Frequently Asked Questions About Asteroid 2005 YU55

Asteroid 2005 YU55 zips by the Earth and moon Tuesday night November 8. This illustration shows how it approaches from the one side, cuts across Earth's orbit and then continues on its way. Click image to see an animation of the event. Credit: NASA/JPL and my own additions

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.

59 Responses

    1. astrobob

      Hey Les,
      It was questions from folks like yourself that help me better understand what people want to know about 2005 YU55. Thank you!

  1. Debra

    Best article I have read on this asteroid so far Bob you have made it very easy to understand this asteroids path

  2. Mac

    Hi Bob, great rundown of Yu55! My question is after running the JPL model I come up with the closest time of approach being on November 9th? Have the coordinates changed or is this just a timezone difference on the JPL model? Thanks again, awesome site!

    1. astrobob

      Hi Mac,
      JPL Horizons uses Universal Time (GMT). Closest approach occurs at 23:28 UT on Nov. 8. When it’s easily visible in early darkness across the U.S. it’ll be Nov. 9 UT.

  3. Milayla

    Ok so it will already have passed us on the 9th it will just still be visable? And once it passes up Tues at 6:30 thats it right? It spends the rest of its time leaving right? No way it can hit or do harm on the way out right? and thanks for all the info bob you help me rest and understand things

  4. Stephan

    Hi Bob,

    thanks for the enlightening article. Well done! I’ll give it a try tomorrow night with my 10″ mirror. Let’s keep fingers crossed that the weather holds and the fog stays down in the river’s valley until after midnight (as it usually does..:-) ).

    Greetings from Stuttgart, Germany

      1. Stephan

        Hi Bob,
        yay, I did it! saw the asteroid move “slowly” along the “foot” of Pegasus around half past one tonight – between the cluster M15 and the two brighter stars 9 and 13 of Pegasus which you mentioned. The night was very clear, the fog waited in the valley – but in order to block out the Moon I put a black T-Shirt over my head and eyepiece πŸ™‚ and looked through the eyepiece with both eyes open. It wasn’t easy because the asteroid was not very bright, but as it was moving in front of the other stars, so it was easier to detect. It took about 5 minutes to cross the field of view of the 26mm eyepiece ( a bit more than a full-moon diameter ).
        Thanks for your helpful star charts – that made it possible for me to aim at the expected spot.

        greetings from Stuttgart, Germany

        1. astrobob

          I enjoyed reading about how you found the asteroid. Congratulations! I agree with you – it wasn’t easy because it was so dim.

  5. les

    Hey Bob. So one quick question. A friend of mine is freaking out and making me worry now that the asteroid is linked to the EAS
    nationwide broadcast and that something horrible is to happen… It is supposed to happen 2pm eastern time so 11am in California. Do u know of anything that could happen science or space wise for that to be true? I just want this week over with really with people saying things about this and then the asteroid and then the whole Nov 11th predictions…I’m kinda jittery…

  6. Stephanie

    Hey Bob I just have a quick question, can Earth’s gravitational pull cause the asteroid to change trajectory and head our way instead? Im very worried because I feel like NASA wont tell us if it was on course with Earth.

    1. astrobob

      Hi Stephanie,
      I have not heard or read that its pass by Earth will cause a change in trajectory. The NASA diagram shows a straight, undeviating path past moon and Earth. The trajectory of the much smaller, much closer 2011 MD in June was drastically changed by Earth’s gravity and shown in diagrams at the time. You might recall there was never any concern it would strike the planet except from the doomsday fringe. While it’s possible Earth’s gravity might cause a small change in 2005 YU55’s path, it definitely won’t pull it anywhere close to or into the planet. The asteroid is simply moving much too fast for that to happen.

  7. Milayla

    The asteroid will have already passed us and be on its way out on 9th at 2:00 right bob? And I just gotta ask dont ppl drive you crazy with all these questions and crazy asumptions?

    1. astrobob

      Depending on moonlight and sky clarity, a little higher magnification will make it stand out better once you’ve found it. I usually start low at around 75x with my 10-inch reflector. Once I pinpoint the asteroid using the chart and its own movement, then I up the power for the pleasure of seeing it a bit brighter (higher powers darken the sky) and watching it move even faster. I’ve seen some near-Earth asteroids that at powers around 200x you can watch move steadily across the field of view in real time.

