Institute of Astronomy


Ask an Astronomer - Solar System

Meteorite identification

Published on 24/06/2013 

Hello there I was wondering if it is possible to test rocks to see if they are from space? I ask because, this is true! Whilst I was gardening today a rock fell into my wheel barrow.  It sounds crazy, but it did happen.  The object is black, looks like coal, and is very light, I suppose like pumice.  If It didn't fall from the sky, I would say it was a piece of normal earth rock, but as it came from above I'm very curious.

It can be quite difficult to distinguish meteorites from Earth rocks.  If it is a couple of centimetres in size or larger and it were a meteorite then I would expect it to have left a dent in your wheelbarrow.  A black exterior is typical since the outside of the rock would be heated to very high temperatures during it's descent, the exterior is often also pitted and can look obviously like it has been melted.  It would not necessarily have been hot to the touch however since it's passage through the atmosphere is very rapid so the interior remains at the temperature of space (extremely cold), and the exterior will cool very quickly.  The lightness would tend to suggest that it is not a meteorite, since meteorites are more often rather dense and heavy, but that isn't an absolute rule.  If you want more information the Natural History Museum in London houses the national meteorite collection and have an identification service that may be able to help.  If you do have a meteorite you are very lucky!

Eclipses on other planets

Published on 24/06/2013 

Hi, I asked my Dad if any other planets in our solar system have eclipses?  He doesn't know but thinks it is a great question so we would be grateful for your help.

Other places in our solar system can have eclipses, though nowhere has eclipses quite like those on we get on Earth.  The moons of the gas giants, Jupiter and Saturn, regularly experience eclipses because Jupiter and Saturn are so large, their eclipses last a lot longer than ones on Earth do though and cover the whole surface, rather like when the Moon is eclipsed by Earth.  In fact NASA's Cassini space probe took an image of Earth while the Sun was being eclipsed by Saturn a few years ago (, and is planning to take another one on 19th July (

Earth-like moons

Published on 18/02/2013 

I wonder what would happen if neptune somehow ended up in earth's orbit and turned the earth into one of its moons? what would happen to the earth assuming neptune kept its new orbit in the habital zone of the sun and what would happen to neptune and its moons. color changes and melting for ex. and would neptune stay blue? thanks

Assuming that we could arrange a bit of magic such that one day we woke up in orbit around Neptune with Neptune occupying Earth's current orbit there would initially not be much change, aside from the rather obvious one of a large deep blue orb taking up a substantial fraction of the sky.  We'll also assume that we arranged our magic transition such that Earth is not too close to Neptune's other moons, so that there are no immediate major impacts, let's put ourselves at the nice healthy distance of a million km from Neptune, giving us an orbital period of about 30 days.  Triton is the only moon we really need to worry about in that sense as it is far larger than all of the rest of Neptune's moons combined (99.5% of all the mass in Neptune's moons is in Triton).  Some of the others are large enough that we wouldn't want to hit them from the point of view of human civilisation, but it would do any long term damage to Earth itself if we did.

The first thing we would notice after the sudden appearance of Neptune in the sky would be the dramatic increase in the height of the tides.  Although our nominal orbit is around 3 times further from Neptune than the Moon presently is from us, Neptune is much more massive, and so the tidal field strength would be around 50 times higher.  This wouldn't directly translate into a tidal range that is 50 times larger, since the much stronger tidal forces would be more effective at deforming Earth's crust as well as the oceans, so the sea floor itself would rise and fall along with the oceans.  It is difficult to say exactly how much larger the tidal range of the water would be than at present, but the mid-ocean tidal range would almost certainly be in the range of a few metres rather than half a metre as it is now.  This would mean that large swathes of low-lying coast around the world, including many of the worlds major cities, would become tidal plains flooded twice a day.  The much greater tidal flexing of the crust would also lead to a significant, and permanent, increase in earthquakes and volcanic activity.  In the long term the extra energy pumped into the Earth through tidal heating would also become a contributor to global warming, though given the other problems I don't think we would notice.  Life on Earth would take a while to get used to these changes, but it certainly would in time, the new tidal plains would become major new habitats, and although modern civilisation would take some heavy knocks I expect that humans would get used to it too.  We could potentially decrease the effect by placing ourselves in a wider orbit, but the tidal effects are always going to be rather larger than at present.

