Why does the James Webb telescope hide behind a sunshield 1.5 million km from Earth?

Because the universe stretched the light it wants to see. Webb was designed for infrared astronomy, to view objects too old, distant or faint for Hubble. Because the universe is expanding, light from distant objects becomes redshifted as it travels, so objects at great distances are easier to see in the infrared. Webb can detect objects up to 100 times fainter than Hubble can, looking back to redshift z≈20, about 180 million years after the Big Bang.

The catch is that infrared is heat. Webb must be kept under 50 K (about −223 °C); otherwise the infrared radiation from the telescope itself would overwhelm its instruments. So it carries a large sunshield of five layers, each about 0.1 mm thick, that blocks light and heat from the Sun, Earth and Moon.

That shield only works if all three bodies stay on the same side. Its position near the Sun–Earth L2 point, about 1.5 million kilometres from Earth, does that: the Sun, Earth and Moon stay on the same side, so one shield is enough.

The price was a spacecraft that could not be fixed. Unlike Hubble, Webb is not intended to be serviced in space, and a crewed repair mission would not be possible. A 2018 review found 344 potential single-point failures: tasks with no alternative or means of recovery that had to succeed for the telescope to work.