## Part 3: Estimating the Diameter of a NEO The asteroid 2005 YU55 is one of many Near Earth Asteroids monitored closely by astronomers. Some of these objects could one day pose a risk to Earth. On November 8th, 2011 the asteroid came within 325,000 km of Earth - within the orbit of the Moon.

Observing a NEO

For this exercise, you will use a set of images of an asteroid during a close encounter with the Earth. How can you find the NEO in your images?

The apparent brightness of an asteroid depends on how reflective its surface is and how much of that surface there is to reflect. What other factors might be important in how bright a NEO looks from Earth?

The diameter of a reflecting object near the Earth can be estimated using this formula: where H is the asteroid's absolute magnitude and A is the albedo. Most Earth-crossing asteroids have a typical albedo of 0.1. To find the absolute magnitude of the NEO, you will first need to find its apparent magnitude by comparing its brightness to that of a known star in your image.

The absolute magnitude H of an asteroid is different from that for a star. It can be calculated using the formula where m is the apparent magnitude, r is the asteroid's distance from the Sun (in AU) at the time of observation, which you can assume to be 1 AU, and d is the asteroid's distance from Earth (in AU), also at the time of observation.

Suppose this object were to hit Earth at escape velocity (roughly 11 km/s). What effect would the impact have?