The European spacecraft Rosetta will fly past the Earth on Friday as it builds up the speed needed to chase down and orbit a comet in 2014.
The probe will get as close as 1,900km (1,200 miles) to the planet, as it accelerates under the pull of gravity on a slingshot manoeuvre out to Mars.
It is the first of three Earth flybys Rosetta will make before travelling through the asteroid belt to Jupiter.
There, it will rendezvous with the Comet 67P/Churyumov-Gerasimenko.
"This last year, Rosetta has really been in a parking orbit and when it comes past Earth, it will get a kick," explained one of the mission scientists, Dr Chris Carr, from Imperial College London, UK.
"It will get an even bigger kick at Mars, where it flies by at just 200km," he told the BBC News website.
European skywatchers with good amateur telescopes stand a chance of seeing Rosetta, if weather conditions are favourable.
It will appear to come out of an area in the sky between the constellations Leo and Sextans.
It will be quite faint, however, as it moves west down to the horizon. The closest approach to Earth, over Mexico, is at 2210 GMT.
The European Space Agency has detailed observation information on its website (see internet links).
It is running a competition for amateur astronomers to take images of the probe. It should be possible to pick out Rosetta's solar panels, which extend over 32m (100ft); the high-gain antenna may also be distinguishable.
The £600m spacecraft will cover some seven billion km (just over four billion miles) on its journey to Churyumov-Gerasimenko.
Once in orbit around the 4km-wide "mountain" of ice and dust, the craft will despatch a small lander called Philae to the surface to study its chemistry.
'Tick the box'
Comets - giant "dirty snowballs", as some have called them - are believed to contain materials that have remained largely unchanged since the formation of the Solar System 4.6bn years ago, and Rosetta data should help researchers understand better how our local space environment has evolved over that time.
Friday's Earth flyby will enable mission engineers and scientists to calibrate some of Rosetta's instruments.
Dr Carr's group, the Rosetta Plasma Consortium, has five sensors on the probe that are designed to study ionised gas particles around the comet. Their data will reveal, amongst other information, the nature of any magnetic fields around Churyumov-Gerasimenko and whether its surface is electrically conducting.
Rosetta will put a lander on the surface of the comet
"Flying past the Earth, we will get a dense plasma, so we'll be able to tick the box that says, 'yes, our sensors really do measure the density of plasmas'," said Dr Carr.
"At the moment we don't know, so it's really important for us to see that our instruments are working well."
Other calibration work will see a navigation camera target the Moon.
This will ensure the spacecraft is set up correctly when it encounters two asteroids, Steins in September 2008 and Lutetia in July 2010.