When to look? In what direction?

Lots of questions coming in, so I thought I would deal with them here.

I live in xxx… Can I see Perseids?

Check out the map below. Unless you live in the red shaded area, you will be able to see the shower. EVERYONE in the United States and Europe with clear weather will be able to see it, provided they are away from city lights and have clear, dark skies. Most other parts of the world will be able to see the shower as well.

When do I look?

You should start to see Perseids around 10 PM local time. The rate will increase throughout the night until just before dawn (3 to 4 am), when you may be able to see as many as 80-100 per hour. Be sure to allow about 45 minutes to allow your eyes to dark adapt.

Where do I look?

Lie on your back on a sleeping bag, blanket, or lawn chair and look straight up and take in as much sky as possible. Do not look at the constellation Perseus, where is the shower radiant is located, as you will see fewer meteors. This is because the length of the meteor gets longer the farther it appears from the radiant; to see nice bright meteors, you need to look some distance away from Perseus, which for U.S. observers is off to the northeast. Looking straight up, towards the Zenith, is a good choice and enables you to take in a lot of sky.

Do not use binoculars or a telescope, as they have narrow fields of view and will greatly reduce your chances of seeing meteors.

Hope this helps and wish everyone lots of meteors!

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Will the Perseid shower be visible from {insert your location}?

I am asked this question over and over again, and it’s a good one. Everyone knows that you have to be in the right place to observe solar eclipses and other astronomical goings-on, so why should meteor showers be any different?

You do have to be in the right part of the planet to view meteor outbursts or storms, because the trails of comet debris are so narrow (hundreds of thousands of miles) that it only takes a few hours for the Earth to pass through the stream. A few hours is not enough time for the Earth to do a complete rotation (which takes 24 hours), so only those people located in areas where it is night and where the radiant is visible will be able to see the outburst or storm. These dramatic events require the viewers to be in the right ranges of both latitude AND longitude.

This is not true for normal meteor showers, like this year’s Perseids. The main stream of particles extends for millions of miles along Earth’s orbit, requiring days for it to cross. All we need is one day to take the longitude out of the visibility calculations, because then the entire planet will experience night while the shower is still going on. That’s the good news.

The kicker is that we not only have to have darkness, but also the radiant – in this case, located in the constellation of Perseus – has to be visible, i.e. above the horizon. The elevation of the radiant depends in part on latitude of the observer, and one can derive – or look up, in this age of Google – a relatively simple equation that gives the maximum elevation of the radiant:

maximum elevation = 90 – |dec -lat|

where dec is the declination of the radiant and lat is latitude of the observer (all in degrees). The vertical lines before dec and after lat mean to take the absolute value of dec – lat.  In order to see meteors from the shower, the maximum elevation must be 0 or greater (preferably more than 15 degrees). In the case of the Perseids, dec = 58 deg, so it is easy to calculate the maximum elevation for various latitudes:

lat maximum elevation
75 73
65 83
55 87
45 77
35 67
25 57
15 47
5 37
0 32
-5 27
-15 17
-25 7
-35 -3
-45 -13
-55 -23

We see that everyone in the northern hemisphere has a shot at seeing Perseids (weather permitting), but folks south of -32 degrees latitude get the shaft.

In the world map above, the red shaded area is the region where the Perseids will not be visible. If you live south of Brazil, at the very southern tip of Africa, or southern Austrailia, you need to take a road trip to the North if you wish to see Perseids. If you want see decent numbers, it will be a long ride, as you need to trek to somewhere above   -17 degrees latitude.

So will I see Perseids? You can find out on your own – look up your latitude (remember, Google is your friend), use the equation above, stick in 58 degrees for the dec, and calculate the maximum elevation. If it is above 15 degrees, you are good.

Remember to get away from city lights. A dark sky is important.

Enjoy the show!

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How low can they go?

