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Explorer1
I wonder what happens if you dial that number at the bottom...
Phil Stooke
I used to do that! For Voyager, not Mariner 10, but it was the same thing. Don Bane from the Public Affairs Office would record a phone message, updated every week during slow times, and daily or more often for busy times like a flyby. It was the equivalent of checking into UMSF every morning.

I'm at LPSC this week... I'll post a few pics when I get back... but regarding Mercury, I asked Sean Solomon if his spacecraft was going to impact on Mercury at the end of the mission so I could have a point to plot on a map. I think he preferred not to think about that just yet... but yes, it will. Still not clear to me if it will be a controlled impact, or just left to strike at an unknown location.

Phil
kwp
QUOTE (Phil Stooke @ Mar 9 2011, 05:10 AM) *
I asked Sean Solomon if his spacecraft was going to impact on Mercury at the end of the mission so I could have a point to plot on a map. I think he preferred not to think about that just yet... but yes, it will.


Hadn't occurred to me before, Phil, quite how morbid your interests can be. With regard to the demise of MESSENGER, do we know enough about Mercury's gravitational field to be able to estimate how stable the MESSENGER orbit will be? I assume Mercury's gravity is more homogeneous than the Moon's, but perhaps that is counteracted by Solar effects and light pressure?

-Kevin
nprev
I'd hardly call that a "morbid" interest, kwp. Phil has been diligently mapping space hardware/man-made crash sites on the Moon, and high-speed impacts are obviously of scientific interest since they frequently expose fresh subsurface material. Knowing Messenger's final impact region would greatly facilitate spotting the nice fresh crater it will make from some future Mercury orbiter (Bepi-Columbo?)
Hungry4info
QUOTE (kwp @ Mar 9 2011, 08:22 PM) *
...do we know enough about Mercury's gravitational field to be able to estimate how stable the MESSENGER orbit will be?
I'm almost certain we know next to nothing about it. All the flybys so far have mostly been equatorial to stay in the plane of Mercury's orbit for alignment with another flyby. And of course there's only been a handfull of flybys.
tasp
The Pioneer Venus craft experienced large perturbations from it's proximity to the sun at Venus distance. That affect will be worse at Mercury, and with the Mercurian orbital eccentricity, the effect will be variable over time, too.


(I am deliberately neglecting Messenger altitude and eccentricity, it is still early here)
DFinfrock
QUOTE (nprev @ Mar 10 2011, 02:41 AM) *
Knowing Messenger's final impact region would greatly facilitate spotting the nice fresh crater it will make from some future Mercury orbiter (Bepi-Columbo?)


I wonder if Messenger can survive in orbit long enough to wait until Bepi-Columbo's arrival, before taking that plunge to the surface. We could get a lot more science out of Messenger's demise, if it could remain in orbit that long, and allow Bepi to watch from orbit as the new crater forms.
gndonald
She's getting closer


kwan3217
QUOTE (Hungry4info @ Mar 10 2011, 01:32 AM) *
I'm almost certain we know next to nothing about it. All the flybys so far have mostly been equatorial to stay in the plane of Mercury's orbit for alignment with another flyby. And of course there's only been a handfull of flybys.


From http://messenger.jhuapl.edu/orbit_insertio...tionkeeping.htm

QUOTE
...and staying there

After MESSENGER arrives in the primary science orbit, small forces, such as solar gravity – the gravitational attraction of the Sun - slowly change the spacecraft's orbit. Although these small forces have little effect on MESSENGER's 12-hour orbit period, they can increase the spacecraft's minimum altitude, orbit inclination, and latitude of the surface point below MESSENGER's minimum altitude. Left uncorrected, the increase in the spacecraft's minimum altitude would prevent satisfactory completion of several science goals.

To keep the spacecraft’s minimum altitude below 500 kilometers (310 miles), propulsive maneuvers must occur at least once every Mercury year - one complete revolution around the Sun, or 88 Earth days. The first, third, and fifth maneuvers after Mercury orbit insertion will occur at the farthest orbital distance from Mercury where a minimum amount of propellant will slow the spacecraft just enough to lower the minimum altitude to 200 kilometers (124 miles). The act of lowering the spacecraft’s altitude in this way has an unavoidable side effect of also lowering orbit period by 13-15 minutes. The second and fourth maneuvers after orbit insertion will increase the orbit period back to about 12 hours by speeding up the spacecraft near its closest distance from Mercury. Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these maneuvers is limited to a few days when Mercury is either near the same point in its orbit as it was during Mercury orbit insertion or near the point where Mercury is on the opposite side of the Sun from that for orbit insertion.


