"The experiment will employ a trio of spacecraft flying in formation in the shape of an equilateral triangle that has sides one million kilometres long, with lasers shining between the craft." The experiment in question refers to
Answer & explanation
Answer: (d) Evolved LISA
The description fits eLISA (Evolved Laser Interferometer Space Antenna), a proposed space observatory for gravitational waves. Three spacecraft would fly in a triangle with arms about one million kilometres long and measure tiny changes in their separation using laser links.
- ✗ (a) Voyager-2 is a single probe travelling through the outer Solar System; it does not fly in a formation of three.
- ✗ (b) New Horizons is also a single probe, sent to Pluto and beyond, not a triangle of laser-linked spacecraft.
- ✗ (c) LISA Pathfinder was a technology demonstrator: one spacecraft carrying two test masses just 38 cm apart, to prove the techniques needed for a future space gravitational-wave observatory.
- ✓ (d) eLISA was designed as three spacecraft, one 'mother' and two 'daughters', linked by laser interferometry, with arms shorter than the original LISA, at 1 million km.
- • Since then The gravitational-wave mission now planned by ESA with NASA is called LISA and uses larger arms: three spacecraft with each side 2.5 million km long (NASA).
Remember · eLISA = three spacecraft, laser-linked triangle with 1 million km arms, to detect gravitational waves. LISA Pathfinder was only the one-spacecraft test.
Sources
- ESA, About LISA Pathfinder ↗ “Housed at the core of LISA Pathfinder are two freely falling test masses separated by 38 cm. … In a full-scale gravitational-wave observatory, the test masses would be housed in two individual spacecraft separated by about a million km”
- Amaro-Seoane et al., Low-frequency gravitational-wave science with eLISA/NGO (Classical and Quantum Gravity 29, 2012; eLISA consortium paper, arXiv copy) ↗ · reference work “The eLISA arms will be shorter (1 Mkm), simplifying the tracking of distant spacecraft, alleviating requirements on lasers and optics”
- NASA, LISA mission page ↗ “The LISA mission consists of three spacecraft flying in a vast triangular formation that follows Earth as it orbits the Sun. Each arm of the triangle stretches 1.6 million miles (2.5 million kilometers)”
Question and answer: UPSC's official GS Paper I (2020, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 1 Oct 2026 (how we verify). ·