I have an idea for a short story that I've been kicking around for years now, but I'm not sure how plausible it is.

Would it pe possible for an asteroid to 'sneak up on us' by making it's aproach from the other side of the Sun, so that it remains undetectable until it is seen transiting the Sun a day or so before a possible impact?

Basically, the observer in the story is looking at the Sun through a scope and sees a sunspot with no penumbra. He takes a few pictures and goes in the house to see what it could be. A few hours later, the object is still there. Taking another picture shows that it has increased in size slightly. Over the next few hours it is realized that whatever the object is, it's moving almost directly from the Sun toward the Earth. If the whole story is to play out over a span of about 36 hours, how big would the asteroid have to be, and how far away would it be 36 hours before impact? Would that make it large enough to see with a 5 inch scope?

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An evil person would do the things I think up.

I think it's extremely unlikely that the object would remain on
the line between the Sun and the observer for more than a few
hours. On the order of one hour is most plausible. It would
slowly drift across the face of the Sun, and a change in speed
might even be noticeable over the course of an hour. It might
appear to be motionless for several minutes, then drift faster
and faster across the Sun until it disappears at the limb. That
would be a possible result of the orbits of the object and Earth,
Earth's rotation, and the object's acceleration toward Earth.
The closer it is, the less time it will likey appear to stand still.

It would be interesting to know whether any geosynchronous
satellites are large enough to be seen silhouetted against the
Sun with a small scope.

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

You could imagine some previously unknown asteroid getting kicked out of an orbit between Mercury and Venus into an orbit that sends it in our direction, so that it is silhoueted against the sun for a few hours just before impacting us. You'll need to try some solar system simulators to tell just how long it might appear as a spot.

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Have you considered a "long period asteriod" in a highly eccentric (very thin ellipse) orbit with perihelion near Earth, very low albedo (a stealthy bloke), and that takes a few weeks between the two sun grazing (few hundredths of an AU) points of its orbit during approach and exit from perihelion?

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For those inclined to oppose human meddling with the structure of the universe or the composition and configuration of objects and groups of objects within the universe, consider:
Whether there is a limit to the magnitude of a modulation of chaos below which order remains invariant? Or, is order but a fiction invented by perspectives applied over finite, however large, time intervals?

Well, the more detailed the orbital requirements for this particular "stealth asteroid" are, the more unlikely it is to exist in the first place, right?

Whew! Stealth asteroid nearly blindsides Earth

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That's not too important in this case. The OP is discussing something for a work of fiction, in which he's looking for possible and unlikely, not probable.

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Forming opinions as we speak

Thanks, ToSeek. Looks like the premise is plausible.

If this were to be on the same orbital plane as the Earth, any way for a claculus dreprived person to figure the speed from 36 hours out? Would 50,000 KPH be reasonable for an average?

__________________
I'm not evil.
An evil person would do the things I think up.

If you put in an out-of-range value for an Earth impact, this site says:

50,000 kph is about 14 km/s, so it's within the range but toward the low end. (I would think lower values would be more likely than higher ones, though.)

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Everything I need to know I learned through Googling.

The problem with the asteroid being stationary in front of the Sun is that such an orbit would make the asteroid spend a good part of its orbit beyond Earth's orbit, where prior detection would have been likely. (Think about where the asteroid would be headed if it didn't collide with Earth.) Unless of course its a small asteroid, but I bet you want a big one for your story

Here's some screen shots for you.
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Here's an asteroid that spends almost its entire orbit interior to Earth's orbit. It would be very difficult to detect as it is usually lost in the Sun's glare.


Yellow: Sun; Gray: Mercury; White: Venus; Blue/Gray: Earth; Red: Mars; Green: asteroid
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Here's an Earth-centered diagram of the orbits. Notice how the asteroid spends much of its final months almost lined-up with the Sun.

Yellow: Sun; Gray: Mercury; White: Venus; Blue: Earth; Green: asteroid
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The asteroid remains lost in the solar glare in the hours and days prior to impact. Then, minutes before impact, it crosses the solar disk. This is the only warning we will get, and probably the best hopes of someone with a 5 inch telescope making the discovery, beating out SOHO and the professional observatories, who probably won't analyze their data until much later, and beating out hundreds of other amateurs who got a solar filter for Christmas and happen to be using it for the few minutes it takes the asteroid to cross the solar disk.

Gray: Moon; Blue: Earth; Green: asteroid
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Here's an alternate impact scenerio. The asteroid doesn't cross the solar disk. It is first observed by probably thousands of people as a slow-moving, but bright satellite in the pre-dawn sky. It resolves into a shape minutes before impacting the dark side of the globe.

Gray: Moon; Blue: Earth; Green: asteroid
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But if you do want it to hover motionless in front of the Sun for a few hours, and you're not concerned about the low probability of that actually happening, here's your impact trajectory.

Gray: Moon; Blue: Earth; Green: asteroid
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And here's a view of the asteroid's solar orbit prior to impact.

Yellow: Sun; Gray: Mercury; White: Venus; Blue/Gray: Earth; Red: Mars; Green: asteroid

The way to think of these "blindsiding" asteroids is that they are actually on orbits very similar to Earth's orbit, but with a slightly different eccentricity than Earth's. As they are moving at virtually the same speed as the Earth, they don't overtake us, they drift out from the Sun or in from that antisolar point. But tony873004's point is that they alternate between these approaches, so if this one is from the Sun, the last one was from the antiSun and would have been seen. Note the period of these passes is 1 year.