Hubble & JAMES Webb, A comparison
James Webb Space Telescope(JWST) is one of the ambitious space missions
that is about to launch, expected in December 2021. JWST is the scientific
successor to Hubble, and its scientific goals were inspired by Hubble’s findings.
Hubble’s science pushed us to look to longer wavelengths to “exceed” what Hubble had already accomplished. More distant objects, in particular, are more strongly redshifted (Hubble’s Law), and their light is pushed from the UV and optical into the near-infrared. Observing these distant objects (such as the first galaxies formed in the Universe, for example) necessitates the use of an infrared telescope.
The capabilities of Hubble and JWST are different. JWST will study the
Universe primarily in the infrared, whereas Hubble will study it primarily in the optical and ultraviolet wavelengths (though it has some infrared capability). Webb’s
mirror is also much larger than Hubble’s. Because Webb has a larger light collecting area, he can see further back in time than Hubble. Hubble will be 1.5 million kilometers (km) away at the second Lagrange (L2) point, while Webb will be in a very close orbit around the Earth.
i. Expanding the Horizon
JWST will primarily observe in the infrared and will be equipped with four
science instruments to capture images and spectra of astronomical objects. These instruments will cover the wavelength range of 0.6 to 28 micrometers (or “microns”; 1 micron is 1.0 x 10-6 meters). The infrared region of the electromagnetic spectrum ranges from 0.75 to a few hundred microns. This means that Webb’s instruments will primarily operate in the infrared region of the electromagnetic spectrum, with some capability in the visible region (in particular in the red and up to the yellow part of the visible spectrum).
The instruments on Hubble can observe a small portion of the infrared spectrum from 0.8 to 2.5 microns, but its primary capabilities are in the ultra-violet and visible parts of the spectrum from 0.1 to 0.8 microns. What is the significance of infrared observations in astronomy? Stars and planets in the early stages of formation are hidden behind dust cocoons that absorb visible light. (The same is true for our galaxy’s nucleus.) Infrared light emitted by these regions, on the other hand, can penetrate this dusty shroud and reveal what is inside.
ii. How big is JWST?
When we speak of “size” it’s all about the primary mirror equipped in the
telescope. JWST includes a primary mirror with a diameter of about 6.5 meters, which will offer it a much bigger collecting area than the mirrors available on today’s space telescopes. Hubble’s mirror is substantially smaller, with a diameter of
only 2.4 meters and a collecting area of only 4.5 𝑚ଶ , providing Webb around 6.25 times more collecting area! Webb will have a much bigger field of view than Hubble’s NICMOS (The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is an instrument providing the capability for infrared imaging and spectroscopic observations of astronomical targets.) camera (covering more than 15
times the area) and much superior spatial resolution than the Spitzer Space Telescope’s infrared sensor.
JWST also comes with a component called Sunshield. Sunshield is a component of the James Webb Space Telescope that is designed to protect the main optics from the heat and light of the Sun. This is a component of a space telescope that unfolds a large metal-coated sheet of material after launch. This material obstructs
the Sun’s light and heat, allowing the telescope to see faint light from stars and galaxies. The sunshield segment consists of the layers and their deployment mechanisms, as well as the trim flap. Webb’s sunshield is about 22 meters by 12 meters (69.5 ft. x 46.5 ft.). It’s about half as big as a 737 aircraft. The sunshield is about the size of a tennis court.
iii. Orbit
The Earth is 150 million kilometres from the Sun, and the moon orbits it at a distance of approximately 384,500 kilometres. The Hubble Space Telescope orbits the Earth at a height of 570 kilometres above the surface. JWST will not orbit the Earth; instead, it will be 1.5 million kilometres away at the Earth-Sun L2 Lagrange point (Lagrange points are locations in space where objects sent there tend to remain stationary. The gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them at Lagrange points. These points in space can be used by spacecraft to reduce the amount of fuel required to stay in position).
Webb’s solar shield will block light from the Sun, Earth, and Moon at the L2
point. This will keep Webb cool, which is critical for an infrared telescope.
iv. Towards the Beginning
Light travels at a constant velocity and hence it takes time to travel. Due to this
fact the telescopes actually works as a time machine, the farther we look the farther we see back in time.
Considering galaxies as the objects for observation, Hubble can see “toddler
galaxies,” and the Webb Telescope will be able to see “baby galaxies.” One reason Webb will be able to see the first galaxies is because it is an infrared telescope. The universe (and thus its galaxies) are expanding. When it comes to the most distant objects, Einstein’s General Theory of Relativity comes into play. It tells us that as the universe expands, the space between objects stretches, causing objects (galaxies)
to move away from each other. Furthermore, any light in that space will stretch, causing its wavelength to shift to longer wavelengths. Because infrared light reaches us, this can make distant objects appear very dim (or even invisible) at visible wavelengths of light. Webb infrared telescopes are ideal for observing these early galaxies.
v. Herschel & Webb
The European Space Agency’s Herschel Space Observatory was an infrared
telescope that orbited the L2 point as well (where Webb will be). The primary distinction between Webb and Herschel is the wavelength range:
Webb covers 0.6 to 28.5 microns, while Herschel covers 60 to 500 microns.
Webb’s mirror is also larger, at approximately 6.5 metres vs. Herschel’s 3.5 metres.
Different sciences chose the wavelength ranges: Herschel looked for the extremes, the most actively star-forming galaxies that emit the majority of their energy in the
far-IR. Webb will search for the first galaxies to form in the early universe, which will necessitate extreme near-IR sensitivity.
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Written by
Sreehari A
M. Sc. Physics
Poetry of Reality 