Pho·​bos | \ ˈfō-ˌbōs How to pronounce Phobos (audio) , -bəs, ˈfä- \

Definition of Phobos

: the larger and innermost of the two satellites of Mars with an irregular shape and a diameter of approximately 15.6 miles (26.6 km) Phobos is covered with a dusty regolith caused by continual meteoric battering. Because of the moonlet's low gravity, almost all the ejecta from these impacts flies off the surface altogether, but it goes into the same orbit around Mars that Phobos is in, and thus falls back onto Phobos in a constant rain. Phobos orbits Mars within a giant torus of this debris, like the "Peanuts" character Pigpen, who walks around inside a cloud of his own dirt. — Henry S. F. Cooper, Jr., New Yorker, 11 June 1990

Love words?

You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.

Start your free trial today and get unlimited access to America's largest dictionary, with:

  • More than 250,000 words that aren't in our free dictionary
  • Expanded definitions, etymologies, and usage notes
  • Advanced search features
  • Ad free!

First Known Use of Phobos

1878, in the meaning defined above

Keep scrolling for more

Learn More about Phobos

Time Traveler for Phobos

Time Traveler

The first known use of Phobos was in 1878

See more words from the same year

Statistics for Phobos

Cite this Entry

“Phobos.” Dictionary, Merriam-Webster, Accessed 20 Oct. 2020.

Comments on Phobos

What made you want to look up Phobos? Please tell us where you read or heard it (including the quote, if possible).


Test Your Vocabulary

Farm Idioms Quiz

  • cow coming home
  • What does 'poke' refer to in the expression 'pig in a poke'?
Spell It

Can you spell these 10 commonly misspelled words?

Syn City

Test Your Knowledge - and learn some interesting things along the way.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!