Explorer 17 was a United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

, launched at Cape Canaveral

Cape Canaveral Air Force Station

Cape Canaveral Air Force Station is an installation of the United States Air Force Space Command's 45th Space Wing, headquartered at nearby Patrick Air Force Base. Located on Cape Canaveral in the state of Florida, CCAFS is the primary launch head of America's Eastern Range with four launch pads...

 from LC-17B on a Delta-B

Delta rocket

Delta is a versatile family of expendable launch systems that has provided space launch capability in the United States since 1960. There have been more than 300 Delta rockets launched, with a 95 percent success rate. Two Delta launch systems – Delta II and Delta IV – are in active use...

 booster, on April 3, 1963, to study the Earth's upper atmosphere

Earth's atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

. It was the first satellite of five Atmosphere Explorers.

Technical specifications

Explorer 17 was a spin-stabilized

Spin-stabilized satellite

A spin-stabilized satellite is a satellite which has the motion of one axis held fixed by spinning the satellite around that axis, using the gyroscopic effect.The attitude of a satellite or any rigid body is its orientation in space...

 sphere 0.95 m in diameter. The spacecraft was vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 sealed in order to prevent contamination of the local atmosphere. Explorer 17 carried four pressure gauges

Pressure measurement

Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....

 for the measurement of total neutral particle density

Particle density

The particle density or true density of a particulate solid or powder, is the density of the particles that make up the powder, in contrast to the bulk density, which measures the average density of a large volume of the powder in a specific medium .The particle density is a relatively well-defined...

, two mass spectrometer

Spectrometer

A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...

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

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 concentration and electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

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

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.

The spacecraft decayed

Orbital decay

Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit.This can be due to drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is larger in the case of increased solar...

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

External links

• NASA's Explorer Missions
• Gunter's Space Page - information on Explorer 17
• Space History Notes