Since the spacecraft will be in a long equatorial orbit, it will spend most of its time relatively far from Mercury, where the local lumpiness in the gravity field matters less and the gravity from the Sun and planets matter more. This is how the station-keeping maneuvers can be planned before the gravity field is mapped out. Of course if the gravity field turns out to be much much different from the expectation, they may have to change their plans, but I am sure that mission ops is flexible enough to handle it.
As old as Voyager
QUOTE (DFinfrock @ Mar 11 2011, 12:59 AM) *
I wonder if Messenger can survive in orbit long enough to wait until Bepi-Columbo's arrival, before taking that plunge to the surface. We could get a lot more science out of Messenger's demise, if it could remain in orbit that long, and allow Bepi to watch from orbit as the new crater forms.


About 5 years ago I proposed to Paul Helfenstein (Cassini science team member) that at the end of its useful life Cassini could use its remaining fuel to leave Saturn orbit and drop Sunwards to impact on Mercury; the impact to then be observed by Bepi-Columbo. He was interested but I doubt it'll happen.
Phil Stooke
Ingenious idea but impossible! Remember, we didn't have a rocket big enough to get Cassini to Saturn without several gravity assists to help it, so the reverse is bound to be impossible with a little bit of residual fuel. And Cassini's fate is decided - burn up in Saturn's atmosphere.

Phil
centsworth_II
QUOTE (Phil Stooke @ Mar 13 2011, 07:16 AM) *
Ingenious idea but impossible!...

Yeah. I know next to nothing about the mechanics of getting from one planet to another but look at how 'hard' it's been getting Messenger from Earth to Mercury. I don't see why it would be any easier to get from Saturn to Mercury and with a craft not designed to do that.
nprev
If I understand things correctly (and I probably don't), it might actually be easier in terms of delta-V to crash into Mercury from the outer Solar System. Saturn's heliocentric orbital velocity is much lower than that of the Earth, so presumably that means less thrust would be required to negate it & 'fall' into the inner system.

However, we're still probably talking about a change in velocity of several (if not tens) of km/sec, plus escaping from Saturn orbit. I doubt that Cassini could have done this even if it was fully fueled at the beginning of the maneuver.
Explorer1
The sun-grazer and long-period comets are an extreme example of this right? A tiny nudge in the Oort cloud is more than enough to send them in a more or less straight line sunward.
nprev
Yeah, basically. The stuff way out there is barely moving in comparison to the planets; probably doesn't take much to negate their orbital motion at all (e.g., gravitational nudges from passing stars over long periods of time, perhaps occasional outgassing from the cometary bodies themselves?)
siravan
If I have calculated correctly, you need a delta-V of 5.4 km/s to go from Saturn's orbit into a Hohmann transfer orbit intersecting Mercury. Dawn could have done it (ignoring distance from Sun issue), but I doubt if Cassini ever had that much of delta-V.
MarkG
At the present time (this Sunday Evening), Mercury would appear from Messenger to be about the same size as the Moon from Earth, with the Sun looming 3 times that diameter.
I think Messenger must halve its current distance to Mercury to enter the Hill Sphere... Soon!
dmuller
The cheapest (in terms of energy) way to get a spacecraft very close to, or into, the Sun is indeed a Jupiter flyby. Scientifically not very interesting because the spacecraft would not spend much time near the Sun during periapsis. See the Solar Probe Plus trajectory options at

http://solarprobe.jhuapl.edu/mission/docs/...018missions.pdf

pages 3 and 4. The Jupiter flyby option requires by far the least C3.
Astro0
Not far now! smile.gif
Click to view attachment

BTW - I love Eyes on the Solar System. Best space Outreach tool ever!
mchan
The orientation of the day side is to the right vs. to the left in Where is Messenger? page on the Messenger website. I had wondered about the latter since Messenger is ahead of Mercury in its orbit waiting for the planet to "catch up" to it. EOTSS appears to have the view in accord with the convention of North pointing up.
djellison
QUOTE (mchan @ Mar 14 2011, 08:43 PM) *
EOTSS appears to have the view in accord with the convention of North pointing up.


