As a NASA spacecraft speeds toward Thursday's rendezvous with Comet Hartley 2, the icy wanderer is putting on a show, blasting out gassy jets and coughing up loads of poisonous cyanide.
The Deep Impact probe will cruise to within 435 miles (700 kilometers) of Hartley 2, marking just the fifth time that a comet has ever been observed up close. And the view should be interesting.
Last week, the spacecraft spotted two massive jets spewing from the comet's nucleus — its core of ice and rock. Comet Hartley 2 also spat out huge quantities of cyanide over a weeklong stretch in September, raising concentrations of the poisonous gas by a factor of five, researchers said.
"We're about to be surprised," said Mike A'Hearn of the University of Maryland, principal investigator of Deep Impact's mission, which NASA calls EPOXI. "This comet is unlike any we've visited before, and we don't know what we're going to find."
The encounter: How it should happen
Comet Hartley 2 is a small, active comet that orbits the sun once every six and a half years. It was discovered in 1986 by Australian astronomer Malcolm Hartley. [Q&A with Comet Hartley 2 Discoverer]
The Deep Impact spacecraft has been chasing Hartley 2 for months, taking pictures and maneuvering to prepare for the close flyby, which is scheduled to take place at 10:01 a.m. ET Wednesday.
Here's a brief rundown of how the flyby should proceed, if everything goes according to plan:
About 18 hours before its closest approach, Deep Impact will begin the encounter phase of its mission. The probe will reorient itself so its three imagers — two in the visible-light spectrum and one in the infrared — can lock on Comet Hartley 2 for the next 24 hours-plus.
This maneuver will point the probe's high-gain antenna away from Earth, meaning Deep Impact won't be able to beam pictures home for a bit, researchers said. Instead, it will store the photos onboard in its two computers.
This lack of multitasking ability reflects the fact that Deep Impact is a recycled, repurposed spacecraft. It was originally designed to serve as a mother ship for NASA's Deep Impact mission, which intentionally fired a probe into Comet Tempel 1 in 2005 to study the object's composition.
Soon after Deep Impact's closest approach with Hartley 2, mission scientists will reorient the probe so it can both track the now-receding comet and start beaming images to Earth.
The spacecraft will continue to take new photos while it sends its cache of comet close-ups home. The first few photos should reach researchers' computers about an hour or so after the rendezvous, scientists said. The complete data dump, however, will take a while.
"We will be waiting," A'Hearn said. "The best images won't reach Earth until many hours after the actual encounter." [The Best Comet Photos of All Time.]
Researchers should get some good looks at Comet Hartley 2. One of Deep Impact's cameras is so powerful that it can distinguish the size difference between a car and a pickup truck from 400 miles (645 kilometers) away, mission scientists say.
Data from the close approach will continue to download through Saturday, but NASA will release preliminary results sooner than that. A news conference is scheduled for 4 p.m. ET Thursday, just six hours after Deep Impact's closest approach to Hartley 2, agency officials said.
Post-encounter: Watching Hartley 2 speed away
Though Deep Impact will zoom by Comet Hartley 2 incredibly quickly — at about 27,000 mph (43,548 kilometers per hour) — the spacecraft will be able to track the comet's tail end with just a slow swivel. The probe will turn about 1 degree per second, according to mission scientists — one-sixth the rate at which a second hand makes its way around a clock face.
This anticipated ease of tracking is one reason mission planners chose to keep its distance from the comet, rather than zip in for a closer, more detailed look.
Another reason: to keep Deep Impact away from the densest parts of Hartley 2's coma, the gassy, dusty cloud around its nucleus. Particles in the coma could damage Deep Impact's antenna if the probe gets too close.
"We've chosen 700 kilometers because we're balancing several desires and several threats here," explained EPOXI project manager Tim Larson, of NASA's Jet Propulsion Laboratory.
Deep Impact will continue photographing Hartley 2 for about three weeks as the comet speeds off into the dark reaches of space, Larson added. After that point, the spacecraft's comet-watching mission will be basically over.
What Comet Hartley 2 can reveal
Researchers hope Deep Impact's flyby past Hartley 2 can give them a good idea of the composition of the comet's icy nucleus. They are also eager to compare Hartley 2 to the four other comets that spacecraft have visited in the past.
Hartley 2, while just under a mile (1.5 kilometers) across, is incredibly active. A close study of its spewing jets and gas clouds could yield clues about comet composition and behavior in general, researchers said.
"We saw outbursts in Comet Tempel 1, as well as many other things," A'hearn said. "We're trying to find out, are all of the new phenomena we saw at Tempel 1 universal across all comets, or are they special to Comet Tempel 1?"
Comets are leftovers from the solar system's formation 4.5 billion years ago, so studying them should give astronomers a better handle on how our cosmic neighborhood came to be, researchers said.
The 2005 kamikaze crash into Tempel 1 exposed what scientists think are primordial layers of matter dating back 4.5 billion years. EPOXI scientists hope an up-close look at Hartley 2 will reveal similar ancient material, perhaps providing key insights about our solar system's youth.
"That's the ultimate goal at the encounter," A'hearn said.