Space Time · Guided activity · 5–10 min

Why does a reply from Voyager take so long?

Distinguish a calculated light-travel time from a reported radio contact.

Name: ____________________ Date: ____________________

Find Voyager 2. Record its Earth distance, one-way light time and position sample date. If a value is unavailable, leave it unknown.
Double the one-way time for a simplified signal round trip. This assumes an immediate reply and ignores motion during transit.
Read the DSN report separately. Record its source time and whether it reports an uplink, downlink, no contact, or unavailable data.

Explain using evidence

Does a calculated travel time prove that Voyager is communicating right now?

Record unknown values as unknown. The app uses a nearby Horizons position sample. Twice its one-way time is an estimate, not a measured command-response delay or communication schedule.

Teacher notes & answer discussion

Have learners label each entry calculated, reported or unknown. Compare timestamps before comparing numbers. Accept unknown as a valid result when either service is unavailable; do not supply fictional replacement readings.

No. Distance divided by light speed estimates propagation time. A DSN snapshot reports ground-station signal activity at its own timestamp. Neither a missing entry nor an old report establishes that the spacecraft has failed.

Sources

NASA/JPL DSN Now and data limitationshttps://eyes.nasa.gov/apps/dsn-now/dsn.html
JPL Horizons documentationhttps://ssd.jpl.nasa.gov/horizons/manual.html

Editorial activity reviewed September 21, 2026. Suggested duration is approximate. No curriculum certification is claimed.