Astronomy/orbit insertion

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Expert: - 5/6/2004


Question
Hello Tom, I've heard it's very difficult to send a probe/satellite towards Mercury (or any inward object, say the Sun itself). I cannot quote a specific source, but I seem to recall hearing this on some program on TV, that the energry expenditure is excessively great to reach an inward planet versus an outer.  I cannot see why. Ok, the Earth is spinning counter-clockwise. So a rocket can simply wait for the correct alignment and simply shoot off towards the center of the solar system and even get a bit of assist from the gravity of the Sun, no?

Thank you
Tim

Answer
Yes Tim,
What you read is exactly true; it's much more difficult...
read that more energy expenditure....to go inward than
outward.  It's not our rotation, its our much slower
revolution velocity around the sun....18.5 miles per second..
compared to Venus and Mercury's orbital speed,
that is the big factor.  Recall that Kepler's 2nd law...
with an elliptical orbit, equal areas are swept out in equal time intervals....
ie. the closer a body is, the faster it orbits the primary
body (Mercury around the sun, in this case).

So while its true that the sun's gravity helps, that would
be an inward pull, 90 degrees to where you want to go
IF you want to intercept an orbiting planet; one still
needs to accelerate to the orbital speed of Mercury's
orbital velocity around the sun...which I'm not sure what it is, but it would be much faster than the Earth's. Thus more energy is required.

Going outbound, the trick is to actually slow down from
18.5 miles per second to intercept, which is much easier to do.  Recall that Mercury orbits the sun in only 88 days,
compared to Pluto's 248 years.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA

Astronomy

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Tom Whiting

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Astronomy has been my hobby/pasttime for over 50 years.  Currently own 3 telescopes, the largest of which is a 30 inch Newtonian truss Dob that is portable.I taught Astronomy/Meteorology at the University Level for 13 years before retiring in 1995. Being retired and home most of the time, I am able to answer all questions relatively quickly, unless it's a new moon weekend with good observing conditions.  No astrology questions please, or questions about alleged UFO picture identifications.

Experience

Experience: Astronomy has been my hobby and study for over 50 years. We currently now own a 30 inch portable telescope (Updated - Pennsylvania`s largest portable telescope). It can be seen on our website at:http://www.velocity.net/~bwhiting and also attend several regional starparties during the year, and have been on 5 total solar eclipse expeditions.

Organizations: President, Erie County Mobile Observers Group for over 15 years.

Publications: Wrote the "Over Erie Skies" newspaper article in our local newspaper for 11 years (1975-86).

Education: Masters Degree- Taught at the University level for 13 years. Retired 20 years -USAF Pilot - KC-135 with 180 combat missions;  Also Eagle Scout, Philmont staff 2 Yrs, Order of Arrow Lodge Chief, Ham Radio (inactive).

Awards: two discoveries: The mini-coathanger asterism in Ursa Minor (the little dipper) And the mini-ladle- another asterism in the bowl of Ursa Minor. Clients: Currently President of the ECMOG as mentioned above.

Education/Credentials
BS  Metallurgical Engineering Grove City College, PAMaster's Degree, Gannon University, Erie, PA Also retired USAF pilot, 20 years.

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