Formula One burst back to life yesterday with Michael Schumacher presenting his new Mercedes and Ferrari's Felipe Massa back on track six months after suffering near-fatal head injuries.
Completing a hat-trick of comebacks, veteran Spaniard Pedro de la Rosa made his first appearance in three years as a fully-fledged racing driver when he tested the new Sauber.
Schumacher, coming out of retirement at the age of 41 and three years after his last race with Ferrari, had to watch as team mate Nico Rosberg was given the honour of starting the first test since the 2009 season ended in November.
The seven times world champion tried out the new W01 'Silver Arrow', an evolution of last year's championship-winning Brawn with a fuel cell doubled in size to satisfy the ban on refuelling, in the afternoon.
Massa cemented his return with his first competitive session since he crashed at the Hungarian Grand Prix in July last year after being hit on the head by bouncing debris from compatriot Rubens Barrichello's car.
Barrichello was also out on track in the new Williams, making a low-key debut with the team deciding against a formal launch.
Britain's Gary Paffett gave the sleek-looking new McLaren its test debut, before world champion Jenson Button and Lewis Hamilton get their hands on it later in the week.
Poland's Robert Kubica made his Renault debut, after moving from BMW-Sauber while Switzerland's Sebastien Buemi gave the new Toro Rosso first run.
The STR5 is the first car Red Bull-owned Toro Rosso have built and designed separately from the Red Bull Racing team, in line with new regulations banning "customer cars".
Principal Franz Tost hailed "a landmark year" for the Ferrari-powered team. "After four years of working in collaboration with Red Bull Technology, the STR5 is the first car that 100 percent down to our own endeavours," he said.
While others shrink, Toro Rosso have taken on an extra 80 staff and expanded their facilities as well as commissioning a wind tunnel in Britain that was bought from Red Bull.
"Making predictions... is a dangerous trap but if pushed, I would say we must aim to finish in the top eight in the constructors' championship," said Tost.
None of the four completely new teams are attending the Valencia test, with Virgin Racing due to unveil their new car online on Wednesday and Lotus firing up their car's engine for the first time next Friday.
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