[meteorite-list] China Reveals Solar Sail Plan To Prevent Apophis Hitting Earth in 2036

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 19 Aug 2011 16:51:36 -0700 (PDT)
Message-ID: <201108192351.p7JNpaXv013460_at_zagami.jpl.nasa.gov>

http://www.technologyreview.com/blog/arxiv/27088/

China Reveals Solar Sail Plan To Prevent Apophis Hitting Earth in 2036
Technology Review (MIT)
August 18, 2011

A small shove could prevent a global catastrophe, according to Chinese
plans

Apophis is a 46 million tonne asteroid that will pass within a hair's
breath of Earth in 2029. However, Apophis's trajectory is likely to take
it through a region of space near Earth known as a keyhole that will
ensure the asteroid returns in 2036.

Nobody knows how close Apophis will come on that pass. But if there's a
chance of a collision, we'll have only 7 years to work out how to avoid
catastrophe.

Today, Shengping Gong and pals at Tsinghua University in Beijing say
they've come up with a plan that will ensure Apophis never returns to
Earth on this timescale .

They point out that keyholes are tiny, in this case just 600 metres
wide. So deflecting Apophis by only a small amount in the near future
will ensure it misses the keyhole and so cannot return to Earth.

There are various ways to deflect an asteroid. Gong and pals say their
preference is to use a solar sail to place a small spacecraft into a
retrograde orbit and on collision course with Apophis. The retrograde
orbit will give it an impact velocity of 90km/s which, if they do this
well enough in advance, should lead to a collision large enough to do
the trick.

Putting a spacecraft into a retrograde orbit about the Sun using little
or no fuel is a pretty neat trick by anyone's standards.. The Chinese
team's calculations demonstrate the point. They show that a 10 kg sail
in retrograde orbit, that hits Apophis a year before 2029, would deflect
it enough to miss the keyhole, thereby eliminating the chance that the
asteroid will return in 2036.

And such a mission ought to be relatively cheap and relatively easy to
deploy.

That sounds easy enough. In practice, however, threading this camel
through the eye of a needle will be extremely tricky. There are all
kinds of variations in the solar wind that could send such a spacecraft
wildly off course.

It also requires a huge sail that will be difficult to unfurl and also
liable to damage during the course of the journey, which will itself
take years.

Then there's the structure and make up of Apophis, which is a complete
mystery. Without knowing the material properties of the asteroid, it's
impossible to determine how the impact will affect it.

So there's a little more work to be done in Beijing before this plan can
get off the ground. Perhaps they should team up with the Europeans we
talked about the other week

Ref: arxiv.org/abs/1108.3183 <http://arxiv.org/abs/1108.3183>:
Utilization of H-reversal Trajectory of Solar Sail for Asteroid Deflection
Received on Fri 19 Aug 2011 07:51:36 PM PDT