The Magic School Bus Lost in the Solar System is the fourth book in the original 1986–2021 book series. It was published in 1990.
Arnold's cousin, Janet, joins Ms. Frizzle's class for the class trip to the planetarium. But after arriving to find that planetarium is closed for repairs, the Friz decides to launch the bus into space and steers it around the solar system, visiting the moon, Mercury, Venus, and Mars before an asteroid knocks out one of the taillights.
When Ms. Frizzle leaves the bus to investigate, her tether breaks. She gets left behind in the asteroid belt, and the students visit the outer planets without her.
With Janet's quick thinking and use of supply, the class manages to turn the bus around, reconnect with Ms. Frizzle and return to the classroom, making a chart and mobile based on their discoveries.
- This book is loosely adapted into the episode "Gets Lost in Space".
- This book is featured in the MSB (Funding Credits) “Visit Your Local Library” book promo for only Seasons 1 & 2.
- In the republished version, the Bus had ZOOM! as its overhead sign.
- On page 23, Arnold thinks, “I really prefer filmstrips”. In republications, he says, "I really prefer video games", to attract a Generation Z audience.
- This book was released as an eBook on LeapFrog devices.