A last-minute technical problem yesterday delayed NASA's unprecedented flight to the sun.

The early morning (US time) 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 today, 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 US$1.5 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.

SUPERHERO-WORTHY SHIELD

Parker's lightweight heat shield is just 11cm thick. But it can withstand 1370C as well as extreme radiation, thanks to its high-tech carbon. Although the corona reaches millions of degrees, it's a wispy, tenuous, environment and so the spacecraft won't need to endure such severe temperatures. The 2.4m shield will face the sun during the close solar encounters, shading the science instruments in the back and keeping them humming at a cool 27C. As one scientist notes, this is a shield Captain America would envy.

SEVEN YEARS IN HOT PURSUIT

The spacecraft's path to the sun runs past Venus. It will fly by our solar system's hottest planet seven times over seven years, using the gravity of Venus to shrink its own oval orbit and draw increasingly closer to the sun. The first Venus flyby is in October, followed by the first dip into the sun's corona in November.

There will be 24 orbits between Venus and the sun, with the final three putting Parker closest to the sun — just 6 million km out — in 2024 and 2025. That's a scant 4 per cent of the 150 million km between Earth and the sun.

BREAKING RECORDS

The records will start falling as soon as Parker takes its first run past the sun. The current close-to-the-sun champ, NASA's former Helios 2, got within 43 million km in 1976.

Parker will come within 25 million km in November and then start beating its own record. During its closest solar approaches, the spacecraft will hurtle through the corona at 690km/h, setting a speed record.