Explorer-1 [Prime], also known as E1P and Electra, was a CubeSat

CubeSat

A CubeSat is a type of miniaturized satellite for space research that usually has a volume of exactly one liter , has a mass of no more than 1.33 kilograms, and typically uses commercial off-the-shelf electronics components...

-class picosatellite built by the Space Science and Engineering Laboratory (SSEL) at Montana State University

Montana State University - Bozeman

Montana State University – Bozeman is a public university located in Bozeman, Montana. It is the state's land-grant university and primary campus in the Montana State University System, which is part of the Montana University System...

. It was launched aboard a Taurus-XL rocket from Vandenberg Air Force Base

Vandenberg Air Force Base

Vandenberg Air Force Base is a United States Air Force Base, located approximately northwest of Lompoc, California. It is under the jurisdiction of the 30th Space Wing, Air Force Space Command ....

, California on 4 March 2011, but failed to achieve orbit after the rocket malfunctioned.

As part of NASA's ELaNA program, E1P was to be launched along with NASA's Glory

Glory (satellite)

The Glory satellite was a planned NASA satellite mission that would have collected data on the chemical, micro-physical and optical properties—and the spatial and temporal distributions—of sulfate and other aerosols, and would have collected solar irradiance data for the long-term climate record...

 satellite, Kentucky Space's

Kentucky Space

Kentucky Space is a non-profit consortium of private and public universities, companies, and other organizations with the goal of designing and leading innovative space missions within realistic budgets and objectives...

 KySat-1

KySat-1

KySat-1 was an American satellite which was to have been operated by Kentucky Space. Designed to operate for eighteen to twenty four months, it was lost in a launch failure in March 2011 after the Taurus rocket carrying it failed to achieve orbit....

 and the University of Colorado-Boulder's Hermes

Hermes (satellite)

Hermes was an American satellite which was to have been operated by the Colorado Space Grant Consortium. Intended to perform technology demonstration experiments in low Earth orbit, it was lost during launch in March 2011 when the rocket that was carrying it failed to achieve orbit.Hermes was a...

 CubeSats.

E1P was a reflight mission of Explorer 1, the first American

United States

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

 satellite, using modern technology including a geiger tube donated by James Van Allen

James Van Allen

James Alfred Van Allen was an American space scientist at the University of Iowa.The Van Allen radiation belts were named after him, following the 1958 satellite missions in which Van Allen had argued that a Geiger counter should be used to detect charged particles.- Life and career :* September...

. The name of the satellite was also adopted from Van Allen, who referred to the satellite as Explorer-1 Prime prior to his death in 2006.

It was originally intended to be launched in 2008 to commemorate the 50th anniversary of the launch of Explorer 1. If it had been successful, E1P would have been Montana's first successful launch of a satellite after the loss of SSEL's MEROPE in 2006.

There was a spare however, and that was ultimately launched with NPP.