Nasa's chief communications officer Jeremy Eggers said it was known as a "tow balloon" and was used to help lift the top fitting of the balloon, which is quite heavy.
"The tow balloon is released once the larger balloon has enough helium to safely hold the top fitting up," Eggers said.
Christchurch man Aaron John described the balloon illuminating the night sky tonight as it floated above the Garden City for around 45 minutes while underway to the stratosphere.
It had disappeared now but was an incredible sight, he said.
There have also been reports of lights in the sky visible from West Auckland.
Vishnu Biju also reported seeing a bright star-like light in the sky south west of Christchurch, and had initially questioned whether it was a UFO.
The balloon inflates to 18.8 million cubic feet, about the size of Dunedin's Forsyth Barr Stadium.
The balloon is made from polyethylene film, which is similar in appearance and thickness to the type used for sandwich bags, but stronger and more durable.
The balloon will ascend to an altitude of 33.5km where the stratospheric winds will propel it at speeds of at least 100 knots through the heating and cooling of the day-night cycle on a weeks-long journey around the Southern Hemisphere, said Debbie Fairbrother, Nasa's Balloon Programme Office chief.
Unfavourable weather conditions meant the last seven attempts to launch the balloon had been unsuccessful.
The balloon will be collecting scientific data from what Nasa describes as "near space".
