Two space probes will on Monday end a lunar mission by crashing into a mountain near the moon's north pole. The impacts conclude a few days of jubilee celebrations, marking 50 years since humankind began to explore the solar system. On 14 December 1962, a Nasa spacecraft called Mariner 2 sped past Venus with radar instruments that confirmed that the cool cloud cover of Venus concealed a surface hot enough to melt lead, and a planet caught up in a runaway greenhouse effect. It was the first ever visit to another planet.

Since then, 12 men have walked on the moon, and robot spacecraft have colonised Mars. There have been sustained US, European and Russian missions to Jupiter and its moons, to Saturn, Titan, Venus and Mercury. Spacecraft have met asteroids, smashed into comets, and orbited the sun. Two missions have passed past the distant outer planets and one of them – Voyager 2 – is right now crossing a strange boundary that defines the beginning of interstellar space.

The adventure has been marked by spectacular failure and astonishing success. The down-to-earth reward – simply in terms of the required advances in data transmission, detection, robotics, computing, thermal control, materials design and lightweight, long-haul energy sources – has been incalculable. The intellectual discovery has been literally out of this world. Sustained and repeated missions have examined the mysteries of Saturn's rings, Jupiter's red spot, atmospheric pressures on Venus, the inhospitable worlds of the outer planets, and the mysterious geology of Mars.

Each separate adventure has added in some way to a new and more profound understanding of this Goldilocks planet of ours, in which everything is just right for complex and intelligent life. The famous Earthrise photographed by Apollo 8 more than 40 years ago, and then the astonishing picture of Earth taken by Voyager 1 in 1990 – that pale blue dot at a distance of 4bn miles – highlighted the fragility of the only known home for life in a vast and mysterious universe. Each image helped drive a new and informed concern for Earth's future.

But the jubilee also signals a loss of momentum. Nasa has no replacement for the space shuttle, and won't be joining Europe on missions to Mars in 2016 and 2018. The European Space Agency must wait for a decision on a new launch rocket, and has shelved a plan to touch down on the moon's south pole in 2018. But any journey into space takes years to plan, and decades to execute. The danger is that at the next anniversary, there will be conspicuously less to celebrate. The Grail mission ends today with a bang rather than a whimper. But Mariner 2, cold and silent for five decades, still orbits the sun: a reminder of bygone marvels, and maybe also a reproach to the future.