You could have north any way you want smile.gif
MarkG
QUOTE (mchan @ Mar 14 2011, 09:43 PM) *
The orientation of the day side is to the right vs. to the left in Where is Messenger? page on the Messenger website. I had wondered about the latter since Messenger is ahead of Mercury in its orbit waiting for the planet to "catch up" to it. EOTSS appears to have the view in accord with the convention of North pointing up.


Actually, Messenger is catching up with Mercury, with higher ellipticity in its current (not for long!) orbit, Messenger's speed at perihelion is greater than Mercury's.
ElkGroveDan
QUOTE (Astro0 @ Mar 14 2011, 09:16 PM) *
Not far now! smile.gif

If it were green, Messenger would look just like that little android character that keeps popping up on my new phone.
mchan
QUOTE (MarkG @ Mar 15 2011, 07:05 AM) *
Actually, Messenger is catching up with Mercury, with higher ellipticity in its current (not for long!) orbit, Messenger's speed at perihelion is greater than Mercury's.

Agree Messenger velocity at perihelion is higher than Mercury. But, it appears to me that Messenger's orbit angular velocity is less than that of Mercury at the rendezvous. The Mercury flybys and DSM burns are tailored to put Mercury and Messenger in near-resonant orbits with each flyby and DSM burn reducing the ratio of the resonance. After the last flyby, Mercury completes 6 orbits and Messenger completes 5 orbits before the rendezvous for the orbit insertion burn. So it appears to me that Mercury is catching up with Messenger.

Would someone who knows the astrodynamics please correct this?
nprev
A minor milestone just occurred to me: Not only will Messenger become the first spacecraft to orbit Mercury, but for the first time we will have orbited every terrestrial planet in the Solar System...in fact, we will have active spacecraft orbiting every major body in the inner Solar System.

Maybe that's not such a minor milestone, actually... smile.gif ...wow!
Ron Hobbs
nprev,

The milestone is that every planet known to the ancients, all the classical planets, will now have been the host of an orbiter from Earth. I think this is a very significant milestone. I find it interesting that Mercury was the last of the classical planets to be explored by an orbiter.

We are nearing the end of the initial reconnaissance of the Solar System. What a time to be alive!

Ron
Holder of the Two Leashes
QUOTE (mchan @ Mar 15 2011, 09:54 PM) *
Agree Messenger velocity at perihelion is higher than Mercury. But...


Planet and spacecraft are both pretty close to perihelion, Mercury already overtook and briefly passed MESSENGER, and now the probe is faster and closing in.
nprev
QUOTE (Ron Hobbs @ Mar 15 2011, 08:44 PM) *
We are nearing the end of the initial reconnaissance of the Solar System. What a time to be alive!


Very true. It took us something like 200,000 years to do that for Earth alone...around 60 years to do the same for the rest of the Solar System.

We make progress of sorts. wink.gif
nprev
And apologies for those who have seen this before, but Ron reminded me just how significant Messenger's impending detailed of exploration of Mercury really is in the Big Picture; here's a column I did once upon a time for Rui Borges' spacEurope blog:

http://spaceurope.blogspot.com/2008/05/map...-previsich.html
tasp
nprev:

The galaxy beckons.


wink.gif
Greg Hullender
QUOTE (mchan @ Mar 15 2011, 08:54 PM) *
Agree Messenger velocity at perihelion is higher than Mercury. But, it appears to me that Messenger's orbit angular velocity is less than that of Mercury at the rendezvous. . . .Would someone who knows the astrodynamics please correct this?

Here's a way to think about it that might be helpful. Imagine three probes. Two of them in very different circular orbits (say, one with a 1-year period and another with a 2-year period) and the third probe in an elliptical orbit that just touches the inner probe's orbit at perihelion and which just touches the outer probe's orbit at aphelion.

At perihelion, probe #3 has to be moving faster than probe #1, and at aphelion, probe #3 has to be moving slower than probe #2. I assume you already know this is because probe #3 has more total energy (kinetic + potential) than probe #1 and less total energy than probe #2.

The reason that the angular velocity as perihelion is greater than the angular velocity of probe #1 is simply because at perihelion, probe #3 is moving tangent to the orbit of probe #1. It has zero radial velocity at that point; ALL of its velocity is angular for that one instant.

--Greg
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