Since our last post in December, ATLAS operations have become smoother and more efficient. We have resolved a host of problems, improved our hardware significantly and our software enormously, and gained a great deal of experience managing our unique asteroid-detection system. We are now reporting asteroid discoveries from each night promptly in the morning, with the result that each new discovery gets followed up by other astronomers across the world on the following night. This is very important, since additional observations of a newly discovered asteroid are essential to calculate an accurate orbit and evaluate any impact hazards. Both amateur and professional observers in many nations routinely participate in this endeavor.
ATLAS has now discovered seven NEOs that received official designations from the Minor Planet Center: two in December, one each in January and February, and three so far in March. They range in size from little 2016 CS247, with a diameter of about 30 meters (100 feet), to 650 meter (2000 foot) 2016 BZ14. This last object came as a surprise: we would not have expected our little ATLAS telescope to catch an asteroid as large as 2016 BZ14 before any other survey. ATLAS is optimized for small objects passing very close to Earth, and surveys using bigger telescopes normally find all of the larger, more distant NEOs. However, 2016 BZ14 shows that on rare occasions a larger asteroid can slip through the cracks.
All of the ATLAS NEOs were big enough to be dangerous – even 2016 CS247 could have destroyed a medium-sized city – but thankfully all seven asteroids passed by Earth at a safe distance, and orbital calculations show that none will be a hazard for the foreseeable future.
Like their sizes, the orbits of ATLAS NEOs exhibit a wide range of properties that illustrate both the versatility of the ATLAS survey and the intrinsic diversity of the asteroid population. The figures below show the orbits of each new NEO in relation to Earth’s trajectory during the period surrounding the discovery. Earth’s orbit is the thick, pale-blue line, and its location on the night of the discovery corresponds to where this line crosses the x axis. The diamond symbols show the position of each asteroid when ATLAS discovered it. The first figure gives the view looking down on Earth from the direction of the North Ecliptic Pole. The second gives the view from the plane of the ecliptic. In both cases the Sun is out of view on the left, and the axes are labeled with distance in astronomical units (AU). It is easy to see that 2016 BZ14 was discovered much farther from the Earth than the other, smaller asteroids, which had to come closer before ATLAS could see them.
During the week of December 10th ATLAS made a trio of new discoveries: two Near Earth Objects (NEOs) and one comet. Their orbits were determined automatically by the ATLAS software, and reported to the IAU Minor Planet Center at the Smithsonian Astrophysical Observatory.
The largest object, numbered 2015XU378, has an estimated diameter of 250 meters, and is classed as a Potentially Hazardous Asteroid (PHA). It passed within 0.066 AU (10 million km) of the Earth on December 8th.
The other NEO, 2015X168, has an estimated size of 50 meters: its perihelion distance is 0.82 AU, which means that during some parts of its orbit it is closer to the Sun than is the Earth.
The comet that ATLAS found, C-2015X7, has a perihelion distance of 3.8 A.U, so no threat to Earth. UH astronomer Richard Wainscoat confirmed the discovery by taking a pair of images (below) using the CFHT telescope. They confirm both the extended shape of the image and its changing position relative to the background stars.
ATLAS has discovered its first near-Earth object (NEO). The image shows the beautiful starfield the asteroid was crossing when we discovered it (more on this below). The new asteroid has received the designation 2015 PE312 from the Minor Planet Center (MPC). Its brightness indicates that it is probably 200-500 feet (60-150 meters) in diameter.
The reason for the wide range in possible sizes is that a large, charcoal-black asteroid could have the same brightness as smaller one made of light-gray rock. We could tell the difference with sensitive observations from an infrared telescope (dark-colored asteroids absorb more of the Sun’s heat and glow brighter in the thermal infrared), but no infrared observations have been made of 2015 PE312 so far.
To receive an official designation (i.e., to be truly ‘discovered’), an asteroid has to be measured at least twice each on two different nights. It’s important to have a rule like this because it’s easy to mistake variable stars, cosmic rays, and other ‘image artifacts’ for asteroids. Watching how objects move from image to image and night to night is what allows us to distinguish real asteroids from lookalikes. ATLAS observed 2015 PE312 eight times in all: four times each on the nights of August 9th and 10th. Although we have learned that one other asteroid survey took images of it without knowing it, ATLAS is the only observatory in the world to report observations of 2015 PE312 to the MPC. If not for us, the object would still be unknown to the human race.
Finding a faint, fast-moving object like 2015 PE312 against the confusion from countless thousands of background stars is quite a challenge — one that ATLAS and other asteroid surveys meet with a lot of hard work and clever computer programming. The starfield image above has all eight detections of the asteroid added in at their correct positions and brightnesses, and indicated with green circles (click on it for higher resolution). The asteroid was moving from upper right to lower left: the detections from the night of August 9 are bunched together near the upper right, then there is a big gap that corresponds to the daytime interval between observing nights, and then the images of 2015 PE312 from August 10 are bunched together at the lower left. You will probably have to click on the image to download a higher-resolution version to see the green circles and the tiny points of light representing 2015 PE312.
An accurate, computerized orbit calculation was required to be sure that the two sequences of faint images were actually the same asteroid. This calculation showed that 2015 PE312 made its closest pass to Earth on August 19, at a safe distance of about 2.7 million miles (4.3 million km, or 0.03 AU). This is about eleven times as far away as the Moon.
We’re now embarked on the long, slow process of commissioning. There’s a lot of things that we need to work on: limits and stops to protect the telescope and mounts in case the software does something stupid, getting the mount on Mauna Loa going with the Pathfinder telescope, mount software, learning how to reduce these huge new images, and starting to discover asteroids. It’s the usual stuff. Next week we’ll activate the telescope on Mauna Loa and start to get some parts built for limits and other things. In the meantime we left Haleakala in the hands of our capable night assistants, Griselda and Mortimer, who provide indispensible help every night we collect data and shake the system into full operation.
Wednesday we worked to explore the image performance as a function of detector position relative to the field correctors. We got pretty good images over the whole field of view, but the seeing was poor. We’re thinking we’ve got to vent this dome – the skin gets really hot during the day and the closed telescope tube takes a long time to cool off.
Thursday we cleaned, cleaned, cleaned! We put things on UPS’s, rebooted everything to ensure that we can run remotely (or robotically), and finally drove down (during the day!), very tired but very pleased. ATLAS is now real.
Sunday we rotated the telescope for better balance, installed the 110 megapixel science camera, installed seven filters: cyan, yellow, B, V, R, I, and H-alpha, rebalanced, and waited for the sky to clear. Finally, around 11pm the fog dropped enough (although it was still extremely humid) and we opened the dome and got our very first images. There is a lot of work yet to do, but the first ATLAS unit has all subsystems operational.