Institute of Astronomy


Ask an Astronomer - Planets

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.

Formation of Earth and life

Published on 07/11/2012 

Hello, I was talking to someone the other day and we got onto the subject of Space as it fascinates me, thinking about space and how life began on Earth is the one thing i can't get my head round, thats why it fascinates me. So anyways, I was wondering, is it possible that the planet we know as home, Earth, wasn't always where we are are now? What I mean is, when I try and think how life began on Earth, maybe Microscopic life to begin with yet life doesn't just start. it has to have something to begin with, you cant put a rock into space, leave it for a few million years and then come back and there will be life on it. What I am wondering is, was life on Earth, whatever it was in the start, frozen on a drifting Asteroid, that Asteroid being Earth, kind of like Pluto is at the moment, just a huge planet of ice and rock, though this huge drifting Asteroid of Ice and Rock was glancing past the sun and got pulled into the circular gravitational pull and now rotates. Obviously of Earth was going on a head on course with the Sun it couldn't be pulled into a a gravitational circle as it would be too sharp a turn, though if it was glancing past the sun it could maybee have got pulled into the gravitational pull, then with the heat of the Sun being just right over the years the Sun defrosted this huge block of ice and rock and slowly thawed out the life, almost like the life was in Cryostasis and now its being defrosted, this can happen as theres a certain moth that does this in the Arctic, gets frozen over winter and defrosts and comes back to life in Spring, just wondered if that has every been wondered and what the answer was, that mabye our Earth and life didn't begin where we think it did, instead Earth actually drifted in from another part of space and got caught in the Suns Gravitational pull and the heat defrosted it and allowed life to begin, well not begin, but carry on now its been thawed out, then over the years the speed of the earth rotating combined with it spinning on all axis sort of moulded into a circular shape planet, kind of like sanding it down. Long question I know but it was on my mind, it also seems a bit more plausable, that life didn't begin in the Milky Way, instead the Milky Way is where our planet ended up and thawed out the life that was on it. Just wondered. Btw I don't study Astronomy, just interests me as its the only thing I can't get my head around so I tend to thing about things, so excuse me if this has already been answered. Thanks for your time, all the best

Thanks for your question.  To answer it, it is useful to split it into two parts; 'did Earth form in the solar system?' and 'was Earth the birthplace of the living organisms that now populate it?'

The answer to the first part of that question is almost certainly yes, Earth did form in the solar system.  The process of planet formation involves quite a bit of jostling about so all of the components that went on to form the young Earth did not necessarily come from near the current orbit of Earth but it would be almost impossible to place a rocky planet into the current orbit of Earth by capturing it.  While it is possible for planets to be ejected during formation, space is really very empty and the probability of an ejected planet passing near to another star is very low.  Even if it did happen and the planet was captured it would end up in a very wide very eccentric orbit (similar to a comet) very different to the current orbit of Earth and although orbits do change over time it would be very difficult to change a comet-like orbit into an Earth-like one.

The question as to whether the genesis of life on Earth was indeed on Earth is a very different one and one that has had a great deal of discussion in one form or another for centuries.  If you look up panspermia you will be able to find far more information than I can give you here, though it is good to be aware that there are some rather crazy ideas out there.  It can be something of a controversial topic but as yet no one has come up with a complete theory explaining how life can arise from non-living material either so it is difficult to really say which is the more likely origin of life on Earth.  Anyway the basic idea of panspermia is that life is spread throughout the universe by asteroids and comets, rather like your suggestion.  As well as the suggestion that life first arrived in the solar system by such a mechanism there is the related suggestion that life could be spread throughout the solar system in the same way if it did first arise within the solar system.  As the only place we know of at the moment that has life is Earth it is difficult to make deductions.  Research is being done into investigating how long terrestrial microbes can survive in space, the Russian Phobos-Grunt mission had a small capsule on board that was designed to test whether the microbes contained within it could survive in space for the 3 year length of the mission, but unfortunately, as you may have heard on the news, that ended up in the Pacific.  When we have more experiments like that, or if life is found somewhere else in the solar system, like Mars or Europa, we can begin to answer the question of where life on Earth arose, and how widespread life is in the universe, in more detail.  All we have to go on at the moment is that pretty much anywhere you look on Earth, no matter how extreme, you find life.

How many solar systems are there?

Published on 24/09/2012 

How many solar systems are present in the Universe and what is the possibility of finding a solar system similar to ours?

Those are very good questions, and ones to which we don't quite know the answer.  There are a number of projects running at the moment that hope to try and provide some answers, such as NASA's Kepler mission and the European CoRoT, as well as various ground-based efforts.  So far they are suggesting that a reasonable fraction of stars (around 10 per cent or even more) have planets, but so far we have not really able to find systems quite like our own.

In terms of the number of solar systems present in the universe, there are something like 300 billion stars in the Milky Way, so if 10 per cent of them have planets there are around 30 billion planets in our galaxy alone, and there are over 100 billion galaxies in the observable Universe for a total of something in the order of 10^21 (that's 1 then 21 zeros) planets in the observable Universe.  There is still quite a bit of uncertainty in that number however, and we don't yet know how many of them would look like our solar system.

What happens when asteroids and planets collide?

Published on 10/06/2012 

Is asteroid 2003/Q0104 going to hit Earth in May 2031, and if not, then by how much will it miss us and what effect could the near miss have on us; could it hit our moon, and if so, what would the effects be for us?


This asteroid is no longer considered to be on a collision course with Earth (or the Moon)! It was removed from this risk category back in 2003 shortly after it was discovered (

If two large objects collide in space, there are a lot of different conditions which need to be accounted for when saying exactly what the effects would be e.g. the size of the objects, their speeds etc.. If the Earth and another object e.g. an asteroid did collide, there are a range of possibilities in terms of the kind of damage it would cause. If the asteroid hit an ocean it would create a mega tsunami while if it collided with land it would cause a large crater, examples of which can be found all over the Earth e.g. Meteor Crater in Arizona, USA. Material thrown up when the crater was formed would also be thrown out of the Earth's atmosphere and would be spread around the neighboring solar system. Depending if the asteroid hit directly or with a glancing blow, this would have a significant effect on the amount of material ejected.

So in summary - there is no reason to panic in 2031! And there are a range of possible outcomes when it comes to collisions of large bodies in the solar system.