approximately 0.75° half-angle and was used for dayglow and nightglow horizon measurements, as well as discrete auroral features which showed strong spatial gradients.

In order that the sensors would respond in a fraction of a second to large changes in surface brightness without any noticeable enhancement in the background count rate, each one contained a 1/100 attenuator and an electronic circuit to back-bias the cathode.