NASA's MAVEN spacecraft is set to enter the Mars orbit Sunday night, two days before India's first interplanetary spacecraft Mangalyaan is due to slip into the martian orbit Tuesday.
MAVEN - the MARS Atmosphere and Volatile Evolution mission - that has completed a 10-month journey spanning 442 million miles or 704 million km, will study Mars' upper atmosphere from orbit, NASA reported.
"I am all on pins and needles. This is a critical event," Jim Green, NASA's director of planetary science, said in a statement.
The mission's combination of detailed measurements at specific points in Mars' atmosphere and imaging provides a powerful tool for understanding the properties of the Red Planet's upper atmosphere.
The orbit insertion manoeuvre will begin with the brief firing of six small thruster engines to steady the spacecraft.
The engines will ignite and burn for 33 minutes to slow the craft, allowing it to be pulled into Mars' elliptical orbit within a period of 35 hours.
Following orbit insertion, MAVEN will begin a six-week commissioning phase that includes manoeuvreing the spacecraft into its final orbit and testing its instruments and science mapping commands.
Thereafter, MAVEN will begin its one-Earth-year primary mission to take measurements of the composition, structure and escape of gases in Mars' upper atmosphere and its interaction with the sun and solar wind.
"The MAVEN science mission focuses on answering questions about where did the water that was present on early Mars go, about where did the carbon dioxide go," said Bruce Jakosky, MAVEN principal investigator from the University of Colorado' Boulder Laboratory for Atmospheric and Space Physics.
These are important questions for understanding the history of Mars, its climate, and its potential to support at least microbial life.
MAVEN was launched Nov 18, 2013, from Cape Canaveral, Florida, carrying three instrument packages.