James Webb Space Telescope

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The James Webb Space Telescope (JWST), previously known as Next Generation Space Telescope (NGST), is a part of NASA’s ongoing Flagship program. It is under construction and scheduled to launch in October 2018. The JWST will offer unprecedented resolution and sensitivity from long-wavelength (orange-red) visible light, through near-infrared to the mid-infrared (0.6 to 27 micrometers). While the Hubble Space Telescope has a 2.4-meter (7.9 ft) mirror, the JWST features a larger and segmented 6.5-meter-diameter (21 ft 4 in) primary mirror and will be located near the Earth–Sun L2 point. A large sunshield will keep its mirror and four science instruments below 50 K (−220 °C; −370 °F).

JWST’s capabilities will enable a broad range of investigations across the fields of astronomy and cosmology. One particular goal involves observing some of the most distant events and objects in the Universe, such as the formation of the first galaxies. These types of targets are beyond the reach of current ground and space-based instruments. Another goal is understanding the formation of stars and planets. This will include direct imaging of exoplanets.

In gestation since 1996, the project represents an international collaboration of the European Space Agency, Canadian Space Agency and team members of other countries led by NASA. It is named after James E. Webb, the second administrator of NASA, who played an integral role in the Apollo program.

NASA has described JWST as the science successor of the Hubble Space Telescope, but not a replacement, because the capabilities are not identical. JWST has the objective to see high-redshift objects, typically both older and farther away than previous instruments could assess. The result was to extend the life of Hubble until JWST as the next generation telescope could go online. This led to a radically altered design for JWST to obtain images deeper into the infrared than Hubble, and beyond the capabilities of the Infrared Space Observatory and Spitzer Space Telescope.

In December 2016, NASA announced that the JWST has passed major milestones, including completion of its primary mirror and integration of science instruments with the payload module, and is undergoing acoustic and extreme vibration testing to simulate launch conditions.

 

 

 

Overview:

The JWST originated in 1996 as the Next Generation Space Telescope (NGST). In 2002 it was renamed after NASA’s second administrator (1961–1968) James E. Webb (1906–1992), noted for playing a key role in the Apollo program and establishing scientific research as a core NASA activity. The JWST is a project of the National Aeronautics and Space Administration, the United States space agency, with international collaboration from the European Space Agency and the Canadian Space Agency.

The telescope has an expected mass about half of Hubble Space Telescope’s, but its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) will have a collecting area about five times as large (25 m2 vs. 4.5 m2). The JWST is oriented toward near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument. The design emphasizes the near to mid-infrared for three main reasons: High-redshift objects have their visible emissions shifted into the infrared, cold objects such as debris disks and planets emit most strongly in the infrared, and this band is difficult to study from the ground or by existing space telescopes such as Hubble.

 

The JWST will operate near the Earth-Sun L2 (Lagrange) point, approximately 930,000 mi (1,500,000 km) beyond the Earth. By way of comparison, Hubble orbits 340 miles (550 km) above Earth’s surface, and the Moon is roughly 250,000 miles (400,000 km) from Earth. This distance makes post-launch repair or upgrade of the JWST hardware virtually impossible. Objects near this point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance and use a single sunshield to block heat and light from the Sun and Earth. This will keep the temperature of the spacecraft below 50 K (−220 °C; −370 °F), necessary for infrared observations. The prime contractor is Northrop Grumman.

 Launch and mission length

Launch is scheduled for October 2018 on an Ariane 5 rocket. Its nominal mission time is five years, with a goal of ten years. JWST needs to use propellant to maintain its halo orbit around L2, which provides an upper limit to its designed lifetime, and it is being designed to carry enough for ten years. The planned five year science mission begins after a 6-month commissioning phase. An L2 orbit is meta-stable so it requires station-keeping or an object will drift away from this orbital configuration.

Orbit

JWST will not be exactly at the L2 point, but circle around it in a halo orbit.

Two alternate Hubble Space Telescope views of the Carina Nebula, comparing ultraviolet and visible (top) and infrared (bottom) astronomy. Far more stars are visible in the latter.

The JWST will be located near the second Lagrange point (L2) of the Earth-Sun system, which is 1,500,000 kilometers (930,000 mi) from Earth, directly opposite to the Sun. Normally an object circling the Sun farther out than Earth would take longer than one year to complete its orbit, but near the L2 point the combined gravitational pull of the Earth and the Sun allow a spacecraft to orbit the Sun in the same time it takes the Earth. The telescope will circle about the L2 point in a halo orbit, which will be inclined with respect to the ecliptic, have a radius of approximately 800,000 kilometers (500,000 mi), and take about half a year to complete.[14] Since L2 is just an equilibrium point with no gravitational pull, a halo orbit is not an orbit in the usual sense: the spacecraft is actually in orbit around the Sun, and the halo orbit can be thought of as controlled drifting to remain in the vicinity of the L2 point.[93] This requires some station-keeping: around 2–4 m/s per year[94] from the total budget of 150 m/s.[95] Two sets of thrusters constitute the observatory’s propulsion system.[96]

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