    1. astrobob

      When it’s ideal for viewing in the U.S. on the evening of the 8th at say, 7 p.m. Central Standard Time, it will be noon or 1 p.m. Nov. 9 in New Zealand. The 1 p.m. time is if you’re observing Daylight Saving Time. New Zealanders will have to wait to see it later on the evening of the 9th when 2005 YU55 will be fainter but still around magnitude 12.

  8. Bobbi

    So is this the time it will be closest to earth? Im quite nervous about it actually….Could it just change its course in a split second and head towards earth…There are sites saying that it will cause 100% distruction of all living things….

    1. astrobob

      No – asteroids don’t do this. They’re like trains barreling down the track on their way along their orbits. Even if you park your car on a train track, you ain’t gonna stop it! This asteroid will fly by Earth and moon without a blink. The doomsday sites are not worthy of a single click.

  9. bret byers

    Bob you stated that 2011 MD in June trajectory was DRASTICALLY changed. So is it passing by the Earth then the Moon ? Because if it is and the trajectory is drastically changed on YU55 and is going to be closer to the Moon then Earth could it not hit the moon cause it to go off orbit which could change weather tides etc ? And does you or ANYONE have any facts to state besides ‘ JPL or NASA says so ” etc They are controlled by the government and it would be nice to have a non biased statement of facts or observations made without all the referals to Nasa and Jpl. This i believe would ease a LOT of peoples minds !

    1. astrobob

      Hi Bret,
      Click on the illustration at the top of today’s “Frequently asked questions” blog and you’ll see exactly how the asteroid will move when it’s near Earth. It will not be hitting anything since it’s following its own trajectory. 2011 MD was much smaller and passed MUCH, MUCH closer to Earth – only 7,500 miles. That’s how its course was changed. Astronomers knew it was going to happen in advance because they can calculate these things knowing speeds, masses and gravitational force. I’ve not heard anywhere nor seen any indication that Earth’s gravity will significantly alter 2005 YU55’s path. Not all asteroids are created equal! 2005 YU55 is much farther from Earth and traveling very rapidly. To see how Earth altered 2011 MD’s trajectory, I highly recommend this article from the Minor Planet Center:
      As for NASA, while they’re not perfect, if it wasn’t for the people there working hard to communicate coordinates, orbital elements and share untold illustrations and photographs on the Web with anyone and everyone, most of the world would be in the dark about ANY of the near-Earth asteroids out there.

  10. stephanie

    hi bob just a few questions…. I read online they been watching the asteroid since the 4th of Nov have they found anything new? and I just gotta know if the asteroid was closer than what it is and will be wouldnt we be able to see it already without a telescope? cause I read that ppl r saying that its actually closer than what you and Nasa are saying?!?!?! and is this getting as much hype and elenin did? my son has been just driving me crazy about it cause kids at his school got him nervous and he wants 100% correct information and he is not getting it from google or youtube….. pls help and thanks in advanced

    1. astrobob

      All that stuff’s more hype, loonyness and general nonsense along the same lines as Comet Elenin. The asteroid’s following its course as usual. We can’t see it with a regular telescope because it in the direction of the sun in the daytime sky. A radio telescope however can “see” it. The latest image of the asteroid just came in this evening from Goldstone. It’s pretty pixelated still because it’s so small and relatively far away yet.

  11. Jack

    Hey bob, in minnesota (central time) what time approximately would I be able to witness it, or would I even be able to? would a high powered binoculars help?

    1. astrobob

      You’re in a good spot. I’ve prepared charts in this blog at
      for between 8 and 9 p.m. Central time. You’ll need more than binoculars to see it, unless you have monster 6-inch aperture binoculars. It’s really for telescopes only — 6 inches and up.

  12. David

    Hey Bob. I have great news my 3yr old toy poodle seems to be going into labor tonight so im going to name one of the puppies “Astroid YuSS” i know it sounds funny right? Ha!

  13. les

    I just went to the fema website to try to ease my mind but only made it worse….why do they keep saying this test and how it will effect you. Somethings not right here. Can u maybe reassure me its nothing space oriented. And if it was coming from the sun would we beable to see it now coming our way??? is there a comet we dont know about??

