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# Re: [Phys-L] Comet Speed in What Reference Frame?

This site claims that speed as relative to the sun. It seems more likely that it’s using ICRS.

http://www.livecometdata.com/comets/67p-churyumov-gerasimenko/ <http://www.livecometdata.com/comets/67p-churyumov-gerasimenko/>

Paul

On Nov 12, 2014, at 2:41 PM, John Denker <jsd@av8n.com> wrote:

On 11/12/2014 10:33 AM, Donald Polvani wrote:
The comet landing by Philae/Rosetta today is tremendously exciting! But it
did get me wondering what reference frame is being used to measure the
comet's speed. News reports quote a comet speed of over 40,000 mph. What
reference frame is this speed measured in? If a frame centered in the
earth, wouldn't the comet's speed become a function of where the earth is in
its orbit (i.e. heading away or towards the comet) as well as where the
earth is in its daily rotational cycle (i.e. turning toward or away from the
comet? What reference frame is normally used when quoting speeds of solar
system objects and space probes?

Let's start by running down the obvious stuff, and see
what (if anything) remains.

1) The only speed that could possibly matter is the /relative/
speed between the comet and the lander. This is the only
speed I've seen quoted in ESA press releases. The touchdown
speed was planned for 1 meter per second.
http://www.esa.int/Our_Activities/Space_Science/Rosetta/Touchdown!_Rosetta_s_Philae_probe_lands_on_comet

Touchdown is a difficult maneuver, for reasons having
*nothing* to do with the 41,000 number

2) If I were tracking a comet, I would use the ICRS.
International Celestial Reference System
http://aa.usno.navy.mil/faq/docs/ICRS_doc.php

3) News reports often use the famous lets-make-stuff-up
reference frame. In most cases where I've had first-hand
knowledge of the events, I found the published reports to
be so different as to almost unrecognizable.

4) I don't know where they get the headline number 41,000 mph
http://www.lasvegassun.com/news/2014/nov/12/european-spacecraft-lands-comet-speeding-41000-mph/
You could get to 40,000 mph by rounding up from 38,000
which is what I get for the speed |v| in the ICRS frame.
You can calculate that yourself given the virial theorem
plus the position of the comet (3 AU today). In another
month it will be at 2.8 AU and very nearly 40,000 mph.

5) If this were a two-body system, it might make sense
to use the earth (or the two-body barycenter) as the
frame of reference, but it isn't so it doesn't. The
comet's motion is dominated by the gravitational field
of the sun, not the earth.

6) If you measure speed relative to the center of the
earth you can get any answer you want, because the
variation in the earth's velocity (due to annual
orbital motion) is larger than any other speed in
the problem at the moment.

7) If you measure speed relative to some chosen observatory
on the surface of the earth, you get to add in another
1000 mph of variation due to the diurnal motion.

8) If you look just at the radial component, you get
a whole different set of numbers.

A month ago the radial component of the earth minus
comet velocity might have been on the order of 40,000
mph, but now it's half that, and in another month it
will be near zero ... not that anybody cares.

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