How to keep LISA's laser on target five million km away

[mickw]

A key technical challenge of the joint ESA/NASA LISA mission has been solved: how to maintain precise pointing of a laser beam across five million km of space.
The next-decade Laser Interferometer Space Antenna (LISA) mission will look for ripples in spacetime -- their existence predicted by Albert Einstein -- known as gravitational waves. A trio of identical spacecraft will fly five million km apart in an equilateral triangle formation, linked by laser beams.
A precision-measuring method called interferometry can combine these laser beams to identify the slightest movement between free-floating metallic cubes within each spacecraft. Motion within a set frequency range will be scrutinised to search out gravitational waves emitted by massive black holes and similarly energetic cosmic objects

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