G3 geomagnetic storm
Strong geomagnetic storm (Kp=7) that hit Artemis II crew in unshielded translunar space.
Last refreshed: 5 April 2026
How dangerous was the G3 storm for the Artemis II crew?
Latest on G3 geomagnetic storm
- Is the Artemis II crew safe from the solar storm?
- Six radiation sensors collected data continuously and NASA confirmed the crew would use about 5% of their lifetime caps, but no specific storm exposure numbers were published.Source: background
- What is a G3 geomagnetic storm?
- A Strong-level storm on the 1-5 NOAA scale, measured at Kp=7 on a 0-9 index. It can disrupt satellites and pose radiation risks to astronauts outside Earth's magnetosphere.Source: background
- Why did NASA not publish radiation dose data during Artemis II?
- NASA treated crew dose data as information not shared during flight. The entire G3 storm passed without a single number reaching the public despite six sensors collecting continuously.Source: background
- What is the Kp index?
- A 0-9 scale measuring geomagnetic disturbance. Below 4 is quiet, 5+ is a minor storm (G1), 7 is strong (G3), and 9 is extreme (G5).Source: quick_facts
Background
A G3 (Strong) geomagnetic storm peaked at Kp=7 overnight on 3 to 4 April 2026, the strongest during a crewed deep-space transit since the Apollo programme. Four Artemis II astronauts were coasting beyond Earth's magnetosphere in unshielded translunar space when the storm struck . NASA published zero crew radiation dose data through the entire event.
The storm was the peak of an escalation chain that began with an X-class solar flare on launch day (31 March), progressed through a G1 watch, a G2 storm with a coronal mass ejection forecast , and an M7.5 flare causing an R2 radio blackout. Six HERA radiation sensors and personal dosimeters aboard Orion collected readings continuously, but no numbers reached the public.
By Day 5 the storm had fully resolved. NOAA forecast maximum Kp of 3.67, well below the G1 threshold . A NASA Q&A confirmed the crew would use approximately 5% of their lifetime radiation caps across the full ten-day mission, the only quantification offered. What fraction accumulated during the Kp=7 peak remains undisclosed.