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lrh_geh
2012-Apr-02, 06:05 PM
What will be the velocity (relative to Earth) when asteroid 2012 DA14 passes near Earth in Feb. 2013? Does DA14 orbit the sun in the same direction as Earth, or in the opposite direction? Could its relative velocity be less than Earth's escape velocity, causing it to veer into collision?

lrh_geh

antoniseb
2012-Apr-02, 06:24 PM
It looks like it will be in the outbound part of its orbit around the Sun, and will pass by the Earth at about 50,000 miles/hr. It is impossible for its relative velocity to be slow enough to cause capture or collision for this, or any asteroid. It orbits in about the same direction as the Earth.

Rhaedas
2012-Apr-02, 06:32 PM
It's a shame its delta-v is so high. A slower moving NEO would have more potential for some type of mission, even if it's just a thrown together one.

lrh_geh
2012-Apr-02, 09:31 PM
50,000 mile/hr / 3600 secs/hr = 13.89 miles/sec. Why is its relative velocity so high? Its period is only one day longer, so I presumed that its orbital velocity would be very nearly the same as Earth's. Its orbit is more eccentric, and not exactly in the same plane as Earth's, but still ....

lrh_geh

It looks like it will be in the outbound part of its orbit around the Sun, and will pass by the Earth at about 50,000 miles/hr. It is impossible for its relative velocity to be slow enough to cause capture or collision for this, or any asteroid. It orbits in about the same direction as the Earth.

Chew
2012-Apr-03, 06:43 AM
50,000 mile/hr / 3600 secs/hr = 13.89 miles/sec. Why is its relative velocity so high?

It's not that high. It's relative velocity at closest approach will be 7.17 km/s (16,040 mph). See NEO Earth Close-Approaches table. (http://neo.jpl.nasa.gov/cgi-bin/neo_ca?type=NEO&hmax=all&sort=date&sdir=ASC&tlim=far_future&dmax=5LD&max_rows=20&action=Display+Table&show=1)

Most of the relative velocity is from being out of plane with the Earth's orbit. It is true DA14's orbit is very similar to Earth's when viewed from above. You are thinking in two dimensions, padawan. Go to the JPL Small-Body Database Browser (http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2012%20DA14;orb=1;cov=0;log=0;cad=1# orb)
and rotate the view until you are looking at the orbits edge on and you will understand better.

lrh_geh
2012-Apr-03, 12:35 PM
It's not that high. Its relative velocity at closest approach will be 7.17 km/s (16,040 mph).

OK. Thanks. So, its delta-V is roughly twice the velocity required to maintain a geosynchronous circular orbit, but less than the ground-to-orbit escape velocity. The theoretical impact velocity figures I have seen evidently include both the orbital delta-V and the added velocity for a fall from infinity.

Rhaedas
2012-Apr-03, 02:23 PM
A rendezvous mission would be difficult, but what if we put a probe right where the asteroid will pass. Certainly there's a lot we could learn from the impact and its debris field, and how much it changes the orbit.

NEOWatcher
2012-Apr-03, 02:27 PM
A rendezvous mission would be difficult, but what if we put a probe right where the asteroid will pass. Certainly there's a lot we could learn from the impact and its debris field, and how much it changes the orbit.
Perhaps you are at a loss for words, but I think you mean to intercept rather than rendezvous.

antoniseb
2012-Apr-03, 02:32 PM
A rendezvous mission would be difficult, but what if we put a probe right where the asteroid will pass. Certainly there's a lot we could learn from the impact and its debris field, and how much it changes the orbit.
I imagine that getting such a mission together in such a short time would be expensive if possible at all, but more broadly, this kind of opportunity comes fairly often, so building and designing a mission to a yet undetermined close-flyby asteroid seems workable. I don't know if it would save any money... but it would make the mission time-line very short.

Rhaedas
2012-Apr-03, 02:49 PM
Perhaps you are at a loss for words, but I think you mean to intercept rather than rendezvous.

I guess rendezvous is only used for ships in particular (Rama was a ship, after all). Intercept would be correct, but I was thinking a more open ended type of mission that matching velocities would allow, such as an orbit, landing, etc. Encounter maybe?

Chew
2012-Apr-03, 05:30 PM
OK. Thanks. So, its delta-V

I understand your use of "delta-V" here to mean "difference in velocity" but please be aware delta-V has a different meaning in orbital mechanics and astrodynamics. (http://en.wikipedia.org/wiki/Delta-V) When people who know orbital mechanics speak of delta-v it is always in the context of propulsion and changing orbits. A better term for what you want to describe is "relative velocity".

is roughly twice the velocity required to maintain a geosynchronous circular orbit, but less than the ground-to-orbit escape velocity.

Yes, but it's the escape velocity at the distance of its closest approach that matters. The escape velocity at 2014 DA14's nominal approach distance of 57,787 km is only 3.71 km/s.

The theoretical impact velocity figures I have seen evidently include both the orbital delta-V and the added velocity for a fall from infinity.

The infinite velocity is the velocity the asteroid would have at its closest approach to Earth if the Earth had no mass, i.e. the Earth were not pulling it in faster. The infinite velocity is used to calculate the impact velocity if it were in fact to hit us. It is analogous to computing the length of the hypotenuse of a right triangle, with the Earth's escape velocity (11.14 km/s) as one leg, the infinite velocity as the other leg, and the impact velocity as the hypotenuse. Vimpact˛ = Vescape velocity˛ + Vinfinite˛

The relative velocity is the velocity the asteroid will have at closest approach and includes the gravitational pull of the Earth. You will notice the relative velocity is slightly higher than the infinite velocity and the closer the approach to the Earth the bigger the difference in velocities. The NEO close approach table shows DA14 will make the closest approach of all the asteroids listed and it also has the largest increase in speed.

NEOWatcher
2012-Apr-03, 08:13 PM
I guess rendezvous is only used for ships in particular (Rama was a ship, after all). Intercept would be correct, but I was thinking a more open ended type of mission that matching velocities would allow, such as an orbit, landing, etc. Encounter maybe?
Actually; I didn't word my response too well either.
I was thinking "intercept" instead of "put a probe right there".
When I said rendezvous I was kind of rewording what you said rather than explaining my substitution.

I think of rendezvous as being able to stay with an object instead of just whiz by or crash into which I apply to the more broader "intercept".