QUESTION:
I am a Technology Education teacher at a middle school. My class and I were
trying to figure out how long, exactly, the trip to Mars would take (I know
it is supposed to land on Independence Day, but we were interested in
finding out the exact time depending on Pathfinder's speed and the distance
it must travel). We began by dividing the distance that was given on your
website (312 million miles) by the speed that was given in our local
newspaper (23,000 miles per hour). We came up with 13565.2173913 hours.
Dividing this by 24 (hours in a day) we came up with 565.2173913043 days.
This can't be right. What are we doing wrong? There are only approximately
210 days between now and 7/4/97.

ANSWER from Pieter Kallemeyn:
The 23,000 mph is the speed relative to Earth (at the time of third-stage
separation), but remember that that the Earth itself is travelling round the
Sun at around 67,000 mph, so we are actually travelling much faster than
23,000 mph relative to the Sun.

The latest navigation solution shows our velocity with respect to the Sun is
33.516 km/sec, or around 75,000 miles per hour! Dividing that into 310
million miles will then give you 172 days, or less than 6 months. The
remaining error from that calculation is caused by the fact that the
Sun-relative velocity decreases (from 75,000 mph to 47,640 mph) as we leave
Earth and approach Mars, so the real travel time is around 211 days.

-- Pieter Kallemeyn, Mars Pathfinder Navigation Team Chief