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algomeysa
2007-Aug-22, 03:57 PM
Apophis is the asteroid that's going to come within spittin' distance of Earth in April 2029, slightly closer than geosynchronous satellites.

Just for fun, I got the orbital elements of it from the JPL website, and was entering them into my old (2000) version of Starry Night.

My idea was, since you can shift your view to the asteroid, I could do an Earth fly-by like a rollercoaster ride.

Well, I probably should have expected this would be too much for my Starry Night program, to accurately predict this 22 years in the future.

It worked out in my simulation to coming within about 2.5 million miles of Earth, so off by about a factor of 100. And a day late, April 14th instead of April 13th.

Don't get me wrong, I'm impressed that my old Starry Night program could do this well looking 22 years into the future.

These were the numbers I used (from the JPL website):

Orbit: Near-Circular Reference Plane: Ecliptic 2000
Mean Distance (a) (in AU) 0.922261415
Eccentricity (e) 0.191059415
Inclination (i) 3.331314642
Ascending Node 204.4591523
Arg of Pericenter 126.3855713
Mean Anomaly (L) 307.3630785
Epoch (Julian Date) 2454200.5

My question is, does anyone have ballpark numbers that would semi-accurately simulate this from, oh, say, early April 2029.

My idea being, probably the Starry Night program is accurate enough to handle it within that short span if it started with good numbers.

Or maybe not; maybe gravity will affect things too much (I know that after it passes Earth, there's that uncertainty whether or not it'll pass through the "keyhole" that will cause another close call with Earth in 2036 or something).

----------------
On another topic, I'm sure this has been mentioned elsewhere on these forums, but if you're a Google Earth enthusiast: Today Google Earth issued an update that adds a "Google Sky" button to the interface. Check it out!

NEOWatcher
2007-Aug-22, 04:14 PM
...Or maybe not; maybe gravity will affect things too much (I know that after it passes Earth, there's that uncertainty whether or not it'll pass through the "keyhole" that will cause another close call with Earth in 2036 or something)...
And one of the Apollo astronauts is following this very closely, I just can't remember who.
The keyhole is in the thousands to one range if I remember correctly.

On another topic, I'm sure this has been mentioned elsewhere on these forums...
Yes it has... I'll leave the search up to you. :D

01101001
2007-Aug-22, 05:15 PM
And one of the Apollo astronauts is following this very closely, I just can't remember who.

B612 Foundation's (http://www.b612foundation.org/) Rusty Schweikart, probably. He's not alone.

NASA Update on the Spaceguard Survey (http://impact.arc.nasa.gov/news_detail.cfm?ID=162)

NEOWatcher
2007-Aug-22, 05:17 PM
B612 Foundation's (http://www.b612foundation.org/) Rusty Schweikart, probably. He's not alone...
Thank you; that's the one.
He's appeared on some documentaries lately that included that subject.

tony873004
2007-Aug-22, 05:57 PM
22 years is probably just too long to accurately propogate an orbit using Keplerian ellipses. Maybe if you grab some data for a few days before the passage you'll have better luck. Here's Apophis' data for April 10, 2029:


$$SOE
2462236.500000000 = A.D. 2029-Apr-10 00:00:00.0000 (CT)
EC= 1.922975748923725E-01 QR= 1.113956586333763E+08 IN= 3.360106355533515E+00
OM= 2.038147325122139E+02 W = 1.267408733353579E+02 Tp= 2462336.955655113328
N = 1.288703865710653E-05 MA= 2.481486413092250E+02 TA= 2.296997790230890E+02
A = 1.379167069091475E+08 AD= 1.644377551849186E+08 PR= 2.793504462729913E+07
$$EOE
************************************************** *****************************
Coordinate system description:

Ecliptic and Mean Equinox of Reference Epoch

Reference epoch: J2000.0
xy-plane: plane of the Earth's orbit at the reference epoch
x-axis : out along ascending node of instantaneous plane of the Earth's
orbit and the Earth's mean equator at the reference epoch
z-axis : perpendicular to the xy-plane in the directional (+ or -) sense
of Earth's north pole at the reference epoch.

