Two Mars probes, a United States spacecraft and one from India, are approaching the Red Planet and being readied for going into orbit around it later this month, scientists say.

The first to take up a circular position above Mars will be NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, scheduled to be in orbit by Sept. 21.

Following MAVEN by just a few days, the Indian Mars Orbiter Mission, or MOM, will arrive in its designated orbit on Sept. 24.

Both spacecraft were launched last year.

MAVEN has traveled more than 440 million miles on its journey between Earth and Mars.

Aboard are instruments designed to look for clues to what became of the planet's atmosphere and water.

"We're the first mission devoted to observing the upper atmosphere of Mars and how it interacts with the sun and the solar wind," says Bruce Jakosky, principal investigator for MAVEN at the University of Colorado, Boulder.

"We'll get a new perspective on the planet and the history of the Martian climate, liquid water and planetary habitability by microbes."

Several previous missions, including NASA rovers on the planet's surface, have found widespread evidence that the planet once had water flowing over its surface.

That water is thought to have disappeared around 3.5 million years ago, and Maven will try to find evidence of how the atmosphere may have been involved in that disappearance, NASA scientists say.

Meanwhile MOM, India's first interplanetary spacecraft mission, is also being prepared for its arrival.

The spacecraft was designed and built by the Indian Space Organization.

A challenge for the MOM spacecraft will be the firing of its liquid-fueled propulsion system for orbital insertion; the system has been sitting idle in the vacuum of space for almost 300 days.

"MOM and its payloads are in good health," ISRO reported in a mission update on Facebook Aug. 30.

MOM will also study the atmosphere of Mars, using a methane gas sniffer. On Earth, methane can have both geological and biological origins, so detecting it on Mars could suggest the presence of Martian microbes, Indian scientists say.

MOM will also look at Mars's minerology, morphology and surface features in a mission expected to last from 6 to 10 months in orbit.

If MOM is successful at entering orbit around Mars, India will be the fourth international entity credited with accomplishing that task.

The United States, Russia and the European Space Agency have all put spacecraft at or on the Red Planet.