  14. les

    But of it were coming from the sun area like u mentioned before. Would we beable to see it 2 days before it came? Im asking u because well i dont know this space stuff…do u know what the hype is about comet 11133??

  15. Milayla

    Are you gonna follow it tomorrow? And so once it hits 6:29 pm ny time its all over? And it will already be on its way out? No way nothing can change? You giys would have noticed that already if that was the issue??

    1. astrobob

      I’ll be following it if we get lucky and it clears. I believe I already answered your question on the visibility period of the asteroid. Nothing’s changed since the recent blog on the topic.

  16. skiewatcher

    Hey bob best article and most informative by far that i have read but i have one question i own a 8″ celestron and i cant see objects that are 10th mag will i really not be able to see this rock ? I live in NYC a suburb not in the big city so theres much light pollution i wish we could have another mass blackout like we did a couple years ago …one more question where exactly should i be looking tomorrow night ? Thanks

  17. In space kinetic masses do from time to time impact each other. 2005 YU55 is small enough for such an impact to alter its orbit, and even a small differential will change things twenty, thirty years hence.

    If one places earth in center of a 200,000 mile target, we are talking (my guess) about a bulls-eye of about 1/2500 of the disc. Not much, but From where does all this certainty come?

    1. astrobob

      You’re absolutely right, otherwise how would all the meteorites get here, right? Yes, rocks hit other rocks from time to time and can change the trajectories of both depending on mass, speed, etc.

  18. Arild

    Hi, I just wondered how much earlier this asteroid would have to have flown by for it to hit the moon.? It could have had a direct impact on the moon if it came by just a little earlier couldn’t it ?

    1. astrobob

      Unfortunately I can’t answer that question, but even if it were lined up so that it looked like it could hit the moon, it may easily have gone under or above it. The animated diagrams unfortunately don’t show how the orbits of both the asteroid and the moon were inclined that evening since they only show two dimensions.

  19. john shields

    even though you said it wouldn’t affect the earth much if it hit,wouldn’t it kill almost all life on earth with the dust cloud it made?

    1. astrobob

      Hi John,
      No, you’d need a bigger asteroid. Even the one that likely killed the dinosaurs left plenty of life in its wake. It’s estimated to have been 6 MILES across. 2005 YU55 was only a 1/4 mile.

  20. Greg Markowski

    Hi John,
    On Tuesday, November 8, between 21:00 and 22:30 EST, I and other members of the York County Astronomical Society in York, PA, were observing Asteroid 2005 YU55. Several of us saw a definite change in magnitude occur several times within a 1 to 2-minute time period – usually while within the same field of view. Although the Moon sky glow was intense, the sky was otherwise clear with minimum cloud interference. The brightening and dimming occurred on what seemed to be a regular basis throughout the evening. We were manually tracking it using our 18″ f/4.5 Dobsonian telescope and a 50mm eyepiece. Unfortunately, we do not have any photographic proof of our observations.

    However, I have seen at least 2 other Astronomy Clubs web sites (from Syracuse, NY and Raleigh, NC) that not only report observing the same magnitude changes, but they in fact have photographic proof on the web! One observer in Raleigh estimated the magnitude change from 8 to 12 when compared to background stars of known magnitude. That’s quite a difference!

    Clearly, something unusual was going on with this asteroid. All “official” sources say it has an 18-hour rotation. But what else would cause such repetitive (and predictable) magnitude change for such a small object except for a rapid (i.e. 2 to 3 minutes) rotation?

    One other interesting side-note during our observation: Since our telescope was stationary, we all exclaimed that although the stars were moving noticeably fast across our field of view in one direction, the asteroid was moving very slowly (almost stationary) in the opposite direction! An amusing illusion!


    1. astrobob

      Hi Greg,
      Great report on the asteroid! Thank you. I’m surprised it would vary so much in brightness. Hopefully NASA’s studies based on the Goldstone and other data will confirm your observations and suggest a cause. It’s not unusual for an asteroid’s brightness to vary as it rotates, but four magnitudes would be exceptional. I’m glad you mentioned the almost-stationary appearance of the asteroid as it plowed east through the starfield. I noticed that too. I even let it drift to see how long it took to finally depart.

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