Symbol meaning

JDCT Epoch Julian Date, Coordinate Time
EC Eccentricity, e
QR Periapsis distance, q (km)
IN Inclination w.r.t xy-plane, i (degrees)
OM Longitude of Ascending Node, OMEGA, (degrees)
W Argument of Perifocus, w (degrees)
Tp Time of periapsis (Julian day number)
N Mean motion, n (degrees/sec)
MA Mean anomaly, M (degrees)
TA True anomaly, nu (degrees)
A Semi-major axis, a (km)
AD Apoapsis distance (km)
PR Orbital period (sec)


If you want it for a different date, create an e-mail to JPL. Address it to horizons@ssd.jpl.nasa.gov . Put the word "job" (no quotes) in the subject line. In the body of the e-mail paste this:


!$$SOF
EMAIL_ADDR=''
START_TIME = '2029-Apr-10 00:00:00'
STOP_TIME = '2029-Apr-10 00:00:01'
TABLE_TYPE = 'Elements'
REF_PLANE = 'Ecliptic'
CENTER = '@010'
COMMAND= 'apophis'
!$$EOF

Change the dates to your choice, making sure that stop time is 1 second after start time. Within 1 minute, you will receive an automated e-mail with your data.

Here's my Apophis page:
http://www.orbitsimulator.com/gravity/articles/a2004mn4.html

algomeysa
2007-Aug-22, 09:20 PM
If you want it for a different date, create an e-mail to JPL.

Thanks; that's useful.

I tried the email, the start and stop time comes back "CT", is that Central Time? I'd have thought it'd be GMT, or maybe Pacific Time, since the JPL is in Pasadena. Or is it on CT because it's defaulting to Houston time?

So, I guess since the Apophis closest approach is 21:50 GMT, it's 16:50 CT, taking Daylight Savings into account (Assuming daylight savings time is still in effect for April 13th, 2029)...

01101001
2007-Aug-22, 09:35 PM
I tried the email, the start and stop time comes back "CT", is that Central Time?

Horizons (Version 3.12) (ftp://ssd.jpl.nasa.gov/pub/ssd/Horizons_doc.ps) (ftp postscript)


PURPOSE:
The Horizons On-Line Ephemeris System provides access to key solar system data and dynamic production of
highly accurate ephemerides for solar system objects. [...]


For cartesian coordinates or osculating elements tables, only CT may be used. For "observer tables", output
may be either UT or TT. To change the UT default for observer tables, append a "TT" when entering the START
time. To switch back, append a "UT" to the start time.
The three time systems are described as follows:
CT: Coordinate Time. The uniform time scale, or independent variable, of the ephemerides. Used for
cartesian and osculating element tables.
TT : Terrestrial (Dynamic) Time. Called TDT prior to 1991. Used for observer quantity tables. This is proper
time as measured by an Earth-bound observer and is directly related to atomic time, TAI. TT periodically differs from
CT by, at most, 0.002 seconds.
UT: Universal Time. This can mean one of two non-uniform time-scales based on the rotation of the Earth.
For this program, prior to 1972, UT means UT1. After 1972, UT means UTC or "Coordinated Universal Time".
Future UTC leap-seconds are not known yet, so the closest known leap-second correction is used over future timespans.

Wikipedia: Coordinate time (http://en.wikipedia.org/wiki/Coordinate_time)

No, I couldn't tell you what time is now, CT.

The documentation makes it seem pretty flexible about specifying time:


Recommended: 1997-May-5 12:30:23.3348 ( 5 MAY 1997 12:30:23.334 )
Acceptable: 1/9/96 3 12 59.2 ( 9 JAN 1996 03:13 )
1 9 96 3,12,59.2 ( 9 JAN 1996 03:13 )
2 jan 91 3:00 12.2 ( 2 JAN 1991 03:00 )
91 MAR 10 12:00:00 (10 MAR 1991 12:00 )
29 February 1975 3:00 ( 1 MAR 1975 03:00 )
10 October 29 3:58 (29 OCT 2010 03:58 )
dec 31 86 12 (31 DEC 1986 12:00 )
86-365 // 12 (31 DEC 1986 12:00 )
JUL 98 ( 1 JUL 1998 00:00 )
JD 2451545. ( 1 JAN 2000 12:00 )
JD2451545. ( 1 JAN 2000 12:00 )
278bc-jan-12 12:34 (B.C. 12 JAN 278 12:34)
AD 99-Aug-12 12:34 (A.D.12 AUG 99 12:34)
bc 278-Jan-12 12:34 (B.C. 12 JAN 278 12:34)

tony873004
2007-Aug-22, 09:40 PM
It doesn't stand for Central Time. As 01101001 points out, it means coordinate time. Just treat it like UT.

algomeysa
2007-Aug-23, 02:12 PM
It doesn't stand for Central Time. As 01101001 points out, it means coordinate time. Just treat it like UT.

So the first time I did this I was off by over 4 million kilometers.

