Tag: orbits

UPDATE on the big UK fireball

By Phil Plait | September 26, 2012 7:00 am

There’s been a bit more news on that amazingly bright and weird fireball seen moving across the skies of northern UK last week.

Marco Langbroek is a paleolithic archaeologist in Amsterdam, and also an amateur satellite tracker – though with modern tech, the term "amateur" is arguable. Anyway, he’s been looking at the track and velocity of the meteor using eyewitness accounts (and the video taken), and thinks he can rule out the cause being the re-entry of human-made debris from a spacecraft. In fact, he thinks the meteoroid (the term for the actual object responsible for the light show) was an Aten asteroid: part of a class of rocks that orbit the Sun on paths that tend to keep them inside Earth’s orbit*.

The key issues here are the slow speed it moved across the sky, and the fact it moved east-to-west. That last part is really important: very few satellites orbit retrograde, or in that direction. Most orbit either prograde – west-to-east, the same direction the Earth spins and also the same direction it orbits the Sun – or in polar orbits (north/south). So right away that makes it unlikely the meteor was from a spacecraft.

However, what has me scratching my head is the slow speed of the meteor. A rock orbiting the Sun retrograde means its velocity will add to the Earth’s, making it move faster as it burns up, not slower. It’s like two cars in a head-on collision; if each is moving 100 km/hr then the resulting collision speed is 200 km/hr relative to either car. You get slower relative collisions if they’re moving in the same direction; they’ll merely bump at low speed relative to one another.

We see this with meteors; the Leonid meteor shower, for example, is made up of tiny particles that move almost in the opposite direction of the Earth, and when they burn up in our atmosphere they move extremely rapidly across the sky. The collision speeds can be 70 kilometers per second!

So why was this meteor over the UK moving so slowly if it were an Aten? Marco thinks he has the answer to that. If the asteroid happened to be at aphelion – the top of its orbit, when it’s farthest from the Sun, also when moving most slowly and in a direction nearly parallel with that of the Earth – it would all add up. The backwards direction and the slow motion would be a natural consequence of this. [UPDATE: I made an error here: the asteroid can orbit the Sun prograde! When it's at the top of its orbit, it can be moving slower than Earth does around the Sun, so when we look at it it appears to move east-to-west. It's like passing a slower car in a faster one; to the driver of the passing car, the slower one appears to be moving backwards when in reality they are both moving in the same direction. I hope that clears up any misunderstanding!]

I’ll note that as far as I have thought about this, I agree with Marco. It’s not conclusive yet, though, but it’s compelling.

Meteors like this are rare. One that gets this bright, is seen by so many people, and drops bits of itself as it burns up are rare enough (the Peekskill meteor in 1992 is the best example of this), but one moving retrograde is even weirder. If Marco is right then I hope even more people submit their observations, pictures, and videos to the International Meteor Organization website. Those observations can help scientists determine the orbit of the object more accurately, and help pin down exactly what the heck this crazy object was.

Image credit: Craig Anderson


* Technically, an Aten asteroid has a semi-major axis less than one Astronomical Unit. Orbits are elliptical, and the semi-major axis is the half-diameter of the orbit along the long axis. Despite this, an Aten can cross the Earth’s orbit if its orbit is elongated (eccentric) enough.


Related Posts:

- VERY bright and spectacular meteor seen over northern UK!
- Video of the daylight California fireball
- Best video of Soyuz rocket burning up so far
- A meteor’s lingering tale

CATEGORIZED UNDER: Astronomy, Cool stuff

What if all the Kepler exoplanet candidates orbited one star?

By Phil Plait | August 14, 2012 7:00 am

This is pretty cool: astronomer Alex Parker took all the planet candidates found by the Kepler telescope – nearly 2300 planets in all – and made an animation showing what they would look like if they all orbited one star.

Dr. Parker had to do some scaling to make this work. For example, the actual size of each planet is known relative to its parent star, which he then scaled to fit the star shown in the animation. He scaled the distance from the star in a similar way. He describes it all on the page for the video.

