Estimate the Earth-Mars Data Link a 10,000-Person Colony Needs for Gmail Alone
Company: Google
Role: Product Manager
Category: Product / Decision Making
Difficulty: medium
Interview Round: Other
A colony of 10,000 people lives on Mars, and Gmail is the only way any of them can communicate with Earth. Estimate how much bandwidth the link between Mars and Earth needs to support them.
```hint Build the day before the second
Model one colonist's daily email in each direction, split by the kinds of messages that drive size. Only then turn the colony's daily total into a data rate and think about when the link can actually be used.
```
### Clarifying Questions
- Should the estimate be the average data rate over a day, or the capacity the link must provide during the hours it can actually connect, and for which direction?
- Can colonists send and receive attachments such as photos, documents and video clips, and do Gmail's usual size limits apply?
- Does mail between two colonists cross the link to Earth, or is there a mail server on Mars that delivers it locally?
- Since email is their only channel, should the estimate include work mail, newsletters and news delivered by email, or only personal messages?
### What a Strong Answer Covers
- A clear statement of what is being sized: direction, average rate versus provisioned capacity, and units.
- A per-person daily traffic model split by message type and direction, with every assumption labeled and adjusted for email being the colony's only channel to Earth.
- Correct conversion from daily bytes to bits per second, including encoding and protocol overhead.
- Mars-specific effects on sizing: limited hours of link contact, long signal delay, changing distance between the planets and periodic blackouts.
- A sanity check, the biggest sensitivity in the model, and product levers that reduce the requirement.
### Follow-up Questions
- Roughly every two years the Sun sits between Earth and Mars and the link goes silent for about two weeks. How does that change what you would build and what colonists experience?
- If the link can provide only a tenth of your estimate, which Gmail features or policies would you change first, and in what order?
- How would your answer change if every colonist's Gmail had to sync directly with servers on Earth?
Overview: An estimation question asking how much bandwidth the Earth-Mars link needs when Gmail is the only way a 10,000-person Mars colony can communicate with Earth. It tests building a per-person email traffic model, converting daily volume into a bit rate, and adjusting for link windows, signal delay and attachment-heavy usage.