[meteorite-list] Interview: Don Yeomans

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Jan 6 13:32:41 2006
Message-ID: <200601061831.k06IV6O06329_at_zagami.jpl.nasa.gov>

http://www.earthsky.org/shows/astrophysics_interviews.php?id=49241

Interview: Don Yeomans
Earth & Sky
January 2006

Don Yeomans is manager of NASA's Near Earth Object Program Office,
whose purpose is coordinating NASA-sponsored efforts to detect,
track and characterize potentially hazardous asteroids and comets
that could approach the Earth. Yeomans spoke with Earth and Sky's
Jorge Salazar about how scientists are anticipating the possibility
of a large asteroid colliding with Earth.

--------------------------------------------------------------------

Salazar: Thank you for speaking with me today. I noticed (Nov. 2005) on
the NASA website for the Near Earth Object Program that there's quite a
bit of scientific interest recently in asteroid Itokawa.

Yeomans: Itokawa is a fairly small near-Earth object that's currently
under study by a Japanese spacecraft, Hiyabusa, which is Japanese for
vulcan. And the spacecraft is in resonance with the asteroid right now.
Although it's not well-known in the U.S., it's rather an exciting
mission, and there is some U.S. participation.

The asteroid is rotating underneath the spacecraft. The spacecraft is
sitting on a line between the sun and the asteroid, and it sort of
hovers there as the asteroid rotates underneath it. So we've been as
close as a couple of miles to the asteroid already, and the images have
built up a map of the entire surface -- we actually have a nice movie of
the asteroid rotating underneath it. Later on, in November, the
spacecraft will come down to the surface, very slowly, and it has a
collection horn of about a meter long or so, and as the collection horn
touches the asteroid's surface, a tantalum pellet is fired into the
surface, and the ejective from the small crater that pellet creates is
captured by this horn, and the sample return capsule captures that
sample, and we come down once again on another location of the asteroid
and do the same thing again, fire another pellet into the asteroid, and
capture that ejecta in a second chamber in the sample return capsule.

In early December of this year, the spacecraft will leave the asteroid
and come back to Earth and drop off the sample return capsule into the
Earth's atmosphere, and it will be collected in the Australian outback.
And so, for the first time, we'll have in our hands, so to speak, in our
laboratories, a sample from an asteroid that can be subjected to
chemical, elemental analysis, of how much iron there is in this sample,
how much calcium, how much magnesium, etc...

The holy grail of asteroid science is to find out what meteorites have
already landed on Earth's surface are most similar to this type of
asteroid. So, with an actual sample from this asteroid, we can say, all
right, this sample most resembles this type of meteorite that is in our
collections. So thereafter, the goal is to say, for all other asteroid
types that are similar to Itokawa, we already know what it is made of,
because we've had this link forged between the asteroid's surface makeup
and a meteorite that's already in our collection. So that's the basic
science involved with the mission. It's called a technology test
mission. They're testing their ion drive engines, they're testing the
sampling technique, they tested the ability to approach the asteroid in
an autonomous fashion using optical navigation images on board the
spacecraft. And so they have these technology tests that they're going
through. And that is the main purpose of the mission. And so the
science, and the return of the sample is actually secondary in terms of
their goals, but it's very much primary to the scientific community,
because it is indeed the first asteroid sample return.

Salazar: What are the scientific goals of the Japanese spacecraft Hiyabusa?

Yeomans: There are several science goals for the mission. One, that we
mentioned, was finding out what type of meteorite on the Earth's surface
is associated with this particular asteroid. Another would be for the
future, what sort of asteroids would be the most valuable to obtain
mineral wealth that is certainly there. When we start building
structures in space, we'll want to know which asteroids have the metals
and the materials that we need to build interplanetary structures. And
finally, we need to know the enemy, because some of these objects will
indeed hit the Earth from time to time, and since their compositions run
the gamut from ex-cometary fluff balls to fractured rock, to solid rock,
to slabs of solid iron -- asteroids can be any on of those categories --
we'll need to know which asteroids are which in terms of their
structure, because if we do find one that has an Earth-threatening
trajectory, we'll need to know what it's made of, how it's put together,
before we engineer an effective mechanism for deflecting it.

Salazar: What are some of the asteroids that pose a threat to life here
on Earth? There's been quite a bit written about asteroid Apophis.

Yeomans: Right -- you mentioned Apophis, which is an asteroid about 300
meters in size that will get very close to the Earth on April 13, 2029
<http://science.nasa.gov/headlines/y2005/13may_2004mn4.htm>. In fact it
will get beneath the geosynchronous satellites -- the same satellites
that are probably used to beam your radio signals to your listeners.
Certainly television and Earth surveying satellites are at
geosynchronous orbits, and this asteroid will get beneath them and
become a third magnitude naked-eye object for a period of time. So
that's kind of exciting. But -- it won't hit the Earth.

