CDS (I) 2024 General Knowledge · Question 9 of 120

Correct — A, It orbits the Sun, about 15 lakh km away from the Earth. Webb was launched on 25 December 2021 on an Ariane 5 from Kourou and flew outward to the second Sun–Earth Lagrange point, L2, roughly 1,500,000 km — 15 lakh km — beyond the Earth on the side facing away from the Sun. L2 is not a parking space. It is a point that itself travels around the Sun once a year, holding station with the Earth, and Webb flies a wide halo orbit about it, swinging between roughly 250,000 and 832,000 km from the point as it goes. Put together, the telescope circles the Sun in step with the Earth — the Sun is what it goes round, not the Earth. That geometry is also the reason L2 was chosen: from there the Sun, the Earth and the Moon all lie in the same direction, so a single sunshield can screen all three at once and keep the mirror and instruments cold enough for infrared work.

• (b)It orbits the Earth, about 15 lakh km away. — Right distance, wrong primary. Nothing orbits the Earth at 15 lakh km — that is roughly four times the Moon's distance and far outside the region where the Earth's pull dominates. Webb's motion is governed by the combined Sun and Earth field at L2.
• (c)It is stationary in space nearly 10 lakh km away from the Earth. — Wrong twice over. Nothing is stationary at a Lagrange point — L2 itself sweeps around the Sun with the Earth, and Webb loops around L2 on top of that. The distance is also about 15 lakh km, not 10.
• (d)It revolves around the Moon nearly 3.5 lakh km away from the Earth. — 3.5 lakh km is roughly the Earth-to-Moon distance, which is what makes this tempting. Webb went about four times further out and has nothing to do with the Moon.

A Lagrange point is a place in a two-body system where a small object can keep pace with both bodies. For the Sun and the Earth there are five; L2 lies about 1.5 million km beyond the Earth in the anti-Sun direction. A spacecraft placed there orbits the Sun with the same one-year period as the Earth despite being further out, because the Earth's gravity adds to the Sun's. In practice craft are not put exactly at L2 but into wide halo orbits around it, which are cheaper to hold and keep the spacecraft out of the Earth's shadow.

Every wrong option here is a real misconception rather than filler, so the item is a good test of whether the geometry is understood or only memorised. The distance figure of 15 lakh km appears in two options, which forces you to decide the harder question — what is Webb going round? Anything that far out is beyond the Earth's gravitational grip, so the Sun must be the primary. The word 'stationary' in option (c) is the other giveaway: an object genuinely at rest relative to the Sun would fall straight into it, and Lagrange points are stationary only in the rotating frame that turns with the Earth, not in space. As of this 2024 paper Webb had been returning science for over a year; it released its first images on 11 July 2022, entered service the next day, and is still observing from L2 today.

• Webb was launched on 25 December 2021 on an Ariane 5 rocket from Kourou, French Guiana.
• It operates in a halo orbit around the Sun–Earth L2 point, about 1,500,000 km beyond Earth's orbit around the Sun.
• Its distance from L2 itself varies between roughly 250,000 and 832,000 km as it goes round.
• It orbits the Sun in synchrony with the Earth — it is neither in Earth orbit nor stationary.
• The first Webb image was released to the public on 11 July 2022 and the observatory entered service on 12 July 2022.

• Reading a Lagrange point as a parking spot where an object can hang motionless in space.
• Attaching the 15 lakh km figure to an Earth orbit — nothing orbits the Earth at that range.
• Confusing 3.5 lakh km, the Earth-to-Moon distance, with Webb's distance, which is about four times greater.

Asked as a single-correct-statement item in which the distance figure is repeated across options so that the real decision is which body the telescope orbits.

• practice — not a real PYQ

  The James Webb Space Telescope carries out its observations from which one of the following locations?

  • (a)Sun–Earth L1 point
  • (b)Sun–Earth L2 point
  • (c)Earth–Moon L4 point
  • (d)Geostationary orbit

  Answer(b) Sun–Earth L2 point — about 15 lakh km from the Earth on the side away from the Sun, where one sunshield can screen the Sun, Earth and Moon together.

• practice — not a real PYQ

  Approximately how far from the Earth is the Sun–Earth L2 point?

  • (a)3·8 lakh km
  • (b)15 lakh km
  • (c)150 lakh km
  • (d)1500 lakh km

  Answer(b) 15 lakh km — roughly four times the Earth-to-Moon distance of about 3·8 lakh km.