NASA’s latest mission to map the cosmos takes off

Meaning:A predecessor is whoever or whatever came before — the person who held the job before you, or the older model that a new one replaces. Hiding inside the word is precede, which means to come before. Here the Hubble Space Telescope is Roman's predecessor: Hubble did this job first, and Roman is the newer telescope taking over, with a field of view more than 100 times wider.

Meaning:Arduous means hard, tiring and long — the kind of difficulty that grinds on for weeks or years, not one tough moment. It usually describes a journey, a climb, or a stretch of work. The article calls the telescope's journey to the launch pad arduous: not the flight into space, but the years of building it and the fight in 2025 over whether it would be paid for at all.

Meaning:Unprecedented means nothing like it has happened before, so there is no earlier example to measure it against. A precedent is an earlier case that sets the pattern, and the un- strips that away. Promising astronomers an unprecedented view of the universe is a claim that no telescope before this one has shown them this much sky in this much detail.

Roman is expected to find about 100,000 planets around other stars, and it will hardly see any of them directly. It will watch the stars instead. Astronomers call this the transit method, and it works for a simple physical reason: a planet is a solid ball of rock or gas, and solid things block light.

Picture a star as a lamp very far away. A planet circling that star is dark. When its orbit carries it straight between the lamp and your telescope, the planet covers a tiny piece of the lamp's face, so slightly less light reaches you. The star's brightness dips — often by less than one percent — and returns to normal once the planet moves past. That dip is a transit.

You can build the same thing in a dark room. A friend across the room points a flashlight at you, and someone else walks a marble slowly through the beam. Your eyes will not catch a thing, but a light meter aimed at the flashlight will: full brightness, a small dip, full brightness again — and another dip each time the marble comes back around. The gap between dips tells you how long one lap takes. The depth of each dip tells you how big the marble is next to the flashlight.

So a telescope that never photographs a planet can still prove the planet is there, measure how wide it is, and time how long its year lasts — all from a star that gets slightly dimmer on a regular schedule.

An astronomer calls a star 100 light-years away nearby. That only makes sense once you know what a light-year measures — and the trap is the word year sitting inside it. A light-year is a distance, not a stretch of time.

Light travels about 300,000 kilometres every second, and nothing in the universe goes faster. A light-year is simply how far light gets in one year at that speed: roughly 9.5 trillion kilometres. Astronomers reach for the unit because the kilometre numbers turn silly — the closest star after our own sun sits about 40,000,000,000,000 kilometres away, which is much easier to say as 4.2 light-years.

Line that up against this article. The Roman telescope parks 1.6m kilometres from Earth, and light crosses that gap in about five seconds. The stars Dr Savransky wants to photograph are up to 100 light-years out, so the light landing on Roman's mirror tonight left those stars around 1926.

That is the part worth keeping. Because light needs time to cross a distance, every telescope shows you the past. Roman's deepest pictures of far-off galaxies are pictures of how those galaxies looked long before Earth existed.