If it had, or if it were predicted to hit the Earth, there are a number
of ways that we could deal with it. The key is to find these things
early. That is the whole goal of the NASA's Near Earth Object program is
to bring under contract observatories such as NEAT here at JPL, to find
these objects early enough so that we can use the observations that they
provide to determine their orbits, predict where they'll be in the future.

For example, in this case we know that it's going to make a close earth
approach. And if we can find them all, and predict where they'll be in
the near future, we have an excellent opportunity to deflect them should
we find one on an Earth-threatening trajectory. So it's just a question
of applying a small change in the asteroid's velocity now so that in
10-20-30 years it will miss the Earth. On the other hand, if we find one
on an Earth-threatening trajectory, say that's going to occur within a
year, there's not a whole lot that we can do about it, frankly, because
it would require too much energy to deflect it in that short a time. But
if we can find them 10, 20, 30, 40 years in advance, then it's
relatively simple -- well not simple, but it can be done.

For example, with the Deep Impact spacecraft we just showed, rather
dramatically, that we can autonomously collide a comet with a fairly
heavy spacecraft, and so you can imagine that a spacecraft like that can
be used to ram an asteroid that was on an Earth-threatening trajectory,
and as long as it was done a few decades in advance of the predicted
impact, it only takes a fraction of a centimeter per second to change
that object's velocity, and it can be done with a fairly modest-sized
spacecraft.

On the other hand, if you wait and don't discover that object for a year
or so prior to impact itself, then you need vastly more energy to change
its path so that it misses the Earth. And if you discover it only a few
months in advance, you can't blow it up with some sort of a nuclear
weapon, because now you've got a shotgun effect instead of a single
bullet, which could be worse. You could use a nuclear weapon, I suppose,
if you found the object early enough, because you could disperse the
pieces of the object so that they would all miss the Earth.

However, there are many more green techniques that could be used to
deflect an asteroid. You could send up a solar mirror, for example, that
could concentrate sunlight on one side of the asteroid head that rotated
underneath you, so that you would oblate the front side of the asteroid
and introduce a rocket-like thrust in the opposite direction. And, over
time, that would change the asteroid's position enough so that it would
miss the Earth. You could, if you wish, could mount a shuttle engine on
the asteroid. You could thrust and change its trajectory enough that
way. But again, you would need several years advance notice. And again,
that is the whole goal of NASA's Near Earth Object program, to find
these objects soon enough that current technology could deal with them.

Salazar: Since Apophis isn't going to hit the Earth in 2029, are there
any other objects that scientists are concerned that might?

Yeomans: Well, actually, that object is of concern. Not in 2029, when it
makes its close approach, but -- once you do have a close approach like
that, it makes computing the subsequent orbital position of that object
more difficult. So, if that object passes through a 600 meter sized
keyhole, in 2029, that is, a location in space that is only 600 meters
wide, if it passes through that, it will indeed hit the Earth in 2036.
Now the chances of it actually passing through this 600 meter sized
keyhole in 2029 is extremely low, and we'll know whether it will or
won't probably next year when we get additional radar data in May of
2006. And if we can't rule it our then, there's an additional radar
opportunity in 2013 that will almost certainly rule out this
possibility. In the unlikely possibility that we don't rule it out in
2013, there's still time to mount a mission to deal with it. This object
illustrates the point rather well. It was discovered early, so we have
lots of options. The first is to wait until 2013, when this whole thing
will almost certainly go away.

Salazar: Could you tell us a little bit more about what scientists know
about Apophis?

Yeomans: Well, as I'd noted, we have optical and radar observations of
the object. So we have a fairly good idea of its size -- it's about 320
meters in size, a decent size but not a real large asteroid. There has
been spectral observations made, and it seems to be similar to what we
call an ordinary condroid meteorite, which means that it's probably
silicate rock for the most part. There isn't yet a good radar shape
model for it yet, though. We know its basic size, and likely structure,
although we don't know the structure for certain, and wouldn't, unless
we had a spacecraft in orbit around it.

Again, if next year's radar data does not eliminate the threat in 2036,
and again in 2013, if we can't eliminate the threat with additional
observations that would show that the object was not going to enter this
600 meter sized keyhole, then there's still time that would first
investigate the surface characteristics, the size, the mass, the
chemical composition of the object. And then a subsequent mission could
be used to deflect it, either with a kinetic impact, or perhaps with
some other technique that actually rendezvoused with the object and
pushed it.

Salazar: Could you detail a bit how asteroids are studied?

