The launch window lasts 65 minutes, according to ULA.
Nasa is planning to launch the $1.5bn probe towards the sun, with an aim to offer the closest-ever glimpse of the star at the centre of the solar system.
Engineers are taking utmost caution with the $1.5 billion Parker Solar Probe, which Thomas Zurbuchen, head of NASA's science mission directorate, described as one of the agency's most "strategically important missions". The launch window for the mission now closes on August 23.
The Parker Solar Probe, named after American solar astrophysicist Eugene Parker, will, as the U.S. space agency describes it, "touch the sun" as it flies within 3.9 million miles of the star's surface.
The heavily shielded Parker Solar Probe will make 24 such close flybys over the next seven years, getting within just 3.83 million miles (6.16 million kilometers) of the sun's surface at closest approach.
Rocket maker United Launch Alliance said it would attempt the launch again today, provided the helium-pressure issue can be resolved quickly.
The probe is set to use seven Venus flybys over almost seven years to steadily reduce its orbit around the Sun, using instruments created to image the solar wind and study electric and magnetic fields, coronal plasma and energetic particles.
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"With each orbit, we'll be seeing new regions of the Sun's atmosphere and learning things about stellar mechanics that we've wanted to explore for decades", Fox added.
"It was just a matter of sitting out the deniers for four years until the Venus Mariner 2 spacecraft showed that, by golly, there was a solar wind", Parker said earlier this week.
It will get more than seven times closer than the current record holder for a close solar pass, a record set by the Helios 2 spacecraft in 1976. It's the first time NASA mission has been named after a living person.
In an orbit this close to the Sun, the real challenge is to keep the spacecraft from burning up.
It said "there was not enough time remaining in the window to recycle".
The probe is protected by a 4in-thick shield that constantly repositions itself between the sun's power and the scientific instruments on board.
At Parker Solar Probe's closest approach to the Sun, temperatures on the heat shield will reach almost 1,371 degrees Celsius, but the spacecraft and its instruments will be kept at a relatively comfortable temperature of about 29.4 degrees Celsius.
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