Assuming we manage to pull ourselves away from our new problems long enough to take a look around the next thing that we would notice would be the lack of our old Moon, which, since Neptune's gravity is much stronger, would become another separate moon of Neptune.

Another thing we would notice would be the monthly total eclipses that would occur every time we passed behind Neptune.  We are used to the usual solar eclipses produced by our Moon, which are only fleeting and cover only a small fraction of Earth's surface, these new eclipses however would last for hours and cover the whole world.

On Neptune it would be the dramatic increase in temperature that would be noticed, at present Neptune has a surface temperature of around -200C and emits two and a half times as much energy from internal sources as it receives from the Sun.  The huge change in the energy balance would certainly affect the Neptunian weather, which would probably become more violent.  It would also almost certainly affect the chemical balance in the atmosphere, though exactly how the colouration would change is uncertain, the methane that gives it its present blue colouration would still be present, but other compounds would likely change.

The changes for Neptune's moons would be dramatic, most of them have large amounts of water and other frozen volatile compounds and elements like ammonia and nitrogen, which would melt, but because they are rather small they would not be able to retain the liquids and gases, and probably would mostly disintegrate due to the outgasing.  Triton would be a different case, almost as large as our moon it may not have hugely strong gravity, but it is enough that it would not immediately loose it's new atmosphere to space.  The new atmosphere would probably be quite thick, composed of nitrogen, carbon dioxide and probably ammonia, and would overly a massive, deep global ocean, since water makes up about 30-40% of Triton's mass, much more than Earth.  In fact if the atmosphere is not too thick, and we could get rid of the carbon dioxide (the ammonia would be destroyed by the Sun fairly quickly) it could become quite a nice second home for us.

Moonrise times

Published on 22/01/2013 

Hello Im a photographer trying to capture a photo of the moon rising above the eastern coast of the atlantic ocean, particularly in central florida. I know there is a full moon next week Jan25-26, what is the best time frame to capture it at horizon?

There are various tools around on the internet that allow one to calculate times of Moonrise and Moonset and Lunar phases, has some quite good ones.  Unfortunately in Florida Moonrise at Full Moon occurs quite close to Sunset, so the photographic conditions might not be ideal depending on the effect you want to capture, you might be better off waiting until a few days after full when the Moon is rising later in the evening, but only you can judge that.

The threat of asteroid impacts to Earth

Published on 22/01/2013 

I am writing a blog post on the threat of asteroids on earth. I have read a number of Science and Nature articles on the matter with the general consensus being that the probability is really low of being hit by a highly destructive 300-meter and above asteroid but that if we were to find an asteroid on a collision course for earth, the current ideas of how to deflect it would highly depend on the composition but also are largely untested. Could you please comment on whether you think funding (NASA or others) should be going towards locating smaller (<1km) objects that are not often seen if they are very close to the sun OR whether the budget should be expanded to test the deflection methods on a variety of non-dangerous asteroids within reach?

The probability of an impact with an asteroid large enough to cause large scale destruction is indeed rather small.  Generally we tend to focus on objects 1km or more in size as those that are particularly dangerous, since although smaller objects could cause a lot of damage it would be more localised (a single continent say) rather than global.  Roughly speaking we think that 1km size asteroids hit Earth about every 500,000 years and the impact rate falls off rapidly for larger objects (since there are less of them), impacts with 5km size objects happen only every 10 million years or so.  The largest impact event in recorded history was the Tunguska event in Siberia in 1908, which involved an object that was probably about 60m across.  The most potentially dangerous asteroid we know of at the moment is 1.1-1.4km (29075) 1950 DA, which has a probability of up to 0.33% of colliding with Earth in 2880, so we have a good while to work out how to deal with it.

There are a number of ongoing projects searching for near Earth asteroids, such as LINEAR and the Catalina Sky Survey.  Also, at the beginning of last year the European Union, in collaboration with the United States and Russia, set up the NEOShield project whose remit is to study appropriate deflection methods, including how the choice of method depends on composition.  One of their aims is also to test some of the proposed deflection technologies, both in the laboratory, and in space.  Both aspects are important, since if we don't know the asteroid is heading our way we clearly can't do anything about it, but it isn't much use knowing about the deadly asteroid if we don't have some way of deflecting it.