A Mars-sized object (~10% the mass of Earth) can change the orbit significantly. It depends on the collision process.

500 km objects will have a pretty small effect.

To append another perspective to this: The entirety of Earth's biosphere makes up an extremely small part of the planet. Essentially, Earth is a massive ball of near-molten rock, with a very thin crust, that is covered in a microscopic layer of water and grime. We're part of that grime.

So when a small pebble (aka a meteor) hits Earth, the planet hardly cares, it's just that the outermost layer, the grime, will be disrupted.

That's how an extinction level meteorite impact can both be so devastating to everything we know, yet also be mostly irrelevant to the planet as an astronomical object.

We don't know the precise impact speed and mass, but 10 km/s is a typical approach speed (leading to a 16 km/s impact due to Earth's gravity). It could be just 2 km/s, it could be 30 km/s, in extreme cases it could be up to 70 km/s. It can't be more for objects in our Solar System, only interstellar objects could impact faster but they are very rare. The conclusion is the same for all realistic speeds.

> I know you are throwing rough numbers around, but wouldn't a 50 meter change compound over 120million years? Or on that solar scale is that so trivial its like a fly hitting a baseball?

It's a one-time change. There is nothing that compounds.

If we put the mass of a fly at ~10 milligrams then it's like a fly hitting a truck.

> Wouldn't a 50 meter change compound over 120million years?

The effect on the size and shape of Earth's orbit would not "compound", it would still be small. The effect on Earth's position around its orbit does add up though.

Even without large impacts the solar system is chaotic, in the mathematical sense. We can make good predictions for the next few million years. But we cannot say, for example, what season it will be in the northern hemisphere exactly 100 million current-day years from now.

As people have said it doesn't compound, because stable orbits are stable. If it slows it down which means the opposite side of the orbit might change by like 50m closer to the sun, as he said negligable but that doesn't mean it starts spiralling towards the sun at 50m per year because as it falls closer to the sun it also gains speed, which causes it to fly back out again to the original distance when it gets back to the same point in it's orbit. The orbit is still stable, simply elliptical (which the earths orbit already is by significantly more than 50m).

As others have said, we can’t know for sure. But there is an indication. Asteroids orbit in stable orbits, for the most part. They aren’t just randomly leaving their orbits and attacking the earth, at least not at this stage of the solar system’s evolution.

Here’s what we think happened. A collision between two asteroids launched a chunk of one into a different orbit within the asteroid belt. Now, there are certain radii in the asteroid belt where an orbiting body interacts with the gravity of Jupiter. This is called an orbital resonance. Jupiter will dump energy into a body in these radii and pump its orbit up. Eventually, the object’s orbit will become more, what we call, eccentric. This means it becomes more elliptical. If the axis of that ellipse crosses the orbit of an inner planet, then the two can collide.

But anyway, to answer your question, we know from the physics how much energy is being pumped into an asteroid that causes it to shift its orbit. We can do those calculations and combine them with what we know about the average velocity of an asteroid in the asteroid belt and get an estimate of the speed of the impactor’s approach.

The current best estimates suggest that the Chicxulub impactor was a stony asteroid (a carbonaceous chondrite) of about 10 km diameter, that impacted at around 20 km/s, at an angle of 45 to 60 degrees from the horizontal, coming from the northeast.