I have to admit, it’s hard to know if there’s anything scientific we can learn from this. It’s fun to play with data, and it does often happen that by doing so you can see hidden relationships, things that aren’t obvious when displayed in normal ways (I’ve had that happen to me as well just playing with data – and Parker is very good indeed at playing with data; see Related Posts below for more cool stuff he’s done). That may very well turn out to be true in this case, or it may simply turn out to be an interesting demonstration. But as he points out, since this is animation was done to scale using all the Kepler planet candidates, one thing you see immediately is that there is always at least one transit going on! In other words, looking at all the Kepler host stars, no matter when you look, there are probably a dozen transits or more occurring at that moment.

That to me is actually shocking. I mean, it makes sense, and given some thought I would’ve realized it on my own. However, the context I always put this in is that just a few years ago we didn’t know of any transiting planets, and in fact for a while after the first were found a lot of astronomers scoffed at the idea. Now, though, the evidence is so overwhelming there is no doubt these transiting exoplanets exist.

And yet in all that time we didn’t know these planets existed, in all that time astronomers were looking for them and didn’t see them, in all that time some were found and other astronomers scoffed, there was never a time when there wasn’t a planet transiting a star.

And that’s just in the tiny patch of sky Kepler looks at; the entire sky is over 300 times bigger. So if there are a dozen or so transits going on in just the Kepler field, as Parker states, that means there are thousands of them going on in the sky even as you read this. Every day, all day, for millions and billions of years.

All those planets, perhaps millions of them, hidden in plain sight. All we needed to do was actually look for them.

So the usefulness of Parker’s animation is clear to me; its impact on me was profound. It reminded me once again that science evolves, and that my own biases – all our biases – must evolve with it. Otherwise, who knows what we’ll miss?

Tip o’ the dew shield to the Scientific American blog.


Related Posts:

- Piano sonata in the key of Kepler-11 (music by Alex Parker based on planet orbits)
- Music of the spheres (more music by Alex Parker, this time based on supernovae)
- New study: 1/3 of Sun-like stars might have terrestrial planets in their habitable zones
- Motherlode of potential planets found: more than 1200 alien worlds!

Space Station Solstice

By Phil Plait | July 25, 2012 7:00 am

This is pretty neat: on June 6, a couple of weeks before the summer solstice, astronauts on the International Space Station pointed a camera to the north and took pictures as they orbited the Earth. Taken over the course of about an hour – 2/3 of a full orbit – this was made into a video where you can see the Sun setting and rising again. What’s cool, though, is the Sun never completely sets. It dips toward the edge of the Earth, then pulls away again:

I love how the Sun shines through the gaps in the solar array.

The geometry of this is fun! Normally, as it orbits the Earth, the ISS passes behind the Earth relative to the Sun, going into the Earth’s shadow. The Earth itself blocks the Sun, so it’s nighttime for the astronauts. Mind you, their orbit is roughly 90 minutes, so this happens on average 18 times per day and lasts for about 45 minutes.

But the ISS orbits the Earth at an angle: the orbit is tilted relative to the Equator by a little over 50°. During the northern hemisphere summer, the Earth’s north pole itself is tilted toward the Sun by about 24°. Combined, this means that for a time around the solstice the ISS can stay in daylight for an entire orbit. The Sun gets very nearly blocked by the Earth, but not quite. I drew a diagram that might help:

The circle represents the Earth. The Sun is off to the left, so the left side of the Earth is lit and the right side is dark. The north pole of the Earth is tipped toward the Sun as shown, and you can see the Equator marked as well. The "terminator" is the day/night line.

I added the rough angle of the ISS orbit – this was done by eye, but shows you how this works. As you can see, the orbit is tilted only a bit from the terminator. Because the ISS is 400 km (240 miles) above the surface, the orbit "pokes over" the edge of the Earth in the diagram (which I exaggerated a bit for clarity). Because of this, the ISS can see the Sun even when it’s over the night side of the Earth: it’s up high enough that the Earth doesn’t block the Sun.

And that’s what the video shows. At the top of its orbit (as shown in the diagram) the Sun gets very close to but not completely blocked by the limb of the Earth’s horizon, and the ISS sees daylight for a full orbit!

