Break out your binoculars or a small telescope, we’ve got a busy week ahead! Watch as a bright asteroid approaches Earth, the Moon steals a star, and Comet ATLAS’s last hurrah.

Radar image of 1998 OR2 taken by the Arecibo Observatory on April 18, 2020.
Arecibo Observatory / NASA / NSF

Is it a coincidence that asteroid (52768) 1998 OR2 looks like it's wearing a mask in this image obtained with the Arecibo radio telescope? Or has COVID-19 completely changed how we see the world? Either way you'll want to make a date with this large near-Earth and potentially hazardous asteroid (PHA) very soon. On the morning of April 29th it will safely zip past Earth at a distance of 6.3 million kilometers (3.9 million miles), or around 16 times the distance to the Moon.

Mask-shaped asteroid
Anne Virkki, Arecibo Observatory staff scientist and lead of the planetary radar group, stays protected during the coronavirus outbreak while demonstrating on the computer screen behind her how Asteroid 1998 OR2 resembles a mask.
Courtesy of Anne Virkki

Other asteroids have swung much closer to Earth but this pass is noteworthy because 1998 OR2 is much bigger than most in its class — it will become bright enough to see in a small telescope.

Discovered by the robotic Near-Earth Asteroid Tracking (NEAT) survey on Maui's Haleakalā in July 1998, the object is a relatively rare L-type stony asteroid that swings to within spitting distance of Earth's orbit at perihelion out to a chilly 3.8 a.u. (around 570 million kilometers) at aphelion. Assuming its reflectivity is similar to other stony asteroids, astronomers estimate 1998 OR2's diameter at around 2.1 kilometers (1.3 miles), making it one of the larger PHAs known.

Orbit of asteroid 1998 OR2
Asteroid 1998 OR2 is a member of the Amor group of asteroids, which have orbits that approach that of Earth but don't cross it. Because 1998 OR2 is large and passes within 7.5 million kilometers of our planet, it meets the criteria to be called a potentially hazardous asteroid.


On April 22nd the asteroid glows at magnitude 12.0, bright enough to see in a 6-inch telescope, but it will peak around magnitude 10.8 during the close-approach window from April 30th through May 3rd. Even under suburban skies a 4.5-inch telescope should capture this celestial roadrunner in the early evening sky as it races across Hydra and Sextans at more than 30,000 km/h. Ardent observers can track the asteroid well into May as it continues through Centaurus while slowly fading. If you don't have a telescope our friend Gianluca Masi will live-stream 1998 OR2's appearance on April 28th starting at 2 p.m. EDT (18:00 UT).

Time-lapse animation of asteroid 1998 OR2
Asteroid 1998 OR2 sails across the sky in this time-lapse animation made on April 9th. Through a telescope the asteroid will look like a slow-moving star.
Ingvars Tomsons / CC BY-SA 4.0

We have two charts that you can use to find and follow the asteroid: one for April 25–28 and the other for April 29–May 1. Each is marked at intervals of 4 hours Universal Time. Be sure to subtract 4 hours from the times shown for Eastern Daylight Time; 5 for Central; 6 for Mountain; and 7 for Pacific. You'll also find charts for a more extensive selection of dates at Gideon van Buitenen's 1998 OR2 link on his site.

Within a few days on either side of closest approach the asteroid's apparent motion will reach ¼° per hour (15 arcseconds per minute), fast enough to discern its movement in just a couple minutes with a magnification of 100×.

Asteroid 1998 OR2 (April 25-28)
Use this chart to find Asteroid 1998 OR2 during April 25th to 28th. The tickmarks are Universal Time. Click on the image for a larger version of the chart.
Sky & Telescope
Asteroid 1998 OR2's path (April 29-May 1)
Use this chart for April 29th through May 1st. Click on the image for a larger version, and here for black-and-white PDFs of both charts.


To spot the asteroid, set a "trap" along its path at a selected time and wait for it to arrive. Bright stars or distinctive star patterns make ideal places to corral the object. The asteroid will look exactly like a star but move slowly to the southeast.

Once you find it, go along for the ride. Picture an irregular, flying roundish island spinning once every 3.2 hours as it saunters around the Sun in 1,344 days. Or imagine a date far in the future when the asteroid could potentially strike the planet, momentarily filling the entire eyepiece field before merging catastrophically with the Earth.

