James Webb Space Telescope Guide

1 - Overview

The James Webb Space Telescope is the largest, most complex observatory sent into space.

Webb’s innovative design tackles the two main challenges for an infrared telescope: It needs a very large mirror to collect enough light, and it has to be kept cold to keep unwanted sources of infrared from interfering with the light being observed. Webb’s key components include:

  • an enormous primary mirror to collect infrared light,
  • a supersized sunshield to keep the telescope cold
  • four scientific instruments to conduct its ambitious science operations. 

The size of the mirror and sunshield present another challenge: Webb must fit into the limited space within a Ariane 5 vehicle. Scientists and engineers devised a creative solution by turning Webb into a piece of enormous origami. The telescope is designed to neatly fold upon itself for launch, then complete a complicated series of steps to unfurl on its way to its observation post a million miles from Earth.

The Webb telescope will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our Solar System.

2 - Webb's Size

Webb’s primary mirror is the most important component of the telescope: the larger the mirror the higher the resolution of the observation is, and more distant objects can be detected.

This mirror towers are more than two stories high, with an area of 25.37 square meters (273.1 square feet), 5 times greater than Hubble’s one, and a mass of 705 kgs (1,555 pounds on Earth).

Once unfurled its protective sunshield measures about the size of a tennis court: 21.2 meters by 14.2 meters (69.5 feet by 46.5 feet). The total mass of the payload is about 6,200 kilograms (about 13,650 pounds on Earth), with a height of 8 meters (28 feet).

3 - Webb's Primary Mirror

The purpose of Webb’s primary mirror is to intercept red and infrared light (0.6—28.8 microns) coming from space and reflect it onto a smaller secondary mirror, which then directs the light into the scientific instruments where it is recorded.

The primary mirror can be thought of as a “light bucket” whose power is a function of size in terms of collecting area: a mirror with larger dimensions has higher sensitivity and it is capable to detect more distant or fainter objects, providing clearer, more detailed images and spectra concerning its predecessors.

With a diameter of 6.6 meters (21.7 feet), Webb’s mirror is composed of 18 hexagonal mirrors, arranged in a hexagonal, slightly concave shape. This configuration is large enough to detect the faint infrared light of galaxies more than 13.5 billion light-years away

The orientation of every single mirror can be corrected independently from each other, in order to obtain a better resolution in every situation.

4 - L2 Orbit

A Sun-Earth Lagrange point is a region in space where the gravitational forces of the two large bodies (Earth and Sun) and the centrifugal force of the telescope balance each other. It exists 5 points that respect these characteristics, and Webb is positioned to the Lagrange point L2.

Webb is not located directly at L2 but instead moves in a halo orbit around L2, almost perpendicular to its orbit around the Sun. Webb orbits L2 at a distance of roughly 500,000 kilometers (300,000 miles), completing one halo orbit every 168 days. To maintain its position, Webb will need to reposition itself periodically using its thrusters.

At L2, Webb is far beyond Earth’s atmosphere, which absorbs (blocks) most infrared light from celestial objects, and is far enough from the Sun, Earth, and Moon for the sunshield to be effective. 

Because L2 moves with Earth as the planet orbits the Sun, Webb’s relative position with respect to the Earth does not change significantly over the year. This simplifies communication between Earth and Webb and makes it possible for Webb’s sunshield to block the Sun, Earth, and Moon at all times.

Webb’s halo orbit around L2 keeps it out of the shadows of Earth and the Moon, reducing temperature fluctuations and allowing it to maintain solar power. The primary disadvantage of L2 is its distance. Unlike Hubble, which was repaired and upgraded five times, Webb is too far from Earth for a servicing mission.

Quick facts:

  • Type of Orbit: heliocentric,
  • Location: Sun-Earth Lagrange point 2,
  • Distance from Earth: about 1.5 million km (930,000 miles),
  • Orbital Period: 168 days to orbit L2; 365 days to orbit the Sun.

5 - Webb's Deployment Sequence

1 - Solar Array

2 - Gimbaled Antenna

3 - Forward Sunshield Pallet

4 - Aft Sunshield Pallet

5 - DTA Deployment

6 - Aft Momentum Flap

7 - Sunshield

8 - Secondary Mirror

9 - Instrument Radiator

10 - Primary Mirror Wings

6 - First Images

The first image shown here is called “Cosmic Cliffs”.

Webb’s seemingly three-dimensional picture looks like craggy mountains on a moonlit evening. In reality, it is the edge of the giant, gaseous cavity within NGC 3324, and the tallest “peaks” in this image are about 7 light-years high.

The cavernous area has been carved from the nebula by the intense ultraviolet radiation and stellar winds from extremely massive, hot, young stars located in the center of the bubble, above the area shown in this image.

A second image shows a chart where the James Webb Space Telescope has captured the signature of water, along with evidence of clouds and haze, in the atmosphere surrounding a hot, puffy gas giant planet orbiting a distant Sun-like star: WASP-96 b.

Two cameras aboard Webb (NIRCam and MIRI) captured the latest image of this planetary nebula, cataloged as NGC 3132, and known informally as the Southern Ring Nebula.

It is approximately 2,500 light-years away. Webb will allow astronomers to dig into many more specifics about planetary nebulae like this one: clouds of gas and dust expelled by dying stars.

Understanding which molecules are present, and where they lie throughout the shells of gas and dust will help researchers refine their knowledge of these objects.

Finally, James Webb Space Telescope reveals Stephan’s Quintet in a new light. This enormous mosaic is Webb’s largest image to date, covering about one-fifth of the Moon’s diameter.

It contains over 150 million pixels and is constructed from almost 1,000 separate image files. The information from Webb provides new insights into how galactic interactions may have driven galaxy evolution in the early universe.