Pretty nifty. And look: your tenth grade geometry teacher may have overstated it a bit when she said some day your life may depend on this stuff… but it does make life a lot cooler when you do understand it.

Tip o’ the spacesuit visor to the ESA G+ page.


Related Posts:

- Space Station star trails
- The green fire of the aurora, seen from space
- Time lapse: The stars from orbit
- Space station gives physics a boost

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Space

Venus, from one side of the Sun to another

By Phil Plait | April 21, 2012 7:00 am

The dance of the planets fascinates me. All the planets orbit the Sun, keeping their own time depending on how far they are from our star. From our vantage point on Earth, circling the Sun once per year, the planets move across our sky slowly, stately, taking weeks or months to get from one side to the other.

Venus is closer to the Sun, and takes only 225 days to orbit it. From Earth, that makes its motion pretty complicated. Sometimes we see it at night, setting after the Sun, and then half an orbit later it passes the Sun in the sky and becomes a morning object. As I write this, Venus is high in the west after sunset, an intense beacon in the twilight sky.

In 2010, starting in March and continuing on until September of that year, Turkish astrophotographer Tunç Tezel pointed a camera to the west and took a photo of Venus every few days. He captured its motion across the sky in this amazing composite photograph:

I had to shrink it a bit to get it to fit, which made Venus look a little dimmer than on the original. Click to encythereanate and see it much better.

In this series, Venus came out from behind the Sun (on the far side of its orbit relative to Earth) near the center of the picture. It moved up and to the right over the next few months, then in late May it turned the corner and started to head to the lower left at that shallow angle. Finally, in the fall, as it came between us and the Sun, it took that last dip on the left (the diagram on this page may help).

There’s a funny thing about Venus’s orbit. Eight Earth years = 2922 days (6 regular 365 day years plus 2 leap years of 366 days). Interestingly, 13 Venus years = 13 x 224.65 (to be more exact) = 2921 days. In other words, the orbital configuration of Venus and Earth cycles every 8 of our years!

Read More

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: orbits, Tunç Tezel, Venus

RV-sized asteroid will buzz the Earth on Monday

By Phil Plait | June 26, 2011 6:48 am

A rock about 10 meters in size will fly past the Earth Monday at 13:30 UTC (09:30 Eastern US time) 17:01 UTC (13:01 Eastern US time) [Note: the time of closest approach was updated this morning, June 27]. It’ll be a particularly close shave — passing just 12,400 km (7430 miles) from Earth’s surface; a bit less than the diameter of the Earth itself — but it’ll miss for sure.

[Update 2: Images and an animation of the asteroid are now up at the Remanzacco Observatory blog; thanks to NickAstronomer on Twitter for the tip!]

We’re in no danger from the asteroid, named 2011 MD, since there’s essentially zero chance it will hit us. Even if it did, it’s too small to impact the surface, and would instead break apart and burn up in the atmosphere. That would be exciting, and make quite a show, but that’s about it.

Here’s a diagram of the asteroid’s trajectory (note that the size of the Earth is not to scale!):

On this scale, the Earth is actually about half the size shown; it was enlarged on the diagram to make it clear. In this smaller diagram here the trajectory is shown from a different angle (edge-on to the Moon’s orbit) with the Earth to scale, and you can see better that 2011 MD will miss us.

You can really see in those diagrams just how much the Earth’s gravity bends the orbit. At this close an approach the Earth’s gravity is significant, and the path of the object will be significantly altered. Just how much it’s changed is difficult to know until observations are made after the event. However, I’ll note the asteroid will be on the daylit side of Earth after it passes, making observations a bit tricky. I expect radar observations will be made using radio telescopes, which don’t need darkness to work and can provide very accurate measurements of the orbital path.

Speaking of orbits, this one is interesting. 2011 MD’s orbit is quite similar to Earth’s around the Sun, taking 396 days to go around once. The orbit is more elliptical than Earth’s, with a semi-major axis of 1.056 AU.

What does that mean? Read More

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