If all goes as planned, NASA's EPOXI mission will send the probe flying through the dense cloud of dust and gas erupting from comet 103P/Hartley 2 at 10 a.m. ET Thursday.
Deep Impact is the probe that previously sent an impactor crashing into comet Temple 1 in 2005. EPOXI is a combination of the names for the two components of Deep Impact's extended mission: Extrasolar Planet Observations and Characterization (EPOCh) and Deep Impact eXtended Investigation (DIXI).
Swinging by the icy visitor at 27,000 miles (43,500 kilometers) an hour on Thursday, the probe will pass within 435 miles (700 kilometers) of Hartley 2's nucleus, taking some of the most detailed pictures yet of this unusually active comet.
The probe is equipped with a high-resolution imager, which can see in infrared wavelengths, as well as a medium-resolution imager, which acts as a backup and can see in visible light.
The spacecraft has been snapping scores of new pictures as it's gotten closer to its target, and what the probe has seen so far is already surprising mission team members. (See pictures of comet Hartley 2.)
Last week, for example, while the spacecraft was still more than five million miles from the comet, cameras began capturing evidence of multiple jets of gas shooting out into space like a sprinkler system.
"We are clearly able to see already that there are two massive jets rotating and turning on and off, and can see variations in compositions as these jets move around," said EPOXI team scientist and University of Maryland professor Jessica Sunshine.
"It's all quite dramatic, and it will only get more so in the next couple of days."
Comet Has Jupiter-Size Green Halo
Comet Hartley 2 was discovered in 1986, and scientists calculate that it orbits the sun about every 6.5 years. This year the comet made its closest pass by Earth since its discovery, swooping in to a distance of a mere 11 million miles (18 million kilometers) on October 20.
Hartley 2 was chosen as a target for the EPOXI flyby because, among the known small comets, this one is by far the most active. (See comet pictures.)
"Strangely, we are seeing more activity than should be producible even if 100 percent of its surface was active," Sunshine said.
One of the most unusual observations made so far took place over ten days in September, when the comet showed a significant increase in the release of cyanide gas, which is common in comet cores.
Both professional and amateur telescopes picked up these gaseous outflows, visible as a giant green halo the size of Jupiter enveloping the comet's 1.2-mile-wide (2-kilometer-wide) nucleus. (Related: "'Biggest' Comet Measured.")
"What caught us off guard was the slow and steady release of [cyanide] over such a long period of time. We have never seen a comet before do that at all," Sunshine said.
Hartley 2 Flyby Carries Risks
Astronomers have struggled to understand how comets work for years, Sunshine said, because the icy bodies are highly unpredictable and variable. This mission should help scientists better understand what comets are made of, how they work, and where they come from.
"A large part of what we are focusing on during this encounter is if we can we tell how much the comet has changed since it formed and [how it has] evolved by being exposed to the sun," Sunshine said.
After the flyby, dwindling fuel supplies will likely bring Deep Impact's comet-chasing career to an end. NASA next plans to use the craft as an astronomical observatory to study extrasolar planets—provided this latest close brush with a comet doesn't put the probe in harm's way.
Because Hartley 2 is so active, scientists expect the nucleus to be surrounded by lots of dust, which can damage camera lenses and clog up instruments, Sunshine said.
Also, unlike during the probe's encounter with comet Temple 1, EPOXI team members plan to keep the cameras focused on the comet for the full mission, possibly putting the imagers at risk of being blinded.
Then there's the slight chance that the comet’s luminous jets may fool the probe's onboard autonavigation system. The probe maneuvers itself into position by tracking bright objects, so the jets could cause the craft to veer off course.
Overall, however, Sunshine believes the risks are minor and are well worth the possible rewards. (Related: "NASA to Visit Asteroid Predicted to Hit Earth?")
"Every day there is something new we learn we didn't know the day before, because we are so much closer," Sunshine said.
"We are about to see [comet Hartley 2] transition from just a dot on the screen to a true geological world, and it's going to happen very quickly, and it promises to be really exciting."