A NASA instrument designed to track carbon in Earth's atmosphere is headed to the International Space Station next week, and the US president isn't happy about it.

Donald Trump slashed funding for the Orbiting Carbon Observatory 3 and four other Earth science missions in his proposed spending plan for the 2018 fiscal year, citing "budget constraints" and "higher priorities within Science". His budget for fiscal year 2019 tried to defund them again.

In both cases, Congress decided to keep the OCO-3 mission going anyway. Now it is set to launch as soon as Tuesday.

OCO-3 was built at the Jet Propulsion Laboratory in La Canada Flintridge, Calif., for less than 100 million dollars, using parts left over from its predecessor, OCO-2. Once the carbon observatory gets to the ISS, a robotic arm will mount it on the underside of the space station so it can keep a close eye on the carbon dioxide in Earth's atmosphere.

That will help scientists answer questions about how and why levels of the greenhouse gas fluctuate over days, months and years.

"Our goal is to get really good data so we can make informed decisions about how to manage carbon and carbon emissions in the future," said Annmarie Eldering, the mission's project scientist at JPL.

Carbon dioxide makes up a tiny fraction of the molecules in our atmosphere - roughly 400 parts per million. But seemingly small changes in the amount of carbon dioxide in the atmosphere have an outsized effect on the planet's temperature.

"Carbon is really effective at trapping heat," Eldering said. "Even changing the ratio from 300 parts per million to 400 parts per million makes a big difference."

OCO-3 is so sensitive that it can detect changes as small as 1 part per million. So if CO2 levels go from 406 ppm one day to 407 ppm the next, the observatory will record the increase.

Eldering, who also worked on OCO-2, spoke to the Los Angeles Times about the difference between the two instruments, the new information she hopes to learn from OCO-3, and how she and her team managed to keep their cool when their project seemed headed for the chopping block.

She said the main science question she hopes OCO-3 will answer is about the movement of carbon dioxide between plants and the atmosphere.