EXPLORER 11

About This Satellite

Explorer 11 launched on April 27, 1961 carrying the first Earth-orbiting gamma-ray telescope. Its detector was sensitive to photon energies above 50 MeV. The achievement was opening a new observing window on the universe, not producing detailed pictures like a modern optical telescope. Only a small number of cosmic gamma-ray events emerged from the early observations.

A tumbling spacecraft scanned the sky

The satellite could not actively point at a chosen target. It was allowed to tumble to obtain a rough scan of the celestial sphere, with detector directions reconstructed to about five degrees. The telescope field of view and its pointing direction describe the sky being sampled; neither is given by the ground-track line on an Earth map.

Twenty-two events were not twenty-two resolved objects

HEASARC reports 22 gamma-ray events alongside about 22,000 charged-particle events in nine hours of space-looking data collected over 23 days. The gamma-ray count is not a catalog of 22 identified stars. Event selection, background rejection, and exposure all matter before a count can become an astronomical result.

Mission reference: NASA HEASARC: Explorer 11 detector and observing results. Editorial review: 2026-09-14.

Why the detector needed to leave the lower atmosphere

NASA explains that the atmosphere absorbs most incoming cosmic gamma rays. Raising detectors above it made this part of astronomy practical. The benefit of orbit was therefore access to radiation that ordinary ground-based optical telescopes cannot directly collect, together with more observing time than a short flight could provide.

Reference: NASA Imagine the Universe: atmosphere and gamma-ray observing platforms.

A small sample established a starting point

NASA contrasts Explorer 11's 22 cosmic gamma rays with the 621 events attributed to cosmic gamma rays by OSO-3 in 1967. That comparison illustrates the early growth of the field, not a fair sensitivity ranking without accounting for exposure and instruments. Later observatories could investigate a much richer sky by collecting larger samples and improving detection methods.

Reference: NASA: early gamma-ray astronomy in context.

Reading this orbital snapshot

Based on TLE epoch: 2026-09-05. Estimated altitude ranges from 463 km at perigee to 1,171 km at apogee, a difference of 708 km. The orbital period is about 101.2 minutes. Eccentricity 0.04919 shows why a single altitude cannot describe the whole path. In a two-body approximation, perigee speed is 1.10 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 28.77 degrees implies approximate geocentric ground-track limits of 28.8 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 107 · GCAT: S00107 (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)

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