There’s been a lot of interest in the space rock that will cruise by Earth and the moon this coming Tuesday night. Many readers have sent me questions about how best to view it and what it might look like, so I thought it would be useful to prepare a list of frequently asked questions. Thanks everyone for the inspiration.
1. How big is 2005 YU55 anyway and what’s it made of?
The asteroid is approximately 1,300 feet or 400 meters in diameter and roughly spherical in shape. It orbits the sun once every 15 months. It’s a dark, rocky body similar to carbonaceous chondrite meteorites that have fallen to Earth. These are very ancient carbon and clay-rich fragments of asteroids from the main asteroid belt between Mars and Jupiter. 2005 YU55 was discovered on December 28, 2005 by Robert McMillan of the Spacewatch Program near Tucson, Arizona.
2. How close will it approach the Earth and moon during the flyby?
On November 8 at 5:28 p.m. CST, 2005 YU55 will pass 201,700 miles from Earth or 85 percent of the Earth-moon distance. Around the same time, the moon will be 150,000 miles from the asteroid.
3. Is there any chance, even remotely, of the asteroid hitting either Earth or moon?
There is zero chance of an impact on either body. The asteroid’s orbit is well known and nothing would suggest it’s going to veer from the predicted path. It’s like taking the train to work; you’re confident it will travel from point A to point B without jumping the rails to another track. The asteroid poses no threat to our planet for at least the next 100 years.
4. Will the flyby happen all at once and then the asteroid’s gone?
Even today, 2005 YU55 is in the daylight sky in the direction of the sun, which is why we can’t see it. As it approaches Earth on the 8th, it will emerge from the sun’s direction, sweep broadside across our orbit and then recede beyond the Earth-moon system.
You can best understand how it moves by a little thought experiment. Imagine waiting at a street corner at night for a car to pass. We first see its headlights in the distance as the vehicle approaches the intersection from several blocks away. The closer it gets, the faster it seems to move and the brighter the headlights appear. As it passes by and drives into the distance, we can still watch the car for many blocks (assuming a straight road) until its tail lights finally fade away in the distance.
Now pretend you’re the Earth and the asteroid’s the car. The asteroid approaches from the sun’s direction (unfortunately making it invisible), passes us in a big hurry, because it’s so close and then recedes back into space opposite the sun. Amateur astronomers with medium to large-sized telescopes will be able to watch 2005 YU55’s “tail lights” in the distance all the way through November 11. While closest approach lasts only seconds, the asteroid will remain visible for several nights.
5. That brings up my next question. How bright will it get?
During the morning and early afternoon hours on the 8th, 2005 YU55 will be very faint – a combination of its small size, slightly greater distance and phase. Since it’s nearly lined up with the sun and Earth at that time, it would look like a tiny (and dim) crescent moon if we could somehow get up close enough to see it. 2005 YU55 will brighten rapidly as it gets closer and its phase waxes to half, gibbous (three-quarters) and finally “full moon”.
Around 2 p.m. CST (9 p.m. Central European Time) on the 8th, it will be fainter than 14th magnitude and a real challenge to see for those living in Europe and Africa where skies will be dark. Three and half hours later at closest approach, the asteroid will have brightened to 11.9 magnitude, well within the range of a 6-inch telescope even with a bright moon nearby. Maximum brightness of 11.2 magnitude occurs around 10 p.m. CST. The reason it’s brighter 4 1/2 hours after closest encounter is because of the phase effect described above. Around 10 o’ clock, the asteroid is nearly opposite the sun in the sky, so more of it is lit up, much like a tiny “full moon” compared to only “half” around 5:30 p.m. And as you’re well aware, a full moon’s a lot brighter than a half!
Since 2005 YU55 remains near “full phase” long after it moves off into the distance, it’ll be as bright as 12.5 magnitude the next night when it’s still conveniently placed for viewing in northern Pisces. By the 11th, the asteroid will have faded to around magnitude 14.5 as increasing distance trumps phase.
6. OK, so what do I need to see it and what will it look like?
You won’t be able to see it with your naked eye or binoculars. Even at brightest – 11th magnitude – it will be some 100 times fainter than the naked eye limit. An experienced observer will see it in a 3-inch or larger telescope, while beginners will need a 6-inch or larger scope. Part of the reason a larger instrument is necessary is because of light pollution from the bright gibbous moon.
7. I’ve got the right size instrument – where do I look?
The asteroid is ideally placed for viewing around the time of closest approach for observers in the U.S., Canada and South America. You can pick up the November issue of Sky and Telescope magazine for an excellent chart or use the ones I’ve prepared, which are based on the magazine’s. Even better, you can use the latest orbital parameters for 2005 YU55 and input them in a sky charting software program to create your own customized finder chart. You’ll find charts, orbital elements and software links in my earlier blog. For European observers, the asteroid will be very faint during early evening hours but brightening up to around 11.5 around local midnight. Unfortunately, it will be low in the western sky at that hour. UPDATE: Online version of the Sky and Telescope chart is up.
8. What will it look like through a telescope?
From the smallest to biggest telescopes, the asteroid will look exactly like a star. It’s too small to show a disk or shape. NASA’s Goldstone radio telescope and the Arecibo radio telescope in Puerto Rico will bounce radio waves off the space rock to determine its precise shape and build a rough map of its surface features.
9. It must be moving pretty fast. Will I be able to keep up with it in my telescope?
At fastest, 2005 YU55 will cover a half degree of sky – the diameter of the full moon – in about 5 minutes. Low to medium magnification of 50x to 75x in a 6-inch telescope will give you a field of view of about one degree or two full moons. Once you find the asteroid and center it in the field, it will take about five minutes to travel to the other side, so you’ll have enough time for several people to step up to the scope for a look before you’ll need to recenter it.
10. What makes this flyby special?
Numerous smaller asteroids fly near Earth and the moon during a given year, but this is the closest encounter of a relatively large asteroid in 35 years. The last time this happened was December 26-27, 1976 when the 656-foot-wide 2010 XC15 buzzed Earth at 190,000 miles. The next will occur when 2001 WN5 passes within 144,000 miles in 2028.
11. Just for fun, what would happen if 2005 YU55 hit the Earth?
What kind of sense of humor do you have anyway?! If it hit Earth, it would blast out a crater 4 miles across and 1,700 feet deep, according to Jay Melosh, professor of Earth and atmospheric sciences at Purdue University. There would be considerable destruction – depending on exactly where it would land – but the planet as a whole would not be affected.
12. OK, I know it won’t hit, but will its gravity affect the Earth?
No. Its gravitational pull is so miniscule, it won’t have any measurable effects on the home planet. You can rest easy.