In the last few weeks, the media has been full of talks about NEOWISE, one of the brightest and most spectacular comets that has ever passed through our solar system and that can still see if it rushes. While the excitement for this interstellar traveler is more than justified, it is also an excellent opportunity to celebrate the Wide-field Infrared Survey Explorer (WISE) space telescope that got its name. The discovery of this particular comet is just the latest triumph in the orbiting observatory’s incredible discovery mission spanning more than a decade, with no signs of slowing anytime soon.
In fact, WISE has been operational for so long now that its mission has evolved beyond its original scope. When launched in December 2009 from California Air Force Vandenberg Base, its primary mission was slated to complete in less than a year. But like many NASA spacecraft that came before, WISE accomplished its original design goals and found itself ready for a new challenge. Although not before, he spent almost three years in hibernation, as the agency decided what to do with him.
Heaven, one image at a time
WISE’s primary mission was to obtain an image of the entire sky as it appears from Earth at multiple wavelengths in the infrared band. Orbiting at an altitude of approximately 500 kilometers and taking care to always point its telescope away from the surface of the planet and the Moon, WISE would take a picture every 11 seconds. Finally, the orbiting observatory had collected millions of images, with each section of the sky photographed at least eight times so that they could be “stacked” for increased accuracy.
The images were captured using a 40 cm (16 inch) diameter telescope and four separate infrared detectors operating at 3.4, 4.6, 12 and 22 microns respectively. To maximize sensitivity, the entire optical assembly was mounted inside a cryostat in the spacecraft and cooled to approximately 17 Kelvin by a block of solid hydrogen. As long as the telescope and detectors could stay at this temperature, WISE boasted a sensitivity thousands of times greater than the previous IR observation spacecraft.
By October 2010, after nine months of observations, the last solid hydrogen had sublimated and the temperature of the telescope began to rise. At this point, WISE had completed a full sky survey and was halfway through its second step. Although the telescope could still operate at a higher temperature, the data it collected could no longer be directly compared to that of the “cold” phase of the mission; as such, WISE’s main mission was closed.
Experimental Extension
Of course, WISE running out of refrigerant was no surprise. Mission planners knew from the start that they were in a race against the clock and that the second survey of the sky probably would not end in time, but they decided to try to get as much useful science out of the telescope as possible in its ideal configuration. . Once the telescope could no longer actively cool down, it was just a matter of finding a task suitable for the spacecraft’s reduced capabilities.
NASA’s Planetary Division proposed a new mission that they called WISE (NEOWISE). WISE had already discovered thousands of asteroids during its initial full-sky study, and it was believed that even with the telescope’s reduced sensitivity, performing another scan could help identify potentially dangerous nearby objects. The experiment continued for four months, long enough to complete another full scan of the sky. After all the data has been downloaded, the spacecraft was ordered to turn off its transmitter and enter hibernation mode on February 1, 2011.
Planetary defense
The story could have ended there. Even if WISE never restarted, the mission would have been a phenomenal success. But almost two years after the day the spacecraft went into hibernation, an asteroid entered Earth’s atmosphere and exploded over Chelyabinsk, Russia.
By releasing energy approximately 30 times when the atomic bomb fell on Hiroshima, the event was a sobering reminder of the danger posed by even relatively small asteroids. Under pressure to find ways to detect and ideally deflect similar objects in the future, NASA decided to restart the successful NEOWISE program.
After establishing communication with the satellite in September 2013, ground controllers ordered the spacecraft to point the telescope into deep space to begin radiating the accumulated heat. After a month, this reduced the telescope to 75 Kelvin, and the IR detectors were recalibrated to the best of their ability.
It is not a perfect solution, as the increased operating temperature and limitations in the telescope software mean that NEOWISE is having trouble identifying objects less than 100 meters in diameter. Still, the observatory has been able to identify hundreds of nearby asteroids since their reactivation; almost 50 of which have been classified as potentially dangerous.
A tired guardian
The Wide Field Infrared Survey Explorer continues to operate under the NEOWISE program to this day. On March 27, he discovered the comet that would eventually bear his name, a testament to the observatory’s continued scientific value a decade after its primary mission came to an end. Now in its 14th full sky scan since its reactivation in 2013, NEOWISE has made nearly a million infrared measurements of more than 37,000 objects in our solar system.
Even considering the success of this post-hibernation program, the fact remains that the observatory hardware is at its best and is being used for a task it was never intended for. History will remember shows like NEOWISE and Kepler’s K2 as some of the brightest mission extensions ever designed, but they shouldn’t be considered replacements for launching new missions.
The Near Earth Object Camera Telescope (NEOCam) is specifically designed to take over WISE as a dedicated observatory to detect and classify asteroids and comets in our solar system, with the ultimate goal of finding ways to deflect or neutralize objects. they are on a collision course with the planet.
Unfortunately, obtaining funds for the mission has been difficult and the project has had several false starts. So until NEOcam or a similar spacecraft can free you from service, WISE will remain in place, doing its best to give us an early warning if any cosmic traveler is heading towards us.
