Named for NASA's chief during most of the 1960s, Webb is about 100 times more sensitive than Hubble and is expected to revolutionise astronomers' understanding of the universe and our place in it.
Webb mainly will view the cosmos in the infrared spectrum, allowing it to gaze through clouds of gas and dust where stars are being born, while Hubble has operated primarily at optical and ultraviolet wavelengths.
The new telescope's primary mirror – consisting of 18 hexagonal segments of gold-coated beryllium metal – also has a much bigger light-collecting area, enabling it to observe objects at greater distances, thus farther back into time, than Hubble.
That advance, astronomers say, will bring into view a glimpse of the cosmos never previously seen – dating back to just 100 million years after the Big Bang, the theoretical flashpoint that set in motion the expansion of the observable universe an estimated 13.8 billion years ago.
Webb's instruments also make it ideal to search for potentially life-supporting atmospheres around scores of newly documented exoplanets – celestial bodies orbiting distant stars – and to observe worlds much closer to home, such as Mars and Saturn's icy moon Titan.
The telescope is an international collaboration led by NASA in partnership with the European and Canadian space agencies. Northrop Grumman Corp was the primary contractor. The Ariane launch vehicle is part of the European contribution.