It will likely lead to a large X-ray telescope being launched in 2028, and to an orbiting observatory to detect gravitational waves going up in 2034. Together, these two ventures will cost in excess of 2bn euros (£1.7bn).
They join a mission already approved known as Juice, which will see a big satellite sent to observe Jupiter and its icy moons in 2022.
The path ahead was set by the Science Policy Committee (SPC) of the European Space Agency (Esa), which is meeting in Paris, France.
The committee’s decision should now give clear direction and certainty to Europe’s research and industrial base.
"These big missions take a long time to put together – of the order of 20 years," said Dr Fabio Favata, head of Esa’s Science Planning and Community Coordination Office.
"Of course, when you fix things you trade flexibility for stability, but this gives the community the opportunity to plan. They now understand what will be the ‘pillars’, what will be the ‘cornerstones’," he told BBC News.
The SPC gathering was asked to approve a set of scientific "themes" that will guide the selection of Esa’s next Large Class mission opportunities. The agency tries to launch one of these flagship endeavours every six years.
The themes are titled the "hot and energetic Universe", and the "gravitational Universe".