EXPLORER 31 (ISIS-X)

About This Satellite

Explorer 31, also called DME-A, launched on November 29, 1965 to study the ionospheric plasma environment. NASA's archive provides electron temperature and density from its Cylindrical Electrostatic Probe experiment. These are measurements near the spacecraft, not photographs of clouds or a map of radio reception on the ground.

A plasma sample needs its observing context

The archive offers one-to-five-minute samples together with position, local time, satellite potential, magnetic-model quantities, and solar and geomagnetic indices. This context helps readers compare measurements taken under different conditions. A model parameter supplied alongside a measurement should not automatically be treated as another quantity directly measured by the probe.

An inconsistent archive date needs verification

The landing page lists a dataset start of October 29, 1965, one month before the launch date stated on the same page. That inconsistency is not evidence of prelaunch orbital observations. Check the actual data files and their documentation before selecting an interval; this profile does not invent a corrected first-observation date.

Mission reference: NASA SPDF/OMNIWeb: Explorer 31 probe archive, variables and sampling. Editorial review: 2026-09-14.

A second spacecraft tested whether instruments could coexist

A 1968 NASA study compared cylindrical probes on Explorer 31 and its launch companion Alouette 2. The experiment investigated whether Alouette's long antennas and powerful sounder pulses interfered with direct plasma measurements. Early comparisons generally agreed within ten percent, with occasional larger differences. The authors explicitly did not guarantee compatibility for all future instruments: the result concerned these measurements and configurations, not a universal absence of radio interference.

Reference: NASA Goddard: Brace and Findlay, comparison of Explorer 31 and Alouette 2 probes.

Coordinated observations were part of the international agreement

The Canada-US ISIS agreement explicitly planned an Explorer satellite alongside Alouette II for coordinated direct measurements. Its scientific objectives covered the changing ionosphere across the solar cycle and the availability of resulting data to researchers. The paired approach was therefore a research design, not merely two unrelated payloads sharing a rocket. The agreement describes the intended programme; measured results must still be established from the mission's experiments and publications.

Reference: Government of Canada: ISIS agreement and coordinated Explorer measurements.

Reading this orbital snapshot

Based on TLE epoch: 2026-09-05. Estimated altitude ranges from 502 km at perigee to 2,785 km at apogee, a difference of 2,282 km. The orbital period is about 119.2 minutes. Eccentricity 0.14227 shows why a single altitude cannot describe the whole path. In a two-body approximation, perigee speed is 1.33 times apogee speed. These are mean-element estimates, not instantaneous measurements. Distance per orbit uses an ellipse approximation.

Saved build snapshot. If a valid TLE is retrieved, this explanation and the orbit panel update together. Otherwise this dated snapshot is retained.

Ground track is not sensor coverage

An inclination of 79.77 degrees implies approximate geocentric ground-track limits of 79.8 degrees north and south. This is a prograde orbit, moving in the same general direction as Earth rotation. The plotted line marks the point beneath the object. It does not represent camera coverage, radio reception, or a guaranteed visible pass.

Accuracy and missing information

The introductory record describes the build snapshot. The orbital snapshot explanation and orbit panel share any successfully retrieved TLE; check the displayed epoch. Reentered objects retain historical data only. Mission history is not rewritten by TLE updates. TLE/SGP4 positions are predictions and can change after maneuvers or updated observations. The 14-day stale-data cutoff is a display safeguard, not an accuracy guarantee. Local passes are geometric predictions, not a visibility forecast. No verified operator or mission status means unknown, not active. Do not use this catalog for collision avoidance or operational flight decisions.

Catalog sources: CelesTrak SATCAT: NORAD 1806 · GCAT: S01806 (source update 2026 Sep 4 2345:57). Supplementary metadata: SatNOGS DB, retrieved 2026-09-05T12:57:25.375Z. Page assembled 2026-09-15. GCAT phase codes describe trajectory phases, not mission health.

Learn more: TLE data · Perigee and apogee · Ground tracks · JOT (Jewawud Live Orbital Map)

Satellite Details