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A long way away, but still close - Off in the distance
my journal
May 2016

The Bellinghman
Date: 2010-12-14 10:51
Subject: A long way away, but still close
Security: Public
Voyager 1 is beginning to reach the heliopause.

Voyager 1 is moving at 17 km/s - or roughly Mach 50. [*]

It's been travelling outwards for 33 years.

It's now a total of ... 16 light hours away.

The nearest star is approximately 2000 times further away.

Space. It's big.

*mobbsy rightly points out that strictly speaking Mach numbers are meaningless in vacuum. Take this as the shorthand for 'the speed that would be Mach 50 at sea level, under standard conditions'.
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User: martyn44
Date: 2010-12-14 11:47 (UTC)
Subject: (no subject)
One step at a time
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Andrew Mobbs
User: mobbsy
Date: 2010-12-14 12:35 (UTC)
Subject: (no subject)
Er... surely that Mach number is entirely wrong, and it'd be near meaningless to give a Mach number for an object in hard vacuum anyway?
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The Bellinghman
User: bellinghman
Date: 2010-12-14 12:55 (UTC)
Subject: (no subject)
I agree that giving a Mach number for something relative to the highly attenuated outer solar system is somewhat dubious. I was using the common shorthand of what Mach number that velocity would be at sea-level in Earth's atmosphere.

In fact, checking, a vehicle travelling at exactly 17 km/s, at sea level where the speed of sound is 340.3 m/s, gives me a Mach number of 49.956

Which is considerably closer than I'd have guessed.

(My rule of thumb is that the speed of sound in air at sea level is approximately 333 m/s - or rather, that 1 km/s is close enough to Mach 3 for government work. The time between lightning and thunder is about 5 seconds per mile distant.)
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Andrew Mobbs
User: mobbsy
Date: 2010-12-14 15:59 (UTC)
Subject: (no subject)
FWIW, Wikipedia gives the speed of sound in the interstellar medium as about 100km/s.
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