I was too lazy to do this manually, so I punched it into GPT 4. I did verify that the math, indeed, checks out.

Edit: Sorry for the edits. GPT assumed the tusk in the calculation, so I had to ask it to remove the tusk for this math to check out properly.

1. The size of a narwhal: Narwhals typically range in size, but males on average can be about 4.7 meters long.
2. The length of a banana: The length of a banana varies, but a typical banana is about 18 centimeters long.

Let's start by converting the asteroid size from narwhals to meters.

If 99 narwhals equal 850 meters, then one narwhal would be approximately 850/99 ≈ 8.59 meters.

Now, this doesn't match the usual size of a narwhal that we have (4.7 meters). There might be a discrepancy in the calculation, but for the sake of this exercise, let's use the size provided.

Now, let's convert the asteroid size to bananas.

850 meters is equivalent to 850 * 100 = 85000 centimeters.

If 1 banana is 18 cm, then:
85000 cm / 18 cm/banana ≈ 4722.22 bananas.

So, the asteroid is approximately 4722 bananas long, if we base the calculation on the given size of the narwhal. But please note, this is a fun approximation and might not be precise.

Colossal banana the size of 99 narwhals to pass Earth Thursday - NASA