PI Perspective - Michael A'Hearn
Until a few years ago, Michael A’Hearn had viewed Ball Aerospace as a specialist in scientific instruments and as an exceptionally visible presence at science meetings. He had been impressed with Ball’s COSTAR optics to repair the Hubble Space Telescope in 1993, the astronomer and University of Maryland distinguished professor recalled, to the point that he had been “amazed that it worked.”
Of course, A’Hearn said, “They’d never built an interplanetary spacecraft before, so they didn’t have any reputation there.”
It turned out A’Hearn would lead the mission cementing Ball Aerospace’s reputation as a deep-space player: Deep Impact, NASA’s $330-million Discovery Program mission to see what was inside the comet Tempel 1. Ball built the dual spacecraft – a “flyby” mother ship and “impactor” bullet – as well as Deep Impact’s visible-spectrum and infrared instruments.
Deep Impact’s success on July 4, 2005 made global headlines. The flyby spacecraft’s photos of its partner exploding against a comet 83 million miles from Earth are iconic. The images and spectroscopic data Deep Impact sent home yielded dozens of scientific papers and greatly advanced our understanding of cometary makeup.
The surprising volumes of microscopic dust showed Tempel 1 to be more a snowy dirtball than the dirty snowball scientists had assumed, and extremely porous, the comet’s powdery crust being at least tens of meters deep. The varying features of the comet’s surface (smooth here, pockmarked there) led to speculation that Tempel 1 could be the combination of two or more smaller comets. The comet certainly formed at very low speed, lending credence to the prevailing theories that such bodies indeed gelled gently in the outer reaches of the solar system.
Scientists used the ejecta plume’s rate of expansion to estimate comet’s mass and density, weighing Tempel 1 in at about 20 billion tons and pegging the comet to be just 40 percent as dense as water. They spotted water ice on the comet’s surface, a first, and found spectroscopic evidence of carbonates, which must form in liquid water at moderate temperatures. Thus Tempel 1 somehow harbors ices frozen in the solar system’s distant reaches, carbonates formed at moderate temperatures, and silicates formed at more than 1,000 degrees, the last of these also a key finding from the Stardust comet mission. It sent theorists of solar-system formation back to their drawing boards.
NASA’s EPOXI mission, which A’Hearn now leads, hopes to answer some of Deep Impact’s unanswered questions. The $33-million effort has used the surviving Deep Impact spacecraft to hunt for extrasolar planets and, in November 2011, it will capture imagery and spectra of the Comet Hartley 2. The planet-hunting phase ended in mid-2008; scientists led by Goddard Space Flight Center astronomer Drake Deming are still preparing their detailed findings.
Common to Deep Impact and EPOXI has been Ball Aerospace’s flexibility in working with scientists, A’Hearn said.
“They have always made it clear that they wanted to do as much as they could to get the science,” he said.
It required – and, with EPOXI, still requires – that Ball Aerospace engineers work in close collaboration with scientists to find solutions maximizing science return, obeying the laws of physics and paying mind budget limits inherent in cost-capped NASA programs.
The design of the impactor spacecraft was one example of such collaboration. Originally proposed as something resembling an aluminum barrel, A’Hearn’s team realized the manhole-sized ends of the proposed spacecraft would waste much of their energy smashing into each other. A sphere would dislodge the most material, they said. Scientists also asked to switch materials – from aluminum to copper – as the vaporizing copper’s infrared glow would steer clear of coveted spectral signatures from water and organic compounds liberated from the comet.
A spherical copper spacecraft was going to be a problem, Ball Aerospace engineers explained. For one, it would be a challenge to package a telescope, electronics and other spacecraft necessities into such a shape. More poignantly, the flyby spacecraft would have to sit on top of it at launch. An impactor made of copper, butter-soft for a metal, would never survive it.
But the engineers came back with several options, and in the end the team settled upon something resembling a meter-tall rivet. Its leading edge would form an arc, shaped with layers of sculpted, honeycombed copper. The hexagonal back end’s aluminum structure would be capacious enough to play host to a full complement of spacecraft components while supporting the flyby during launch.
Later in the program, during instrument testing, A’Hearn and others on the science team noticed what looked like spurious signals coming from the infrared detector, whose data would be critical in meeting the mission’s primary goal of seeing what’s inside a comet. Ball Aerospace optical engineers found that the company that had made the detector had beveled its edges, allowing stray light to enter and bounce around inside it. Engineers came up with a mask blocking light from the beveled edge.
Around the same time, the team realized the surface of Tempel 1 most directly facing the sun would be much hotter than Ball’s instrument designers had expected, to the point that the heat would saturate the detector and blind it during certain planned imaging sequences. Engineers came up with the idea of mounting a sliver of millimeter-thin glass across the center of the spectrometer’s entrance slit. It would turn away most of the hottest light like a good pair of sunglasses. Within about six weeks – an extremely quick turnaround by space-engineering standards – Ball’s instrument team had installed and tested both fixes.
Fast forwarding the EPOXI mission, Ball Aerospace’s close collaboration with A’Hearn’s science team continues. The Hartley 2 flyby will happen at higher relative speed – 12 kilometers per second, versus Tempel 1’s 10.3 kilometers per second – and from the opposite side of the comet than was the case with Tempel 1. In addition, Hartley 2 will be about a third the distance closer to the sun at the time of flyby than was the case with Tempel 1, which will make for a very different thermal environment.
“One of the things that Ball has been doing to enable the science is looking at how the thermal properties change as we change the geometry of encounter to optimize the use of the IR spectrometer and thereby optimize imaging,” A’Hearn said. “How rapidly do the instruments heat up, and how frequently do we have to turn away to let them cool off?”
EPOXI scientists and engineers are also working out how quickly the spacecraft will be able to rotate to track the comet, which will, in part, dictate how close the spacecraft can pass Hartley 2, A’Hearn said.
“It’s a matter of attitude control and how much the reaction wheels can drive the spacecraft,” A’Hearn said. Contingency planning also plays a role in such considerations, he added: “Will all four reaction wheels be working? Or do we assume we have to do it on three in case one fails?”
In addition, he said, Ball Aerospace engineers are working with scientists to understand the probability of the spacecraft surviving Hartley 2’s ballistic sandblast. On the Tempel 1 flyby, the spacecraft went into “shield mode” during its closest approach, effectively turning a well-armored cheek to the comet. That meant the flyby spacecraft could only image one side of the celestial body.
“Now science says we want to see both sides of this comet, so we want to track it all the way through,” A’Hearn said. “That exposes unshielded parts of the spacecraft, and introduces the issue of how much damage we’re willing to take.”
Unlike the Deep Impact mission, which was designed to communicate with the Deep Space network in real-time on approach, EPOXI must also rotate each time it downlinks data. Scientists and Ball Aerospace attitude-control engineers are now working together to understand how fast the spacecraft can turn to send home data and how such actions will impact scientific observations, A’Hearn said.
“Dealing with scientists on those sorts of issues is the kind of thing that Ball is particularly good at,” A’Hearn said.