The Wide-field Infrared Survey Explorer (WISE) has been featured often in these pages, usually in terms of brown dwarfs and the possibility of uncovering a small star or brown dwarf closer than Proxima Centauri. But while we still have no evidence of such, we do have abundant data on brown dwarfs, as well as a useful compendium of objects that come close to the Earth.
For WISE, launched in 2009 and placed into hibernation in 2011 upon completion of its primary mission, was reactivated in 2013 as NEOWISE. The goal is now the observation of asteroids and comets both near and far by way of characterizing their size and composition.
Amy Mainzer (JPL), NEOWISE principal investigator, points to the mission’s success:
“NEOWISE continues to expand our catalog and knowledge of these elusive and important objects. In total, NEOWISE has now characterized sizes and reflectivities of over 1,300 near-Earth objects since the spacecraft was launched, offering an invaluable resource for understanding the physical properties of this population, and studying what they are made of and where they have come from.”
The WISE/NEOWISE archive is publicly available through IRSA, the NASA/IPAC Infrared Science Archive. The animation below shows detections during NEOWISE’s four years of operation under the current mission parameters. All told, 2.5 million infrared images were collected during the fourth year and are now combined with the prior three years of NEOWISE data in the archive. 10.3 million sets of images are available, and a database of more than 76 billion source detections extracted from those images. The April, 2018 NEOWISE data release can be found here.
Image: This movie shows the progression of NASA’s Near-Earth Object Wide-field Survey Explorer (NEOWISE) investigation for the mission’s first four years following its restart in December 2013. Green dots represent near-Earth objects. Gray dots represent all other asteroids which are mainly in the main asteroid belt between Mars and Jupiter. Yellow squares represent comets. Credit: NASA/JPL-Caltech/PSI.
Obtaining measurements of the diameters and albedo of near-Earth objects through infrared observation, NEOWISE has, in its four years of tracking asteroids and comets, scanned the skies a total of eight times, observing 29,375 objects, a total that includes 788 near-Earth objects and 136 comets since the WISE to NEOWISE transition. Ten of these objects have been classified as PHAs, or potentially hazardous asteroids, based on both their size and the proximity of their approach to Earth’s orbit. In fact, the first PHA found by WISE was 2013 YP139, discovered a mere six days after observations resumed in December, 2013.
We’re now well into the ninth sky coverage period for NEOWISE, with the mission extended through June of 2018. Although potentially hazardous asteroids obviously get greater attention, not all of the mission’s news has been made among this population. In mid-2017, for example, we learned that mission scientists had found about seven times more long-period comets measuring at least one kilometer across than had previously been predicted. Long-period comets take more than 200 years to complete a single revolution of the Sun.
That was a finding with interesting implications:
“The number of comets speaks to the amount of material left over from the solar system’s formation,” said James Bauer, lead author of the study and now a research professor at the University of Maryland, College Park. “We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”
Although discovered in the NEOWISE era, the paper on this work actually draws on data produced during the original WISE mission, an example of how the spacecraft continues to let us examine objects both near and far, including those that have been perturbed out of their orbits in the Oort Cloud, pristine material from the Solar System’s era of formation. That there are so many more long-period comets than predicted would seem to reinforce the idea that cometary delivery of icy materials from the outer Solar System must have been common.
The paper on the long-period cometary population is Bauer et al., “Debiasing the NEOWISE Cryogenic Mission Comet Populations,” Astronomical Journal Vol. 154, No. 2 (July 2017). Abstract available.