# STEREO-A

*Spacecraft*

One of a pair launched together and set drifting to either side of the Earth, so that the Sun could be seen from two places at once and coronal mass ejections could be measured in three dimensions rather than guessed at from a disc. Its twin fell silent in 2014; this one is still going, and passed back between the Sun and the Earth in 2023.

| | |
| --- | --- |
| Agency | NASA |
| Launched | 2006-10-26 - still working |
| From the Sun | 144,230,109 km (+15,039 km/d) |
| From the Earth | 166,617,269 km (+133,307 km/d) |
| A signal takes | 9 min |
| Travelling at | 30.3 km/s |

Distances and speed are computed from JPL Horizons trajectories and are true of 2026-09-05, changing at the rate given beside each. The interpolation itself is good to a few kilometres; what ages these figures is the date, not the arithmetic.

## Path

A loop round the Sun slightly smaller than the Earth's, so it runs slightly faster and slowly laps us: years ahead of the Earth, then round the far side, then back.

## Along the way

- 2006-12-15 — Swung past the Moon and out into orbit round the Sun
- 2011-02-06 — With its twin, the first view of the whole Sun at once
- 2023-08-12 — Passed back between the Sun and the Earth, 17 years on

## Elsewhere

- On the live map: https://www.orbitalmap.space/?world=solar-system&focus=stereo-a
- The mission's own page: https://science.nasa.gov/mission/stereo/
- This object as a page: https://www.orbitalmap.space/o/stereo-a/
- Every object: https://www.orbitalmap.space/o/
- What this site is and what it reads: https://www.orbitalmap.space/llms.txt