Yeomans: Usually when we have observations of objects, they're
telescopic, visual observations, that is the object is located in the
sky with respect to background stars that happen to be in the
neighborhood. So you determine the right ascension and declination,
which, you know, is similar to longitude and latitude here on Earth. So
you have an angular position of where the object is in the sky. And the
third component is the distance between you, the observer, and the
object. That is provided by radar. You can actually use one of two
planetary radars -- one in Goldstone, in the Mojave desert in
California, and the other in Arecebo in Puerto Rico. And what is done
with these giant radio antennas is to send a pulsed beam of radiation to
the asteroid, it bounces off the asteroid and comes back and is
received. And by noting how long it takes for that signal to leave your
antenna, bounce off the asteroid, and come back, and be received by your
antenna, by measuring that time, and you know the speed of light, that
gives you the distance from your antenna to that asteroid. And that
gives you the third component. You already have the other two on the
plane of sky from your angular observations done with telescopes. And
then you have the distance, provided by the radar. So that's an
extremely powerful data type. And so radar data can give us a much
better understanding of this object's orbit than would be otherwise
available only from the more traditional optical observations of the object.

Salazar: In the event that an large asteroid is found to be headed for
Earth, what can be done about it to avoid disaster?

Yeomans: Well, there hasn't been a great deal of work done on asteroid
mitigation techniques. Some papers have been done though. But basically,
if you find the object soon enough -- and again, that's NASA's goal --
then you have a number of technologies available to you. The easiest is
to simply run into it, like Deep Impact, and slow it down, or speed it
up, depending on which direction you're headed from, and then monitor
its motion. And if the first impact didn't do it, then you'd do another
one and monitor its motion. And so if you had plenty of time, then the
kinetic impact, or impacters would probably be the easiest and the cheapest.

If you wanted to actually rendezvous with the object, that would take a
longer time, because you'd have to match positions and velocities with
the asteroid. And if you wish to use this solar concentrator that we
talked about, that would require a rendezvous mission. If you wish to
land on the surface and use a shuttle engine to deflect the object's
motion, that would require a rendezvous as well. So these techniques
that require a rendezvous require a lot more time, and are quite a bit
more expensive than the kinetic impacter which actually just runs into
it. Or, if you actually wanted to just use a flyby and some sort of a
nuclear device that would shatter the object -- you'd actually have to
bury the device into the surface before you shattered it. If you have
the time, the kinetic impacter is the cheapest and easiest.

Rendezvous with a solar concentrator, or rendezvous and just simply
using a spacecraft tug to push it out of the way a little bit -- that's
a concept that Rusty Shweikerk and the B612 folks put out some time ago
in Scientific American, that would work. There are other techniques
which have been suggested. You can use a mass driver, sitting on the
surface of the asteroid, that electrostatically threw rocks and material
off the surface and, the equal and opposite thrust would move the
asteroid over time. The problems with all of these techniques that
actually sit on the surface of an asteroid is that you can only do it at
certain times, because the asteroid's rotating, and you want all of the
thrust to be in the same direction. So you'd have to pulse-thrust --
every time the object rotated once, you would thrust, wait for it to
rotate to the same position, and thrust again. So using a technique that
requires landing probably doesn't make much sense. If you have a
standoff solar concentrator, the object can rotate underneath you, and
you would oblate the surface that is closest to the mirror and introduce
a thrust that is in the same direction all of the time. That would be a
cute technique. But again, that requires a rendezvous.

So, if you have the time, I think the kinetic impacter is the best way
to go. If you don't have enough time to deal with it in that way, then
you'd have to try and shatter it with some sort of an explosive device
so that the pieces would have time to disperse and miss the Earth. You
don't want to have a shotgun effect. That would require that your blast
be at least a year out. The easiest are blast techniques that would try
and shatter a small asteroid, or kinetic impacters which would try and
nudge it one way or the other, so that in 10 or 20 years, when it was
predicted to hit the Earth, it would simply be out of the way. If you
have a large asteroid that was on an Earth impact trajectory -- first of
all we would discover it decades in advance, so that we would have
plenty of time to deal with it. It's the small ones that are vastly more
numerous than the large ones, that are most likely to sneak up on us.
One of the issues that NASA is wrestling with now is, should they extend
the Near Earth Object Discovery program, like NEAT, and others, LINEAR,
LINEOS, should those programs be expanded to try and find out some of
the smaller objects as well as the larger ones. I mean we have
discovered lots of small ones, but there's a vast number of them that we
haven't. And so that's the current issue for NASA headquarters to
wrestle with.

You've got this peculiar duality, that near Earth objects, and asteroids
in particular, are the closest objects to the Earth -- I mean even
closer than the moon, and represent a danger. And at the same time,
they're also the easiest objects to land on -- some of them are easier
than the moon itself. So, while some of them are a threat, others may
well be a boon to future space exploration. We may be able to hop on
some of these objects and mine them, and use the materials that are
there to build future space habitats. So, we've got both friends and
foes in our near Earth objects.
Received on Fri 06 Jan 2006 01:31:06 PM PST