A balloon-borne mission launching from the Southwest will aim a telescope at the approaching Comet ISON.
|Mission Update: Due to unusual weather patterns, BRRISON launched the evening of Saturday, September 28th. However, after a too-fast slew back to stowed position, the telescope was stuck in place and the team failed to gather mission data.|
BRRISON will carry two cameras, one for ultraviolet/visible and one for infrared. The infrared one is sensitive to 2.5 to 5 microns, primed for observations of carbon dioxide and water. Astronomers want a good look at the comet’s ratio of these two compounds, because it could tell them how bodies in the Oort Cloud (the far-out territory where ISON hails from) differ from those in the Kuiper Belt. This trip is thought to be ISON’s first into the solar system, so the comet should have pristine material on its surface that’s being vaporized for the first time. Plus, because the comet is whizzing so close to the Sun — it’ll pass about one solar diameter above the photosphere — stuff that normally doesn’t outgas might appear in the coma and tails.
At the wavelengths it’s observing, BRRISON should have a spatial resolution of 1.2 arcseconds per pixel, enough to resolve about 1,100 miles at ISON’s current distance, says principal investigator Andrew Cheng (Johns Hopkins University Applied Physics Lab). The comet’s nucleus is only a couple of miles wide, so we won’t be seeing its craggy surface; it’s the growing aura of volatiles that’s the focus. Still, the comet’s brightness is lower than originally anticipated, and that will affect measurements.
ISON isn’t expected to hit peak brightness until late November, but BRRISON doesn’t have the luxury of waiting. One, it can’t look closer than 40° from the Sun — which would eliminate dates after November 12th. But two, the launch crew won’t be around. There’s a sweet spot wind-wise in late summer and early fall, when winds are slowest in the stratosphere over Ft. Sumner. After that, NASA moves its crew to Antarctica to begin the launch season there. And because ISON is primarily a Northern Hemisphere object, the frozen continent is not an option.
The second hurdle is launch time. Balloons launch in the early morning hours when the winds are calmest. ISON will be visible then, but it would be best observed at night, because Earth's thermosphere still stretches overhead at BRRISON’s altitude and adds daytime emission that astronomers have to work around. At the mercy of the winds and weather, the instrument might have to be brought down before its scheduled daylong flight is completed. Given that ISON will set in mid-afternoon and won’t rise again until the wee hours of the morning, the team will need some luck to catch the comet at night.
Despite its name, BRRISON won’t just look at the comet. It has a long list of potential targets, from Jupiter and Comet Encke to Mizar and water-bearing asteroids. BRRISON is very much a pathfinder mission, says team member Eliot Young (Southwest Research Institute). It’s not the first high-resolution balloon mission — Sunrise has produced some of the highest resolution images ever taken of the Sun — although it is the first planetary science balloon mission in roughly 50 years (Stratoscope II last launched 1971).
But there’s hope that the mission will help herald a new era of balloon observations. One of BRRISON’s main goals is to characterize a fine-pointing system designed to compensate for image motion at the tenth-of-an-arcsecond level, Young explains. And while NASA’s longest balloon flights have only circled Antarctica for 50-plus days, new superpressure balloons wouldn’t have to lose and gain altitude with day/night cycles and could conceivably stay up twice as long. If the agency goes ahead with plans for these long-duration missions, a 6.4-ft (2-meter) balloon-borne telescope with a fine-pointing system like BRRISON’s could conceivably provide hundreds of hours of observations that rival the Hubble Space Telescope's in imaging resolution, he says.
As a semi-aside, I geeked out a little when I heard the specs for the balloon. The balloon starts out tall and skinny (150 feet across) at launch, but as it rises, it expands to balance the atmosphere’s falling pressure. At altitude it will be 459 feet across, swollen to a pumpkin shape and banded head to open toe with the seams where the banana-peel-shaped, polyethylene film gores are heat-sealed together. The balloon plus payload at altitude will be 670 feet tall, 20% taller than the Washington Monument.
Then there’s aiming the telescope attached to this plastic monster. Researchers joke that pointing several thousand pounds of suspended payload is akin to stabilizing a Hummer hanging from a 1,000-foot tether. There are a couple ways to handle the problem; BRRISON does it with a frictionless bearing and flywheel that spins to counteract the spinning balloon.
BRRISON is far from being the only observing endeavor for ISON. NASA is spearheading the Comet ISON Observing Campaign, and observatories (both professional and amateur) from around the world are getting involved.