An eye to the sky

continued quest As the search for finding 'earth-like' planets beyond our solar system continues, a possible successor to the Hubble telescope will be

An eye to the sky

A  big Bus size 11-tonne telescope launched 25 years ago at a cost of $1.5 billion was named after US astronomer Edwin Powell Hubble (1889-1953) who discovered for the first time in the 1920s that our universe is expanding. 

Since then, the low-Earth orbit (559 km) Hubble has been going around the Earth every 96 minutes from full sun to complete darkness. The initial projection of life span for this telescope was just 15 years. After successfully completing 25 years of service on April 24, 2015, Hubble is expected to serve at least another 10 years. This is an incredible achievement for a telescope which rewrote the textbooks of astronomy during its 137,000 trips around the Earth which captured data and images of more than 38,000 objects in space. But this sojourn was as difficult as its pre-launch preparations. The historians of astronomy have chronicled the Hubble journey as ‘almost failed before it had even begun’ and ‘blurred after its launch’.

Astronomer and Princeton Professor Lyman Spitzer had written about the feasibility of building, launching, and operating a satellite observatory to study the universe in visible light in his visionary report ‘Astronomical Advantages of an Extraterrestrial Observatory’ in the early 1946. He had hypothesised a technology that could probe the universe with utmost clarity through a telescope above earth’s atmosphere. He argued that light from distant stars and galaxies travels trillions of miles across space clearly until it reaches the last 100 miles of atmosphere above the earth, where it gets blurred.

But Lyman had to wait until August 23, 1966 for the Lunar Orbiter-I to dispatch a picture of the Earth from the vicinity of the moon to get support from fellow astronomers for his pet project ‘Large Orbital Telescope’. The National Academy of Sciences gave its approval for the design and development of the ‘Large Space Telescope (LST)’ in 1969. The LST scientists, largely from NASA, were constrained by the budget cuts while designing this unique telescope. The Science Working Group recommended that LST carry a large complement of interchangeable instruments.

The design team of NASA began their work from 1978 with a target date of launch in 1983. While the astronauts began their training to work best in low gravity for space telescope missions, the optics company PerkinElmer commenced building the Optical Telescope Array Assembly with a 2.4 m mirror. But the project overshot the budget and time when the mirror inside the telescope got mismanaged and the launch date was pushed to 1986.

On January 28, 1986, the Challenger space shuttle broke apart within 73 seconds of its flight killing all the seven crew members. The same space shuttle was scheduled to carry the Hubble Space Telescope into low-Earth orbit later that year. The Hubble had to wait till 1990 for a ride on Discovery space shuttle and the telescope commenced its operations on April 24 that year.

Hubble’s trouble did not stop with the delayed launch. The primary mirror was a product of exquisite workmanship, but for the curvature around the edges which was flat by an infinitesimally small amount of 2.2 micrometres (a micrometre is a thousandth of a millimetre). In real terms this deviation was negligible compared to the size of the mirror, but the images reflected from these edges were blurred to a large extent, as the light rays bouncing off from this surface were not properly focused. None of the tracking adjustments made from the Earth could correct this problem.

During the very first service mission of the Hubble in 1993 through Endeavour space shuttle, astronauts carried small, coin-sized mirrors which could reflect light from Hubble’s primary mirror and carefully positioned them in front of the misshapen mirror and corrected its curvature. The crew also replaced the telescope’s main camera with its modified version, and fitted a second device to connect Hubble’s other scientific instruments. After all these ‘modifications’, the first image aired by Hubble was extraordinarily beautiful.  Then onwards, every place Hubble pointed out in the sky, was something new and remarkable.

Hubble was further serviced on different time periods to replace broken stabilising gyros, life expired solar panels and power unit, and to add new scientific instruments. Further, addition of two star trackers, five major leave-behind equipment subsystems and more than eight custom tools were added to support astronauts during service missions. Upgrading and repairing a spacecraft like Hubble which undergoes very hot and very cold environments within 96 minutes was a challenging task for the astronauts.  The instruments and tools were required to be built compatible with the bulky gloves on the spacesuits of the astronauts.

The servicing of the Hubble faced one more major trouble when the space shuttle Columbia disintegrated on re-entry in February 2003, killing seven of its crew members. NASA took a decision that all space shuttles should be able to reach the International Space Station on emergencies. Since ISS is on a different orbital altitude from Hubble, the single mission of a space shuttle could not have covered both the destinations. Due to this constraint, the servicing of Hubble Telescope suffered for nearly seven years. In 2009, the space shuttle Atlantis was allowed to fly exclusively on Hubble mission. With no future servicing planned for the Hubble as of now, NASA expects it to complete its mission life as is.

Hubble’s accomplishments are in plenty through its many million pictures. Astronomers could discover for the first time the regions of space with flat discs of dust around the newborn stars. They believe that these discs of dust are the building material for planets of other solar systems.  Thousands of galaxies have been filmed by the Hubble. Some of the faintest light rays recorded by the Hubble are believed to have originated from the galaxies when the universe was just 500 million years out of its 13.8 billion years of existence. 

The big questions bothering the astronomers now are what would happen to Hubble and what next after Hubble? A section of the astronomers still believes that there is life for Hubble for at least a decade from now and expect it to retire when the cost of its operation exceeds the scientific output.  On such an occasion, the orbit of Hubble starts sinking lower due to atmospheric drag.  During the peak activity period of the sun, which occurs once in 11 years, the atmosphere expands to sink the Hubble further.  The next solar maximum is due around 2024.  While entering the atmosphere, most of the telescope will be burnt out, with only a few fragments are expected to fall on the earth’s surface.

Successor to Hubble: JWST
Astronomers expect a successor for Hubble to be placed in orbit before its fall. NASA is planning to launch in 2018, James Webb Space Telescope (JWST), named after its Chief Administrator during the Apollo programme. This telescope shall have a 6.5 m mirror made up from 18 hexagonal panels which are folded at the time of launch to be unfurled at the time of deployment. Once in orbit, the huge mirror is expected to see much fainter objects than Hubble. This telescope shall be placed in a location 1.5 million km from Earth, in the opposite direction to the sun, with an orbit period of one year.

The revised budget estimate for JWST is capped at $8 billion with more than 40 per cent already spent on the hardware development and testing. The technological feat of JWST is in its giant sunshield of a tennis court size which separates the warm and cold sections. The sun-facing side carries solar panels to provide power for the telescope’s instruments, the side facing away from the sun is kept cool, and operates at a temperature of -2200 °C.

JWST has the ability to look further back in time with its infra-red observatory which can detect ancient galaxies which are receding away from us at an enormous speed. The light rays emitted by them are stretched to longer wavelengths while receding, making them appear more reddish. Astronomers expect JWST to detect even the first stars and galaxies that formed after the big bang, through the light rays emitted by them billions of years ago.

As the search for finding ‘earth-like’ planets beyond our solar system continues, JWST shall watch some of those potential planets as they cross their stars. The light rays originating from these planets, or reflected through their atmospheres, might indicate the seasonal changes on the ground, weather patterns and signs of vegetation.

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