A second attempt will occur Sunday at 3:31 a.m. Eastern time. The spacecraft — which NASA touts will one day “touch the sun” — is on top of a Delta IV Heavy rocket built and operated by United Launch Alliance, a joint venture between Boeing and Lockheed Martin.

The weather is not ideal, with only a 60 percent chance of favorable conditions. The attempt will be streamed online on NASA TV starting at 3 a.m.

Why does NASA keep trying to launch at such an early hour?

The main constraint on the launch time is to get the spacecraft to its first rendezvous with Venus in November. At this hour, the launchpad is pointed in the right direction to get to Venus.

It takes a lot of energy to get to the sun — more than 50 times as much as it takes to reach Mars, NASA explains. That is because Earth is moving fast in its orbit around the sun — 67,000 mph. In order for the spacecraft to spiral inward, it has to shed some of that energy, and that means launching at high speed in the direction opposite to Earth’s motion.

The spacecraft is small, about the size of a car and weighing 1,400 pounds, but there is a third stage to give an additional velocity boost.

Another constraint is making sure that the spacecraft passes as quickly as possible through the Van Allen belt, a region around Earth of intense radiation. The radiation can scramble the spacecraft’s electronics. Those considerations also helped dictate when the launch window begins and ends.

What happens if it can’t lift off again Sunday?

Try, try again. The deadline is Aug. 23, when Venus moves too far out of position.

On Monday, odds improve to an 80 percent likelihood of good weather, according to Air Force forecasts.

What is the Parker Solar Probe and what will it study?

In short, it will study the sun’s outer atmosphere and the particles it produces known as solar wind.

Who is the spacecraft named after?

Eugene Parker, the astrophysicist who predicted the existence of solar wind 60 years ago.

This article originally appeared in The New York Times.

KENNETH CHANG © 2018 The New York Times