Then I ran those numbers from JPL for Apophis at 21:50 PM UT 4/13/2029, but was still off by over 800,000 kilometers.

Then I realized; the problem wasn't Apophis, it was that by 2029 Earth wasn't where my Starry Night program thought it would be.

So I emailed the JPL Bot for the coordinates for Earth at 21:50 PM UT 4/13/2029, and with that, I created a new Earth.

---
Apophis:
Orbit: Near-Circular Reference Plane: Ecliptic 2000
Mean Distance (a) (in AU) 1.0222774071428738368017818620677
Eccentricity (e) EC= .2235291185698543
Inclination (i) IN= 3.596106338765123
Ascending Node OM= 203.7581117056388
Arg of Pericenter W = 98.20652081685415
Mean Anomaly (L) MA= 287.6151087265046
Epoch (Julian Date) 2462240.409722222
----
Doppelganger Earth (ID "399"):
Orbit: Near-Circular Reference Plane: Ecliptic 2000
Mean Distance (a) (in AU) =1.0008616769854649748893062605202
Eccentricity (e) EC= .01655832698662039
Inclination (i) IN= .004318632233069560
Ascending Node OM= 180.7209649960635
Arg of Pericenter W = 285.0302264407362
Mean Anomaly (L) MA= 96.20575330580610
Epoch (Julian Date) 2462240.409722222

With this, Apophis made its approach to Doppelganger Earth at 21:45 PM UT 4/13/2029...
And came within 38,157 kilometers.

Close enough!

I tried to make a Doppelganger Moon as well, but thus far I don't have it set up quite right...
---
Thanks for your help with this; that emailing the JPL-Bot thing is a useful thing to know.

tony873004
2007-Aug-23, 06:18 PM
You might want to try referencing the Moon from the Earth, rather than the Sun. In the e-mail, change Center='@010' to Center='@399'. And for the Moon, instead of Command = 'moon' try Command = '301'.

Here are some other id numbers:
Mercury = 199
Venus = 299
Earth = 399
Mars = 499
Jupiter = 599
Saturn = 699
Uranus = 799
Neptune = 899
Pluto = 999
Earth's first (only) moon = 301
Mars' first moon (Phobos) = 401
Mars' second moon (Deimos) = 402
Jupiter's first moon (Io) = 501
etc...

Spacecraft are available too. They have negative id numbers:

Pioneer 10 spacecraft = -23
Pioneer 11 spacecraft = -24
Lunar Prospector (LP) spacecraft = -25
Stardust spacecraft = -29
Deep Space 1 Spacecraft DS1 DS-1 = -30
Voyager 1 spacecraft = -31
Voyager 2 spacecraft = -32
Mars Express spacecraft MEX = -41
Genesis spacecraft = -47
Mars Odyssey spacecraft = -53
Ulysses spacecraft = -55
Infared Space Observatory ISO = -64
Deep Impact spacecraft = -70
Galileo spacecraft = -77
Giotto spacecraft flyby = -78
Spitzer Space Telescope = -79
Cassini spacecraft = -82
NEAR spacecraft = -93
Hayabusa spacecraft Muses-c = -130
Huygens spacecraft (Cassini mission) = -150
Chandra Observatory = -151
Nozomi spacecraft = -178
INTEGRAL spacecraft = -198
CONTOUR-A (SPACECRAFT FRAGMENT) = -204
CONTOUR-B (SPACECRAFT FRAGMENT) = -205
CONTOUR-C (SPACECRAFT FRAGMENT) = -206
Rosetta = -226
Opportunity spacecraft MER = -253
Spirit spacecraft MER = -254
Galileo Probe = -344
NEOCP J002E3 = -998

More objects here:
http://ssd.jpl.nasa.gov/data/ELEMENTS.NUMBR , http://ssd.jpl.nasa.gov/data/ELEMENTS.UNNUM , AND http://ssd.jpl.nasa.gov/data/ELEMENTS.COMET for a complete list of comets and asteroids.
http://ssd.jpl.nasa.gov/sat_eph.html

You may substitute an object's name for its id number. Sometimes this works, sometimes it doesn't. But if it doesn't work, you'll get an e-mail telling you why it didn't work, and offering suggestions. So if you want the data for Sedna, but you don't know its ID number, just try typing Sedna.

Also, if you replace the word Elements with Vectors, instead of giving you orbital elements, it gives you position and velocity vectors.

publiusr
2007-Aug-24, 06:52 PM
Rusty is featured in the latest Ad ASTRA mag. I don't know as I would agree with him. A comsat mass gravity tractor? I'm not sure I buy it.