space headache
n. A debilitating headache experienced by astronauts during space travel.
After quizzing 17 seasoned astronauts [researchers] found more than two-thirds suffered from headaches on missions yet were headache-free back on earth.

The disabling headaches appeared unique described by the crew as "exploding" and were generally unrelated to common space motion sickness.

The Dutch researchers say space headache should be seen as a separate medical condition.
—“Space such a headache,” Sydney MX, June 03, 2009
Astronauts need to add space headache to their list of occupational hazards, say researchers. …

The researchers believe there are a number of reasons why space travel could cause headaches, the root cause being microgravity.

Microgravity is known to cause lower oxygen levels in the blood and this may be the trigger for space headache.
—“Space headache a real phenomenon,” BBC News, June 02, 2009
2009 (earliest)
Headache is a common, but rarely voiced, complaint during space flights, usually attributed to space motion sickness (SMS). …

The vast majority of headache episodes (76%) were not associated with symptoms of SMS. We conclude that space flights may trigger headaches without other SMS symptoms in otherwise 'super-healthy' male subjects. We propose to classify space headache as a separate entity among the secondary headaches attributed to disorders of homeostasis.
—AA Vein, “Space headache: a new secondary headache,” Cephalalgia, June 01, 2009