A last-minute technical problem has delayed NASA's unprecedented flight to the sun.
The early morning Saturday launch countdown was halted with just one minute, 55 seconds remaining, keeping the Delta IV rocket on its pad with the Parker Solar Probe.
Rocket maker United Launch Alliance said it would try again on Sunday, provided the helium-pressure issue can be resolved quickly.
As soon as the red pressure alarm for the gaseous helium system went off, a launch controller ordered, "Hold, hold, hold".
Once on its way, the Parker probe will venture closer to our star than any other spacecraft.
The $US1.5 billion ($2.1 billion) mission is already a week late because of rocket issues.
Saturday's launch attempt encountered a series of snags; in the end, controllers ran out of time.
Thousands of spectators gathered in the middle of the night to witness the launch, including the University of Chicago astrophysicist for whom the spacecraft is named.
Eugene Parker predicted the existence of solar wind 60 years ago. He's now 91 and eager to see the solar probe soar. He plans to stick around at least another few days.
But engineers are taking utmost caution with the pricey probe, which Thomas Zurbuchen, head of NASA's science mission directorate, described as one of the agency's most "strategically important missions."
The next launch window opens at 3.31am (0731 GMT) on Sunday, when weather conditions are 60 per cent favorable for launch, NASA said.
By coming closer to the Sun than any spacecraft in history, the unmanned probe's main goal is to unveil the secrets of the corona, the unusual atmosphere around the Sun.
The corona about 300 times hotter than the Sun's surface and hurls powerful plasma and energetic particles that can unleash geomagnetic space storms, wreaking havoc on Earth by disrupting the power grid.
These solar outbursts are poorly understood, but pack the potential to wipe out power to millions of people.
The probe is protected by an ultra-powerful heat shield that is 11.43cm thick.
The shield should enable the spacecraft to survive its close shave with the fiery star, coming within 6.16 million kilometres of the Sun's surface.
The tools on board will measure the expanding corona and continually flowing atmosphere known as the solar wind, which Parker first described in 1958.