Breaking up a comet

South Valley native Anne Elson will have a front-row seat in the
control room
It’s not every day that you get to play chicken with a comet, but that’s exactly what Gilroy native Anne Elson will get to do as part of a team from NASA’s Pasadena-based Jet Propulsion Laboratory in a few days.

Elson – a 1965 graduate of Gilroy High School – is part of a team working on mission Deep Impact, a $300 million co-venture between JPL, the University of Maryland and Ball Aerospace – yes, owned by the same company that makes jam jars – that will be studying the make-up of comets.

The scientists are currently tracking a two-part spacecraft that will separate late on the evening of July 2. One piece, about the size of a washing machine and made almost entirely of copper, will ram the side of a comet the size of Manhattan, while the other takes pictures in high resolution, medium resolution and infrared formats. Elson will narrate the action live from the NASA control room on the night of July 3, she said.

Among the most primitive elements in the solar system, comets are thought to contain matter that may have been present at its very beginnings. Unfortunately, scientists know very little of their actual makeup. NASA scientists believe they are made up of ice, dust and gas, but have conducted little up-close study. Several unmanned spacecraft have taken photographs of them, which were later beamed back to Earth, and one even entered the tail of Haley’s comet to get a snapshot. Deep Impact will reveal, for the first time, what the innards of a comet look like, according to the JPL mission Web site.

“We don’t know if the comet is light and frothy or if it’s solid like ice,” said Elson, who worked as a leader on software and spacecraft development teams during the completion of the project. “Depending on the composition of the comet, it might dig a hole in the comet about the size of the Rose Bowl stadium – about seven stories deep – or, if it’s really light and frothy, it might just disappear.”

The comet in question is a body named Tempel 1, named for discoverer Wilhelm Tempel in 1867. It orbits the sun every five and a half years and, at the time of impact, will be 0.9 astronomical units from Earth or roughly 83.66 million miles. By comparison, our planet is about one astronomical unit – equivalent to 92.96 million miles – away from the sun.

Earth has been steadily moving away from the comet since the Deep Impact spacecraft was launched in January, situating us a comfortable distance from the main event. But all that extra space also breeds delay.

And delay – an average of 1.7 minutes now, according to Elson – means that scientists back on Earth cannot control the impact or fly-by spacecrafts in their final stages.

Instead, Elson’s team created trajectory software that gauges the impactor’s position, maps key points on the asteroid and tells the impactor when and how to hit the hurtling giant, repeating the trajectory analysis at three key points in the last moments before impact. It will have to strike the comet in a well-lit area observable from the fly-by unit.

If all goes according to plan, the impactor will fly straight for a well-exposed spot, relaying pictures back to Earth until it crashes into the mass.

The fly-by unit will also gather information, snapping images of the debris that comes out of the comet and showing scientists the size of the hole they will have created. If things go wrong, the impactor could hit an obscured spot on the comet or miss it altogether.

Years of planning and waiting have gone into this mission, which began in late 1999. Elson, who began her work on the project in summer 2000, said the mission has a projected success rate “in the high 90s,” but she is currently involved in developing contingency plans.

“Basically, one of our big questions is, what if separation doesn’t work?” asked Elson. “We put together a whole other timeline, and we’ll be ready if that’s the case. If the spacecrafts don’t come apart, we’re going to fly both of them into the comet and get what we can.”

Another team is working on the second problem engineers fear they may encounter.

“The other thing we’re worried about is called live impactor late separation,” said Elson. “That’s where you know your impactor is healthy, you know there’s separation, but let’s say that something went wrong going into the separation mission. The originally planned separation was for 24 hours out, but this one we’re redoing the whole timeline for so we could do it from 10 hours out instead.”

They have to have the contingency plan in place to make sure the fly-by unit reaches a safe distance from the impact.

One thing that Elson and others working on the project can’t predict is what the impactor will encounter on its way toward the comet. No one is sure of just how much debris surrounds a comet, and a large-enough piece could knock the impactor off target without giving it time to recover.

The fly-by unit is also in jeopardy during the 800 or so seconds that it’s expected to take pictures, because it will have to fly into a field of comet debris, according to a message posted on the JPL Web site by Alan Delamere, a staff consultant and engineer at Ball Aerospace and Technologies Corp. in Boulder, Colo.

Whatever happens, those with cable access to NASA-TV will get the chance to see it live.

Elson and the rest of the crew at NASA will be on live from 8:30pm to midnight on June 3, with an anticipated impact at 10:52pm.

For more information on Deep Impact, visit DeepImpact.

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