Most schools have at least one famous ex-pupil – an author or a painter perhaps.

But few can boast a CEO of a leading satellite tech company in Silicon Valley who helped NASA to find water on the moon.

Scientist-turned-entrepreneur Will Marshall is a former pupil of Tunbridge Wells Grammar School for Boys (TWGSB) and is now the CEO of Planet Labs.

The Californian-based company provides daily satellite data that helps businesses, governments and researchers better understand the physical world and take action over climate change.

Interested in science and astronomy from the age of seven, Mr Marshall was further inspired by his physics teacher at the school in St John's Road who handed him a copy of Stephen Hawking’s A Brief History of Time.

For his craft, design and technology project, Mr Marshall built an astronomical telescope that was so impressive that the school arranged for him to visit and show it to TV astronomer Sir Patrick Moore.

After school, he read physics and space science technology at Leicester University, before doing a PhD in theoretical physics at Oxford.

He then moved to America and did a postdoctorate at Harvard University, before joining NASA, the US civil space program.

Mr Marshall described the most pivotal moment of his professional life was the 2009 LCROSS project he did with a team at NASA, which looked for and found water on the moon, after 73 previous missions had failed.

The discovery led to him being on the front cover of Science Magazine and prompted speculation that human settlement on the moon might one day be possible.

But while he was at NASA, he realised there was a problem with the huge expense and size of satellites being built by the space agency, which prompted him to want to change the way things were done.

Working initially with friends out of a garage in California, he set up a company - Planet - to manufacture inexpensive and more compact satellites, which could be made in large quantities, that he called CubeSats.

The first one was launched in April 2013, and since then Planet has put more than 500 satellites into low orbit.

Last week, Mr Marshall took time off from his busy work schedule to speak to today's students at TWGSB by zoom link.

He explained how today his satellites continually photograph the 57.3 million square miles of Earth’s land mass, although each is no bigger than 10 by 10 by 30 centimetres.

They circle the globe daily, sending back high-resolution images which are used to improve climate monitoring, agricultural crop yield prediction, urban planning and disaster response.

Mr Marshall said: "If you look at all the big problems facing the world, the challenges are basically around things like solving poverty, ending hunger, giving everyone access to clean water, and stopping climate change and deforestation.

"With Earth imaging – and particularly this idea of having many more satellites up there to image the entire Earth every day – we thought we could demonstrably help tackle those global challenges."

Mr Marshall explained that his interest was never just in pure science but in finding ways to help humanity.

He advised today's students: "Don’t do something for money. Your career choices really matter.

"People say 'I’m just going to do this for a year or two… work in a bank or something. I'll make enough money and then I’ll go and do X.

"But then 20 years later they find they are still doing banking. They got stuck in it and never even wanted to do banking!"

Mr Marshall said: "Those first choices really matter – vote with your feet from the get-go, otherwise you get stuck in the wrong career that you didn’t want to have.

"There are lots of noble professions to help the world – teaching is one of them, science might be another."

He said: "Apply yourself to something that is meaningful and impacts the world.

"Don’t do it for the money."

The school's marketing manager Lucy Tipler said: "It was a very exciting for our students to hear directly from such a brilliant space scientist!"