NASA will reach a major milestone on Tuesday as July 4, 2017, marks 20 years of continuous Mars exploration.
Each mission has helped to pave the way for the next and has led to many scientific discoveries, including evidence that liquid water once flowed on Mars.
NASA has been sending spacecraft to the Red Planet since the 1960s, but there were many gaps between missions.
The continuous exploration began on July 4, 1997, when NASA’s Pathfinder spacecraft landed on Mars. Since this date, there has always been at least one active mission exploring the planet either from the ground or from orbit.
Pathfinder remained operational for only a little over three months, but it was crucial to the future of Martian rovers.
"Without Mars Pathfinder, there could not have been Spirit and Opportunity, and without Spirit and Opportunity, there could not have been Curiosity," Pathfinder Project Scientist Matt Golombek of NASA's Jet Propulsion Laboratory, Pasadena, California, said.
Spirit and Opportunity were the next missions to explore the surface of Mars with both arriving on the planet in January of 2004.
The twin rovers explored different parts of the planet, but both had the same goal of finding evidence that liquid water was once present on Mars.
“Both rovers have found evidence of ancient Martian environments where intermittently wet and habitable conditions existed,” NASA said.
Contact with Spirit was lost on March 22, 2010, but Opportunity is still operation and driving around the Martian surface to this day.
A small, immobile probe called the Phoenix Lander also conduced research near Mars’ north pole between May 28, 2008, and Nov. 2, 2008, before contact was lost. This mission helped NASA understand the polar regions of the planet, an area that had previously been unexplored.
The most recent rover to arrive on Mars is the Mars Science Laboratory, more commonly known as Curiosity.
Curiosity touched down in 2012 and is equipped with advanced instruments, such as a laser to vaporize and study rocks from a distance.
“Early in its mission, Curiosity's scientific tools found chemical and mineral evidence of past habitable environments on Mars,” NASA said.
While rovers explored the Martian surface to peek into the planet’s history, satellites have been orbiting from high above to take observations from space and send them back to Earth.
Mars Odyssey has been the longest operating satellite in the history of Mars exploration, arriving in 2001 and still operating to this day.
Over the years, it has helped scientists identify where certain minerals and chemicals are on Mars, as well as serving as a communications relay between Earth and the rovers on the planet below.
The Mars Reconnaissance Orbiter has also provided a wealth of data about the planet. It is equipped with the most powerful camera ever flown on a planetary exploration mission and can photograph objects as small as a dinner plate.
Information from these satellites will also be used by NASA to determine the landing spots for future rovers and eventually where humans will step foot on the planet.