by S. Prasad Ganti
The perilous journey on the ground as well as in the space is complete. Continuing our journey with the James Webb space telescope, which was launched on Christmas day at the fag end of 2021. Some deployment tasks have been completed in space. The solar panels have been unfurled. So have the heat shield and the complex mirror. Each of them is a huge undertaking and a potential point of failure. More importantly, it has reached its home at the L2 Lagrangian point about a million miles away from Earth on the other side of the Sun-Earth axis. Picture given below from the internet shows the L2 Lagrangian point in relation to the Earth and the Sun.
The telescope will be parked here or rather will be moving along with the Earth around the Sun. With some tiny orbiting around the L2 point in a perpendicular plane. With respect to the Earth, it will appear at the same point all the time. Hopefully for the next decade or even longer. The L2 Lagrangian point is named for the French mathematician Joseph Lagrange. It is a point at which the combined forces of gravity due to the Earth and the Sun are supposed to be minimal. It is difficult for me to visualize the forces involved. Lagrange solved this “three body” problem mathematically to arrive at five such points in space named L1 through L5 shown in my favorite picture above. L2 is the most appropriate place for the task at hand, which is to see the vast swath of Universe away from the Sun.
The telescope itself is a piece of engineering marvel. It took ages and cost a lot of money to engineer, construct and test. And finally send it to its home at the L2 point about a million miles away. Several nuances of engineering about this marvel fascinate me. And how human thinking strives to make things better and better. A true successor to the current Hubble telescope, which had a shaky start with a faulty mirror corrected by floating astronauts in space. Hubble has lived beyond its expected lifetime and contributed enormously to our knowledge of the Universe. Webb will be a quantum jump over the Hubble. Comparing and contrasting with Hubble, it weighs much less at 6.5 tons vs. 11 tons for Hubble. Yet its mirror is more than double the size, weighing about a tenth. The mirror has been designed using the divide and conquer technique. Casting larger mirrors is fraught with exponentially larger difficulty and is fragile enough to be sent into space as one piece. It has 18 hexagonal segments, each made of an ultralight metal called Beryllium. This metal has been mined in Utah and exists largely in powdered form and is difficult to work with in forming larger pieces. Each segment is polished to extremely low tolerance and is coated with gold. Gold is a good reflector and never loses its sheen. Hence gold captivates humanity as well ! Another favorite picture of mine given below, courtesy NASA.
The mirror could not be sent into space as one piece due to the real estate limitation of the largest rocket which was used to launch. It had to be folded. A segmented design helped towards this end. And the unfolding went very well once it reached space. What is left before Webb starts doing real science is the process of tuning the giant mirror. It is expected to take the next three months. Why such a long time ?
Firstly, the mirror has to be absolutely perfect to help us take a good peek at the Universe. Each of the 18 segments has an actuator connected to a motor. Commands are given to the motor to move the segment for alignment. And motors do generate heat while operating. Heat is something which Webb cannot tolerate. Since it will be observing the Universe in the infrared region, which is basically heat, it needs to be kept very very cold. The huge sun shield on the side of the telescope facing the Sun does a lot of the shielding. Any heat from the telescope’s internal instruments will not be acceptable. Hence the process is very slow.
The motors can be asked to move only very small distances (about a millimeter) and can operate for only a short period of time each day. It is supposed to be the same speed as the growth of the grass. We are watching the paint dry ! The process is to do some alignment and take pictures of a reference star and then check for clarity. And repeat the process until sufficient clarity is obtained. This iterative slow process will happen over the next three months and then we will hope to have some excellent results and the knowledge thereof. We waited so long. Another three months in the quest for perfection !