Answer:
Scientists are getting better at finding them
Step-by-step explanation:
The PHAs have always been there, but improvements in technology have enabled us to detect more of them.
A potentially hazardous asteroid (PHA) is a space rock that could come within eight million kilometres of Earth and has a diameter greater than about 140 m.
Detection problems:
- Most PHAs are quite small.
- They are extremely faint (> 22nd magnitude)
- They are travelling quite fast.
- Telescopes can detect them only in clear skies at night
- Telescopes can scan only a small portion of the sky at a time
What's happening now
- An expanding army of home astronomers is covering more of the sky with telescopes that can detect fast-moving objects of magnitude 23.
- Scientists are continually developing wider-angle telescopes that can detect fainter objects.
- In the planning is a space-based telescope that can detect PHAs as far away as the Asteroid Belt.