Blogs / Bad Astronomy

Orbits can be a bit complicated. As the International Space Station orbits the Earth every 90 minutes or so, the Earth is spinning underneath it… and not only that, the orbit of the ISS is tilted by about 50 degrees to the Equator. All of this means that any one spot on Earth doesn’t see the ISS every 90 minutes, and in fact it can be days or weeks between favorable overhead passes.

To help you figure all this out, NASA has created a nifty applet to help you determine when the ISS (and a handful of other satellites, including Hubble) are visible at your location. You can enter your country or zip code and it will tell you when the next visible pass of the ISS occurs. You may have to click "Next Sighting" a few times to get one that’s at a decent time, but keep at it.

ISS and the Shuttle rising over the trees. I shot this picture in 2007.

As it happens, the next week or so yields many favorable overhead passages of the ISS in the US — think of it as a holiday celebration (even though there’s that pesky I in ISS). For me, in Boulder, the next good sighting is on Monday July 6, when it passes very close to directly overhead at 10:17 p.m. (there are a few sooner but they are early in the morning when I tend to be asleep).

This application also provides you with a map of the sky to help you out.

And while it’s nice and all, I still prefer to use Heavens Above, a fantastic resource on the web for satellite passes, sky mapping, and tons more. All you need to do is put in your latitude and longitude as accurately as you can (Google Maps will help there) and it will give you a table of dozens of visible satellite passes, including ISS, Hubble, and a gazillion more.

For you Americans out there, the Fourth of July is a great time for skygazing; even though the Sun doesn’t set until late, a lot of folks will be out after dark anyway. So why not stay outside a few extra minutes and watch the real fireworks?

Hey, I just found out the 109th edition of the Carnival of Space is being held at none other than my friend, fellow TAM speaker, and Hollywood scientist Jennifer Ouellette’s Twisted Physics blog!

Go over there, read about space and astronomy and tell her you say "hi".

Scientists using data from the recently-Moon-smacked Kaguya spacecraft have found evidence of radioactive elements on the lunar surface, including, for the first time, uranium!

That’s pretty cool. It’s a little unexpected to me, too. One of the key aspects of the Moon is that it’s not as dense as the Earth — in fact, it’s just a bit more than half the Earth’s density — so you don’t expect it to have a lot of denser materials. Also, dense materials tend to sink and lighter ones float in a liquid, and we know the Moon was once entirely molten. So most of the uranium (which is among the densest of all elements) should be near the Moon’s core, not on the surface.

Most, but not all. It appears there is some on the surface!

[Update: I received some comments and email (hi Emily!) about this. I was trying to simplify the story about dense stuff sinking and I suppose I left off too much; of course, since we find uranium on the Earth’s surface it can’t all sink down. Basically, uranium likes to bind with oxygen, and that makes it into a less dense molecule which doesn’t just drop all the way down to the core, and that’s why it can be found on the Earth’s surface. I appreciate the comments on this — they keep me honest and usually give me a chance to learn something.]

The way you detect uranium is by gamma rays (having an old prospector pan for it probably won’t get you far, even if he has a Geiger counter). The nucleus of a uranium atom is packed with protons and neutrons; Uranium-238, for example, has 92 protons and 146 neutrons all coexisting in a little ball. Because of the weird ways of the quantum mechanical world, this nucleus is unstable, and after a while it spontaneously ejects a little clump of two protons and two neutrons: a helium nucleus, or what’s called for historical reasons an alpha particle.

Along with this alpha the uranium also emits two gamma rays, photons of light with very high energy. These photons go screaming off in some random direction. In this case, enough gamma rays were sent to the then-still-orbiting Kaguya’s gamma ray detector that it was able to see them. Different elements decay in different ways, and so they send off gamma rays with different energies, too. Kaguya’s detector was built to distinguish between these different gamma rays, and that’s how the scientists knew they had found uranium.

And it’s also how they found other elements like thorium, potassium, oxygen, magnesium, silicon, calcium, titanium, and iron, too. We already knew that there was oxygen and a few of those others on the lunar surface, but this method of mapping from orbit is effective in finding these particular flavors (OK, isotopes) of the elements.

What does this mean? Practically, not a whole lot for now. It’s not like the first astronauts to go back to the Moon will get rich mining uranium, or even have enough to use it for power. In the future, however, any colonists will have to do large-scale mining to get access to aluminum, oxygen, and other things they need to build and survive on the Moon. Maybe they’ll find uranium in useful quantities then. It’s not a good reason to go back to the Moon, but it’s something for future space travelers to remember.

But what this does do is give us more clues into the birth, history, and evolution of the Moon; how it’s changed since it formed over 4 billion years ago. We’re pretty sure the Moon formed when a Mars-sized object slammed into the Earth, and the ejecta coalesced into our satellite. But we can always learn more details, and those help us understand just how this happened, and what has changed since.

More knowledge is always good. You never know where it may lead. This is another step in understanding the Moon, and one day it’ll be the home for a lot of people. The more we know, the better.

Prospector image from ToOliver2’s Flickr photostream.

Wow, are we up to the 108^th Carnival of Space already? It seems like just last week we were at 107!

Oh, wait.

Anyway, it’s hangin’ at Starts With a Bang, and as always has lots of astronomically-bent spacey articles for you to enjoy.

Enjoy.

NASA has uploaded video of the launch of the Lunar Reconnaissance Orbiter on YouTube. It’s pretty cool. I love how they are routinely putting webcams on the rockets now!

It fascinates me to watch how the rocket flame changes in color and shape as the air pressure drops with altitude. And that view of the Earth! Wow.

LRO will enter lunar orbit at 05:43 a.m. EDT (09:43 GMT) Tuesday, June 23. I think it will mark our return to the Moon to stay this time. It’s a first step, but a good one.

The Japanese Space Agency JAXA has released images taken from the lunar probe Kaguya’s final moments. Here’s one of a crater in a hillside taken minutes before the spacecraft slammed into the Moon:

There are seven images released, linked from this page (translated into English using Google). The last image is mostly black with just the tops of the hills lit by sunlight; Kaguya’s impact point was on the near side of the Moon in an area where the Sun had just set. You can see all the long shadows in the images!

I still haven’t heard much news about the impact, but it may take some time before all the data are processed and studied. In the meantime, LRO is on its way to the Moon, and will continue the task of mapping our satellite. LRO’s mission is to pave the way for future exploration of the Moon, leading up to another giant leap for mankind.

I wonder if, someday, people will take tours of all these impact sites, the way people today visit Plymouth Rock? I like to think so.

Tip o’ the Whipple Shield to Emily Lakdawalla.

[UPDATE (22:00 GMT): LRO launched successfully at 21:32 GMT, delayed 20 minutes by weather. Everything looks great so far!]

Reminder: LRO (NASA’s latest Moon probe) is due to launch today at 17:12 Eastern time (21:12 GMT). There are two more launch windows today, one at 17:22 and the other at 17:32.

Follow the news on Twitter from NASA itself, from the LRO feed, the LCROSS feed, or from me.

I’m sure the launch will be on NASA TV as well.