The first attempt was postponed after failure of radar systems and the second hampered by bad weather.
The unmanned Falcon 9 rocket is delivering the observatory, nicknamed DSCOVR, for NASA, the National Oceanic and Atmospheric Administration and Air Force.
But the second part of the SpaceX mission, to prove it could land its leftover booster on an ocean platform 600 kilometres off the Florida coast for later re-use, had to be abandoned due to rough seas. SpaceX will still attempt a “soft landing” in the ocean, but is not expecting to succeed given the terrible weather conditions.
DSCOVR will travel 1.6 million kilometres to a gravity-neutral position in direct line with the Sun and will keep watch to provide advance warnings of incoming geomagnetic storms that could disrupt power and communications on Earth.