TRANSIT 4A

About This Satellite

Transit 4A was an early US Navy navigation satellite and a landmark in spacecraft electrical power. The Department of Energy identifies it as the first US spacecraft powered by nuclear energy, using a radioisotope thermoelectric generator developed by the Atomic Energy Commission. Its historical significance is not limited to the orbit plotted in a modern tracking catalog.

Decay heat supplied electricity without a turbine

The generator converted heat from plutonium-238 decay into electricity through thermocouples, without moving parts. This is an RTG, not a fission reactor: its heat source is radioactive decay rather than a controlled chain reaction. It supplied electrical energy to the spacecraft; it was not a nuclear rocket engine accelerating the satellite around Earth.

The 2.7-watt figure belongs to the generator

DOE reports an electrical output of 2.7 watts for Transit 4A's RTG. That figure should not be labelled as thrust, transmitter output, or the entire spacecraft's power budget. The same historical account describes its long broadcasting life, but a record of past transmissions is not evidence of a signal available today.

Mission reference: US Department of Energy: Transit 4A and early radioisotope space power. Editorial review: 2026-09-14.

A new shape provided room for solar cells too

Johns Hopkins APL dates the launch to June 29, 1961. Its development history describes Transit 4A and 4B as drum-shaped, providing more surface area for solar cells than the earlier spherical design. Nuclear power did not mean solar hardware was absent. These spacecraft also introduced the programme's 150- and 400-MHz operating frequencies; a frequency multiplier produced 400 MHz from a lower-frequency transistor stage.

Reference: Johns Hopkins APL: Transit 4A/4B design and radio-frequency development.

Navigation experiments also improved knowledge of Earth

APL credits Transit 4A and 4B together with helping determine previously unevaluated gravity-field harmonics and establish geographic ties between continents and islands. The work revealed an approximately one-kilometre error in Hawaii's mapped position. That was a correction to geographic knowledge, not a claim that the island suddenly moved by a kilometre or that Transit 4A alone made every measurement.

Reference: Johns Hopkins APL: Transit geodesy and geographic reference improvements.

Reading this orbital snapshot

Based on TLE epoch: 2026-09-04. Estimated altitude ranges from 862 km at perigee to 979 km at apogee, a difference of 116 km. The orbital period is about 103.4 minutes. Eccentricity 0.00798 indicates a nearly circular path. In a two-body approximation, perigee speed is 1.02 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 66.81 degrees implies approximate geocentric ground-track limits of 66.8 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 116 · GCAT: S00116 (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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