Asteroid 1998 OR2's next close approach occurs on April 16, 2079, when it will pass more than three times closer, missing Earth by a mere 4.6 times the distance to the Moon.


On Saturday evening, April 25th, the three-day-old lunar crescent will occult the 3.5 magnitude star Epsilon (ε) Tauri, also known as Ain. The name derives from the Arabic for "eye," and Ain represents The Bull's left eye; the right eye is Aldebaran.

The star will disappear behind the lunar limb in late twilight from locations in the northeastern U.S., including Burlington, Vermont; Boston, Massachusetts; and New York City, while observers in the eastern half of Canada and the Upper Midwest will witness the disappearance in bright twilight (small telescope required!) and reappearance during mid-twilight when binoculars might suffice. By the time darkness falls in western North America the Moon will have left the star behind, but their close proximity will make an attractive binocular sight.

Lunar occultation view in Stellarium
The scene in Duluth, Minnesota, at 9:04 p.m CDT, April 25th, several minutes after Epsilon Tauri reappears.

The occultation occurs at the Moon's earthlit limb, where the contrast between star and Moon is most dramatic. Some locations may experience a grazing occultation with the star popping in and out of view from behind mountains and crater walls that project beyond the lunar limb.

Lunar occultation, artist's view
The view from New York City at 9:54 p.m. on April 25th, seconds before the Moon occults the star.

If you're not in the occultation zone you'll still get to see the Moon slide just to the north of Epsilon Tau in a beautiful setting that includes the Hyades and Venus. The star is an orange giant 13 times the Sun's diameter and about 150 light-years away. It's orbited by a super-Jupiter, an exoplanet 7.6 times more massive than our own Jupiter. During the International Astronomical Union's NameExoWorlds contest in 2014, when 31 exoplanets received formal names, Epsilon's planet was one of the lucky recipients — it now goes by Amateru, after the Shinto goddess for the Sun, Amaterasu.

Boston — disappearance at 9:44 p.m., moonset before reappearance
New York City — d: 9:55 p.m, moonset before reappearance
Burlington, VT — d: 9:39 p.m., moonset before reappearance
Montreal — d: 9:36 p.m., moonset before reappearance
Thunder Bay, Ont. — disappearance at 9:35 p.m., reappearance at 10:07 p.m.
Duluth, MN — d: 8:46 p.m., r: 8:59 p.m.
Winnipeg, Man. — d: 8:32 p.m., r: 9:01 p.m.
Grand Forks, ND — d: 8:44 p.m., r: 8:55 p.m.

Below you'll find occultation times for several cities (local daylight-saving times are accurate within 1 to 2 minutes). You can also simulate the occultation (or lack thereof) for your location by using Stellarium. In the Sky and Viewing Options window be sure you reduce the default value under Relative Scale to 0.25 so the stars become points. Also, uncheck the Scale Moon box under the SSO tab. Search for the asteroid, zoom in, and run the clock back and forth to recreate the event.


Finder chart for Comet ATLAS
Map showing the path of Comet ATLAS daily at 0h UT (8 p.m. EDT on the previous date) from April 25th through May 14th. Click on the image for a larger version of the chart and here for a black-and-white PDF.
Sky & Telescope

As promised, we have a new chart of Comet ATLAS (C/2019 Y4) to share. This is the comet we'd pinned our hopes on to brighten to around third magnitude before its May perihelion. Sadly, the object continues to fission and dissipate. No one likes to see a potentially bright comet fade away, but its evolution has been fascinating to watch through the telescope.

Comet ATLAS fragments
This Hubble composite image made on April 20th of the nuclear region of Comet ATLAS shows four major fragments, two of which (C and D) have crumbled into clouds of debris.
Quanzhi Ye / NASA / ESA

Recent images from the Hubble Space Telescope reveal several distinct chunks of the original comet's nucleus along with debris clouds of calving fragment cascades. While Comet ATLAS continues to slowly fade — now around magnitude 9.5 with a 4′ to 5′ coma and a faint tail pointing east — it's still bright enough to view in an 8-inch telescope. For how long is anyone's guess!

Quelle: Sky&Telescope