As the smallest planet in the Solar System crossed the face of the Sun on Monday 9 May, one of ESA’s smallest satellites was watching. Proba-2, smaller than a cubic metre, monitors the Sun from Earth orbit with an extreme-ultraviolet telescope. It was able to spot Mercury’s transit of the Sun as a small black disc roughly four pixels in diameter.The Mercury transit was visible from Earth starting at 11:13 GMT and ending at 18:42 GMT. The total transit time was about 7 hours and 31 minutes.Solar transits – where a celestial body is seen to pass across the solar disc from the perspective of Earth – are relatively rare events. Mercury undergoes around 13 transits a century, and Venus two transits every 120 years.