Real-life meteor showers are not like what you see in movies – there are no flaming rocks barreling out of the sky blasting holes in buildings, or sending cars hurling many yards through the air. Most meteor showers are caused by debris left behind by comets, icy particles mixed with dust and organics that stand no chance of surviving their kamikaze death dives through Earth’s atmosphere. The meteors that actually do make it through, becoming meteorites when they strike ground, are very, very few in number and originate from asteroids (and much more rarely, Mars and the Moon). There are only a handful of recorded falls each year.

So how low can a Perseid get? The NASA all-sky cameras can provide the answer, at least for the bigger Perseids (inch or so across); the smaller particles burn up higher. Our two station camera network can determine the trajectory of a meteor through triangulation, and tell us the start height of the meteor (the location where it is first seen) and its end height (the location where it disappears or “burns up”). Both cameras observed 80 Perseids last year and 24 so far this year, which gives us enough data to tackle the problem.

We start out by taking the end heights of the Perseids and throwing them into 1 mile wide altitude bins. This results in the following graph:

Looking at the plot, it is apparent that most large Perseids burn up at about 56 miles (90 km) altitude. Some ablate as high as 65 miles (104 km), whereas others may get as low as 47 miles (76 km) altitude. We see none getting down to 45 miles or lower, which gives this old ground dweller a warm fuzzy feeling – I can enjoy the shower, secure in the knowledge that the meteors are going poof way up there.

It turns out that our friends the Perseids don’t get very low at all, ending their interplanetary journeys at least 46 miles above our heads.

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Mysterious flashers in the night…

Even though NASA’s all-sky cameras are designed to detect meteors, nothing is perfect. Planes, lightning, clouds passing in front of the Moon, and bugs crawling on the plastic dome in twilight can trigger a detection, bringing either laughter or scowls when the video is reviewed the next day. Comparing vids from more than one station eliminates 95% of these “falses” – hard to imagine a bug that can be present at the same time on the MSFC and the Walker County cameras, which are separated by 90 miles! Even so, we occasionally pick up non-meteor events in both stations. These have to be caused by objects tens of miles up in the atmosphere, or even higher. Planes don’t operate this high, so that leaves one obvious explanation – satellites.

Anyone who has spent any length of time outdoors in the evening has seen a satellite, most likely the International Space Station. Very often, the satellite first appears as a faint dot of light over in the west, gradually increasing in brightness as it moves in a slow arc across the sky. Sometimes, it disappears in the haze near the eastern horizon; other times, the geometrical circumstances are such that the satellite will abruptly dim and disappear long before, passing into Earth’s shadow. Our all-sky cameras frequently detect the International Space Station, as it is the largest object orbiting Earth and reflects enough sunlight to outshine even the planet Venus.

However, there are instances when you look up and see a short, bright flash. These, too, are caused by satellites, or rather, sunlight reflecting off the solar arrays. Sometimes, a satellite’s attitude and orbit are oriented so that when it passes over your location, the solar array will reflect sunlight down to you, which will produce a star-like flash of light lasting a few seconds. If you pay close attention, you might be able to see that the “star” moves a little bit before it fades out.

The most famous of these “flashers” are the satellites that make up the Iridium constellation. They are communications satellites in low Earth orbit, used for satellite phones, and they have 3 very shiny, door-sized antennas that reflect quite a bit of sunlight. Iridium flares are very bright and very common; so common, in fact, that we can now predict when they will occur with great accuracy. You can go here to get predictions for your location.

Other satellites can produce flares as well, but because their shapes and properties are not well known, they cannot be predicted. But, you can use tools like the Heavens Above website to figure out which satellite caused that bright flash in the night, provided you make careful note of the time when it occurred. Last night, both NASA all-sky cameras caught a flare from Okean-O, a big Ukranian oceanographic research satellite launched back in July of 1999. Here’s an artist’s drawing:

And here’s the vid of the flare:

So the next time you see a flash in the night sky, don’t run to the eye doctor or call the History Channel’s  UFO Hunters. You probably saw a reflection from a satellite, and a little web research can even enable you to find the culprit and solve the mystery.

Takes about 5 minutes!

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