A comet streaking through space with a European robot lab riding piggyback will skirt the sun this week, setting another landmark in an extraordinary quest to unravel the origins of life on earth.
Scientists hope the heat of perihelion - when the comet comes closest to the sun in its orbit - will cause the enigmatic traveller to shed more of its icy crust.
If so, it could spew out pristine particles left from the Solar System's birth 4.6 billion years ago, they believe.
And if Comet 67P/Churyumov-Gerasimenko undergoes this dramatic change, Europe's Rosetta spacecraft will be orbiting nearby, ready to pounce on any clues of how our star system came into being.
"This is the time most of the action happens," said European Space Agency (ESA) expert Mark McCaughrean of the weeks-long peak of comet activity.
The ancient celestial voyager will reach its closest point to our star - some 186 million kilometres - at about 0200 GMT on Thursday, before embarking on another 6.5-year egg-shaped orbit.
Things have been heating up for weeks, with gas and dust blasting off the comet's surface as solar heat transforms its frozen crust into a space tempest.
This is "the greatest opportunity to catch material and analyse it if you're looking for rare species of molecules," especially organic ones, McCaughrean told AFP.
"We want to look at the more pristine material that might come out" from beneath the layer of icy dust stripped from the surface.
Most exciting would be if the duck-shaped comet's "neck" - which hosts a 500-metre crack - were to break in two to reveal the raw insides.
"That's really the Holy Grail... to see the interior of the comet," said McCaughrean, though most scientists believe a break-up is unlikely this time around.
In any scenario, ground teams working on the 20-year-old Rosetta mission will likely have to wait weeks, if not months, to analyse new data.
For one thing, there has been no word from Philae, their eyes on the ground, since July 9, and its status is unknown.
Right now, 67P with its precious cargo is hurtling through space at 34.17km per second.
Rosetta has had to move farther away to avoid the confounding effects of the dust storm on its star-tracking navigation system.
The spacecraft now orbits at some 200-300km from the comet, compared with less than 10km at its closest in October last year.
"If we were right next to it, bathing in the material, they [scientists] would be super happy," said McCaughrean - but with a high risk of losing Rosetta.
"You have to do a compromise between spacecraft safety and getting as close as possible," added Philae project manager Stephan Ulamec of German space agency DLR.