প্রকাশ: 25/01/2022
NASA's James Webb Space Telescope, designed to give the
world an unprecedented glimpse of infant galaxies in the early stages of the
universe, arrived at its gravitational parking spot in orbit around the sun on
Monday, nearly a million miles from Earth.
With a final five-minute, course-correcting thrust of its
onboard rocket, Webb reached its destination at a position of gravitational
equilibrium known as the second Sun-Earth Lagrange point, or L2, arriving one
month after launch, NASA officials said.
The thruster was activated by mission control engineers at
the Space Telescope Science Institute in Baltimore, with radio signals
confirming Webb was successfully "inserted" into its desired orbital
loop around L2.
From there, Webb will follow a special "halo" path
that keeps it in constant alignment with Earth but out of its shadow, as the
planet and telescope circle the sun in tandem. The prescribed L2 orbit within
the larger solar orbit thus enables uninterrupted radio contact, while bathing
Webb's solar-power array in non-stop sunlight.
By comparison, Webb's 30-year-old predecessor, the Hubble
Space Telescope, orbits the Earth from 340 miles (547 km) away, passing in and
out of the planet's shadow every 90 minutes.
The combined pull of the sun and Earth at L2 - a point of
near gravitational stability first deduced by 18-century mathematician
Joseph-Louis Legrange - will minimize the telescope's drift in space.
But ground teams will need to fire Webb's thruster briefly
again about once every three weeks to keep it on track, Keith Parrish, the
observatory's commissioning manager from NASA's Goddard Space Flight Center in
Maryland, told reporters on Monday.
Mission engineers are preparing next to fine-tune the
telescope's primary mirror - an array of 18 hexagonal segments of gold-coated
beryllium metal measuring 21 feet, 4 inches (6.5 meters) across, far larger
than Hubble's main mirror.
Its size and design - operating mainly in the infrared
spectrum - will allow Webb to peer through clouds of gas and dust and observe
objects at greater distances, thus farther back in time, than Hubble or any
other telescope.
These features are expected to usher in a revolution in
astronomy, giving a first view of infant galaxies dating to just 100 million
years after the Big Bang, the theoretical flashpoint that set the expansion of
the known universe in motion an estimated 13.8 billion years ago.
Webb's instruments also make it ideal to search for signs of
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.
NEXT STEPS
It will take several more months of work to ready the
telescope for its astronomical debut.
The 18 segments of its principal mirror, which had been
folded together to fit inside the cargo bay of the rocket that carried the
telescope to space, were unfurled with the rest of its structural components
during a two-week period following Webb's launch on Dec. 25.
Those segments were recently detached from fasteners and
edged away from their original launch position. They now must be precisely
aligned - to within one-ten-thousandth the thickness of a human hair - to form
a single, unbroken light-collecting surface.
Ground teams will also start activating Webb's various
imaging and spectrographic instruments to be used in the three-month mirror
alignment. This will be followed by two months spent calibrating the
instruments themselves.
Mirror alignment will begin by aiming the telescope at a
rather ordinary, isolated star, dubbed HD-84406, located in the Ursa Major, or
"Big Dipper," constellation but too faint to be seen from Earth with
the naked eye.
Engineers will then gradually tune Webb's mirror segments to
"stack" 18 separate reflections of the star into a single, focused
image, Lee Feinberg, Webb's optical telescope element manager at Goddard, said
during Monday's NASA teleconference.
Alignment is expected to start next week when the telescope,
whose infrared design makes it super-sensitive to heat, has cooled down enough
in space to work properly - a temperature of about 400 degrees below zero
Fahrenheit (-240 Celsius).
If all goes smoothly, Webb should be ready to begin making
scientific observations by summer.
Sometime in June, NASA expects to make public its
"early release observations," a 'greatest hits' collection of initial
images used to demonstrate proper functioning of Webb's instruments during its
commissioning phase.
Webb's most ambitious work, including plans to train its
mirror on objects farthest from Earth, will take a bit longer to conduct.
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.
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