# Explorer 4

> American research satellite launched in 1958

**Wikidata**: [Q370053](https://www.wikidata.org/wiki/Q370053)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/Explorer_4)  
**Source**: https://4ort.xyz/entity/explorer-4

## Summary
Explorer 4 was an American research satellite launched on July 26, 1958, as part of the Explorer program. It was the first satellite to carry a scientific payload into orbit, specifically designed to study the Earth's radiation belts and magnetic fields. The satellite operated until its orbit decayed in October 1959.

## Key Facts
- **Launch Date**: July 26, 1958
- **Launch Vehicle**: Juno I
- **Launch Site**: Cape Canaveral Launch Complex 5
- **Mass**: 25.5 kilograms
- **Orbital Period**: 110.2 minutes
- **Apoapsis**: 2,213 kilometers
- **Periapsis**: 263 kilometers
- **Orbital Inclination**: 50.3 degrees
- **Orbital Eccentricity**: 0.127936
- **Semi-Major Axis**: 7,616.2 kilometers
- **Decay Date**: October 23, 1959
- **Payload**: First satellite to carry a scientific instrument (a cosmic ray detector)

## FAQs
### Q: What was the primary mission of Explorer 4?
A: Explorer 4 was designed to study the Earth's radiation belts and magnetic fields, making it the first satellite to carry a dedicated scientific payload into orbit.

### Q: Which rocket launched Explorer 4?
A: Explorer 4 was launched by a Juno I rocket from Cape Canaveral Launch Complex 5 on July 26, 1958.

### Q: How long did Explorer 4 remain in orbit?
A: Explorer 4 operated in orbit until its decay on October 23, 1959, after approximately one year in space.

### Q: What was unique about Explorer 4's payload?
A: Explorer 4 carried a cosmic ray detector, marking it as the first satellite to use specialized scientific instruments for research purposes.

### Q: What was the significance of Explorer 4 in space exploration?
A: Explorer 4 was a pivotal early satellite, demonstrating the feasibility of conducting scientific research from space and contributing valuable data on Earth's radiation environment.

## Why It Matters
Explorer 4 played a crucial role in the early days of space exploration by proving that satellites could be used for scientific research beyond simple orbital mechanics. Its cosmic ray detector provided the first direct measurements of radiation in space, advancing understanding of Earth's magnetic field and its interaction with solar particles. As the first satellite to carry a dedicated scientific payload, Explorer 4 set a precedent for future space missions, paving the way for more sophisticated research satellites. Its successful operation and data collection demonstrated the potential of space-based science, influencing the design of subsequent satellites and the broader field of space research.

## Notable For
- **First Scientific Payload**: Carried a cosmic ray detector, marking the first satellite to use specialized instruments for research.
- **Early Radiation Study**: Provided critical data on Earth's radiation belts and magnetic fields.
- **Orbital Longevity**: Operated for over a year, contributing to long-term space science.
- **Technological Milestone**: Proved the feasibility of conducting scientific experiments in space.
- **Programmatic Precedent**: Established the Explorer program as a model for future space research satellites.

## Body
### Mission Overview
Explorer 4 was launched on July 26, 1958, aboard a Juno I rocket from Cape Canaveral. Weighing 25.5 kilograms, it was designed to study cosmic rays and the Earth's radiation environment. The satellite's primary instrument was a cosmic ray detector, which collected data on high-energy particles in space.

### Orbital Characteristics
Explorer 4 entered an elliptical orbit with an apoapsis of 2,213 kilometers and a periapsis of 263 kilometers. Its orbital period was 110.2 minutes, and the orbit had an inclination of 50.3 degrees. The satellite's semi-major axis was 7,616.2 kilometers, and its eccentricity was 0.127936.

### Scientific Contributions
The satellite's cosmic ray detector provided the first direct measurements of radiation in space, contributing to our understanding of Earth's magnetic field and its interaction with solar particles. This data was crucial for early space weather research.

### Operational Life
Explorer 4 remained operational until its orbit decayed on October 23, 1959, after approximately one year in space. Its successful mission demonstrated the potential of satellites for scientific research and influenced the design of future space missions.

### Legacy
Explorer 4 is remembered as a pioneering satellite that laid the groundwork for space-based scientific research. Its achievements in radiation detection and orbital longevity set standards for subsequent satellites in the Explorer program.

## References

1. Jonathan's Space Report
2. Freebase Data Dumps. 2013