The US space agency (Nasa) will make a second attempt on Tuesday to put a high-resolution carbon dioxide observatory in orbit.
The first satellite was destroyed on launch in 2009.
Since then, scientists and engineers have built a near identical spacecraft, which will launch from the Vandenberg Air Force Base in California.
'It's been a long walk back to where we are now,' David Crisp, the mission's Science Team Leader, told BBC News.
'We've delivered the spacecraft, but we've still got a number of challenging steps ahead of us before this system is on orbit, operating and returning science data.'
The Orbiting Carbon Observatory-2 (OCO-2) will ride to orbit on a Delta II rocket
Lift-off is timed for a 30-second window at 02:56 local time (09:56 GMT; 10:56 BST).
The $468m ($275m) OCO-2 mission is going to trace the global geographic distribution of carbon dioxide in the atmosphere - to try to identify precisely where it is emitted and absorbed.
Humans are adding nearly 40 billion tonnes of carbon dioxide to the atmosphere every year, principally from the burning of fossil fuels.
Only about half of this sum stays in the atmosphere, where it drives a warmer climate.
About half of the other half is absorbed into the ocean, with the remainder pulled down into land 'sinks'.
Exactly where, though, is highly uncertain.
It will likely include underappreciated areas of forest and grassland, but getting to the answers is complicated by the variability in the performance of sinks from year to year.
'Understanding what controls that variability is really crucial,' said Dr Mike Gunson, the OCO-2 project scientist at Nasa's Jet Propulsion Laboratory.
'If we can do that today, it might inform us about what might happen in the future.
'Will those processes continue? Or will we see an abatement in their ability to absorb carbon dioxide, and does that increase the amount of CO2 in the atmosphere, obviously having climate change impacts.'
The observatory carries a single instrument - a spectrometer that breaks the sunlight reflected off the Earth's surface into its constituent colours, and then analyses the spectrum to determine how much carbon dioxide and molecular oxygen is present.
Combining the data on these two gases can be used to work out atmospheric concentrations.
Current CO2 concentrations in the atmosphere stand at about 400 parts per million.
OCO-2's precision should enable it to detect changes of one or two carbon dioxide molecules out of the 400.
However, to locate the sources and sinks, scientists will need to combine this information with models that estimate how CO2 is being moved and mixed through the air.
The mission follows on the heels of the Japanese Gosat (Greenhouse gases Observing SATellite) venture, which has been doing a similar job since 2009, although at a lower resolution than OCO-2 will manage.
The Nasa scientists say they have learnt a huge amount from the Gosat experience, and expect the US satellite's science return to be hugely boosted as a result.
Dr Crisp commented: 'Our science team has been working very closely with the Japanese, and this has provided a critical series of opportunities to develop and then validate the algorithms we will use to analyse the data.
'We are now so much further ahead of where we would have been had we launched successfully in 2009. In fact, we now think that within a few months of [Tuesday's] launch, we'll be producing a product that will be far better than anything we could have produced in the nominal mission of [the original satellite].'
One technique proved by Gosat that will be pursued by OCO-2 is the ability of a space-borne spectrometer to detect the glow induced in chlorophyll by sunlight.
Being able to sense a fluorescence in plants' key photosynthetic molecule makes it possible to assess the health of vegetation.
Over time, researchers should be able to tell, for example, if some forests are becoming stressed and therefore less efficient at absorbing CO2.
Europe has carbon missions of its own coming at the end of the decade.
The French space agency (Cnes) is developing a concept called MicroCarb, which, like Gosat and OCO-2 before it, will measure carbon dioxide concentrations.
Cnes is also working on a concept with the German space agency (DLR) called Merlin. This satellite would study the distribution of methane in the atmosphere.
And the European Space Agency (Esa) has approved a project called Biomass. It will employ an orbiting radar system to 'weigh' the amount carbon stored in the world's forests.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos