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What kinds of astronomical events can pose a serious threat to life on Earth in the near future (1000-10,000 years). If there are none in the near future, when is the soonest event that is predicted to threaten life on Earth?
I am ignorant about astrononomy and so far I have only thought of an asteroid collision, solar flares, and effects from the sun dying in several billion years (and a visit by some hostile aliens).
Essentially, anything could happen! We can't really predict events such as you've described. The only event we can really be certain of is that the Sun will inevitably die in about 5 billion years. When it's almost run out of hydrogen to fuse it will expand into a red giant, becoming so large it will take out the Earth. There's a good video demonstrating this here: http://science.howstuffworks.com/life/evolution/life-on-earth-end1.htm
You could add geomagnetic reversal and gamma ray bursts to your list. Also see: http://science.howstuffworks.com/life/evolution/life-on-earth-end1.htm
Adding to Oceanescence's mention of Gamma Ray Bursts, someone's recently done some calculations: Gamma Ray Bursts Limit The Habitability of Certain Galaxies, Says Study
In fact, according to the authors of the new paper, there is a 90% chance that a GRB powerful enough to destroy Earth’s ozone layer occurred in our stellar neighborhood some time in the last 5 billion years, and a 50% chance that such an event occurred within the last half billion years. These odds indicate a possible trigger for the second worst mass extinction in Earth’s history: the Ordovician Extinction. This great decimation occurred 440-450 million years ago and led to the death of more than 80% of all species.
Gamma ray bursts are thought to emerge from the poles of supernova, so a star's spin axis would have to be in the right direction for it to hit us. Betelgeuse, a nearby supernova candidate has it's spin axis at an inclination of about 20° to the direction of Earth, so that star is not likely to get us when she blows. However, we don't have accurate information on the direction of the spin axis of most stars.
