they studied the structure of planets
They were the Mariner 1 and Mariner 2 spacecrafts.
Mariner 2
Mariner 9
Mariner
space probes
Yes, there have been probes sent to Mercury. Messenger and Mariner 10.
Mariner 4 was the first ship to visit Mars.
Yes. It was one of the Mariner probes.
They have already sent two probes near Mercury: Mariner 10, and MESSENGER. The BepiColombo is a proposed joint mission between JAXA and ESA, which will launch a further two probes.
2 probes. Mariner 10 orbiter in the 1970s and the current MESSENGER spacecraft which will orbit and map the entire surface.
There have been two space probes: Mariner 10 and Messenger.
There have been many space probes to Venus. Here are a few important ones: * Mariner 10 (NASA) * The Venera series of probes (Roscosmos) * Magellan (NASA) * Venus Express (ESA) (check related link)