Deep Impact - the US space agency (Nasa) mission to crash a projectile into a comet - is returning blurry images from one of its instruments.
Deep Impact will punch a crater in Comet Tempel 1
But engineers believe they can probably fix the glitch and that the spacecraft itself is in good basic health.
Nasa said test images taken with the High Resolution Instrument (HRI) showed it "has not reached perfect focus".
The probe will eject a 372kg impactor into the path of Comet Tempel 1 to blast a deep hole in its nucleus.
The event will be recorded by cameras and other instruments on the flyby spacecraft, or "mothership".
Soon after its launch on 12 January 2005, Deep Impact entered a commissioning phase, during which team members verified the health of the spacecraft's subsystems and instruments.
During this phase, engineers carried out heating of the HRI to remove normal residual moisture from its barrel.
The moisture was absorbed into the structure of the instrument during the spacecraft's last hours on the launch pad and its transit through the atmosphere to space.
At completion of this "bake-out" procedure, test images were taken which exposed the problem.
The HRI is the main scientific instrument on the mothership. It is comprised of a telescope with a 30cm (11.8 inch) aperture, an infrared spectrometer, and a multi-spectral CCD camera
A special team has been formed with the aim of bringing the instrument into full focus.
"It appears our infrared spectrometer is performing spectacularly, and even if the spatial resolution of the High Resolution Instrument remains at present levels, we still expect to obtain the best, most detailed pictures of a comet ever taken," said Deep Impact principal investigator, Michael A'Hearn, of the University of Maryland in College Park, US.
Change of focus
At the beginning of July, Deep Impact begins making final targeting manoeuvres before releasing the impactor towards its quarry.
The mothership will then capture images and gather data from a distance of 500km (300 miles) as the one-by-one-metre copper-fortified impactor shatters the surface of Tempel 1.
The projectile will collide with Tempel 1 on 4 July - 24 hours after its release - travelling at a relative speed of 37,000km/h (23,000 mph).
It could punch a crater in the comet big enough to swallow Rome's Coliseum.
Comets are the "undercooked leftovers" that remained when a sprawling cloud of dust and gas condensed to form the Sun and planets 4.6 billion years ago.
Mission scientists hope the mission will answer basic questions about how the Solar System came to be, by offering a better look at the nature and composition of these frozen balls of ice and rock.
It has been suggested that comets first delivered the basic building blocks of life - carbon-based molecules and water - to Earth.