Asteroid on collision course

American Association of Variable Star Observers (AAVSO)
Mon, 09/24/2012 - 14:28

In a story I'm working up, an asteroid is launched(long story made short) from the asteroid belt in a collision course with earth. From the viewpoint here on earth, it is in the milky way of Gemini and Orion. Visual magnitude about 15, 16, at its distance. Dim. This is an alternate history in our equivalent of 1990 or so; before the interest in NEOs and the umpity dozens of surveys conducted to track them down. The planetoid itself is rather large, with an H of 10 or so.  As it approaches, it'll get brighter and brighter, natch'.

My question is, how bright would it have to be before its noticed by astronomers, professional, or (most likely) you folks and other amateurs out there, like comet hunters. Visual magnitude, I mean. 8, 10? Naked eye visibility? Your best guess from what you know about observing, and the professional astronomers' capabilities with their big scopes would be much appreciated.

I would think it would have very little lateral motion if its on a collision course(its headed to arrive one month after "launch", and where the earth will be then), and if photographed by a survey, would appear to be just another star in the star clouds. So it would most likely be noticed by a lucky amateur and his 17" dobsonian.

Thanks for your help.


Well, I used to be very much into watching comets back then and they generally got to 8th magnitude or so before somebody swept them up visually.  This generally did not involve big telescopes.  Often a 4 inch scope or big binoculars was what was used.  Sometimes, as big as 10 inch.  I recall seeing more than one successful comet hunter posing with a really ratty small telescope.  Usually, an alt-az rig.

The magnitudes are not entirely comparable because comets are not necessarily stellar like an asteroid and the comet mags are misleading.  Some of the hunters might well have noticed such a stellar object though, once it got bright enough, because they were so familiar with the star patterns.  Somebody like Alcock in the UK might have spotted your object in time to give a little warning.

I personally would recommend a fresh long period comet.  It is easier to have it simply come streaking out of nowhere to provide the drama.  GW

It seems to me that they did

It seems to me that they did indeed tend to be that bright before disovered, though it's been a while.  I used to call a hotline at Sky and Telescope which gave a weekly prerecorded message of all the latest comet discoveries and so forth.  It was very exciting.

Just today, I heard there is a comet called Comet ISON which is a Sun-grazer and may get very, very bright next year.  They have found it more than a year ahead of perihelion.  Before the surveys, perhaps we would have had just weeks to get ready to see it.  It's hard to say.  That makes two big comets we may get next year, with PannStarrs in first, in the Spring.

Prepare your novel soon and you can tie it in!

I remember the excitment when Hykutake showed up in 1996.  A little different orbital scenario, but at any rate we had around 8 weeks after discovery before it passed quite close by us.  On the other hand, we had more than a year to await Hale-Bopp, because it was such a bright thing that it was spotted way out there.


American Association of Variable Star Observers (AAVSO)
Asteroid-Earth Collision Story

I can point to a number of flaws in the story concept as it stands.

For many years now a number of highly enthusiastic amateur astronomers have been monitoring the milky way photographically for novae. A number of these amateur surveys reach to 11th-12th magnitude, some even beyond. An 8th or 9th magnitude body would have been likely discovered long before reaching that brightness level, especially against the sparce background of the winter milky way.

It is also difficult to imagine any scenario that could produce a new orbit that would not be a long elliptical arc requiring the earth-bound body to take a quite extended period of time to reach us and dramatically increasing the probability of its discovery if as large as proposed in the storyline. And no matter what the orbital elements might be, the asteroidal body would not appear to stand still among the stars. When objects, like comets, are seen to barely move against the starry background it is not just a result of their moving at us, but also that they are at a very great distance from earth.

In fact, the most favored scenario for story purposes would likely be to have the aliens simply nudge a single asteroid into a new orbit having its perihelion point within that of the earth's orbit. Under the right circumstances the asteroid could be said to have reached perihelion hidden in the glare of the Sun and then approaches earth from the sunward side. Quite real asteroids with earth-crossing orbits have occasionally done so from time to time.

A few words regarding Comet ISON might not be out of place here either, especially following all the internet hype surrounding it over the past week. Comet ISON is not a Sungrazer, a name appropriate only for members of a very specific family of comets, of which ISON is not a member. The technically correct term to be applied to ISON is "Sun-skirting" comet, of which only a very few are known and most seem to have no relationship with one another.

The matter of its brightness when near perihelion next November likewise needs further clarification. Odds are that IF the comet even survives to reach that point in its orbit (if it turns out to be dynamically "new" its performance could well prove a dismal failure, as Kohoutek did in 1973) a peak brightness of perhaps -6 would be about the upper limit it would attain. Further, this would occur only very briefly (a single day) with the comet situated just 2-degrees, or so, from the Sun. For most observers and the public any real show Comet ISON might put on will take place later in December as the comet withdraws from the twilight regions and likely grows a quite impressive tail.

J.Bortle   (BRJ)