# What is the largest body in the solar system we could meaningfully and accurately adjust the orbit of?

There is a lot of science fiction and emerging science that move comet and asteroids as part of the main plot. Pretty much everything in our solar system, is in orbit around the sun, or in orbit around an object orbiting the sun. If you want something to be somewhere else, in essence you change it's solar orbit to match (or collide) with your desired location.

We have several man made bodies that we have placed in lots of different orbits. we have even caused some to leave the solar system. Given our current (2015) tools and knowledge what is the largest object would could meaningfully and accurately adjust the orbit of?

Where "meaningfully and accurately" = bringing into a given orbit (specific and calculated) around any body that it is not currently orbiting, in a timely fashion (i.e. 5 years or less)

Of course the biggest challenge is getting your tools and knowledge away from earth and to the body you want to adjust the orbit of, that is mater of economics. Assuming you have the budget to get what you want off of Earth, what is the biggest thing you could move accurately?

• I'm wondering if this still might be too broad. It depends on whether it is possible to be clear about what a meaningful, accurate change is. Maybe there are some large things we could change the orbit of by a few meters, such that in a few centuries it would be likely to hit something specific - but most such opportunities are in places where their orbits are heavily influenced by a lot of other things and accurate orbit prediction down the road is pretty hard. – kim holder May 8 '15 at 18:59
• "2015 tools and knowledge" is very vague; by my interpretation of it, the answer is "a very small artificial satellite". – Russell Borogove May 8 '15 at 19:06
• A body being so small that adjusting its orbit might be possible will be too small to be detected with telescopes from earth or by telescopes in an earth orbit. – Uwe Apr 3 '17 at 15:42

If "meaningful" means measurable then seeng a half-percent change in the period of a tiny double-asteroid at a few AU is pretty close to as big as possible.

@PearsonArtPhotos question Using DART to measure G came to mind when I came across this "classic" question, so let's connect the dots.

DART will be the first demonstration of the kinetic impact technique to change the motion of an asteroid in space. The DART mission is in Phase B, led by JHU/APL and managed by the Planetary Missions Program Office at Marshall Space Flight Center for NASA’s Planetary Defense Coordination Office.

DART is a planetary defense-driven test of one of the technologies for preventing the Earth impact of a hazardous asteroid: the kinetic impactor. DART’s primary objective is to demonstrate a kinetic impact on a small asteroid. The binary near-Earth asteroid (65803) Didymos is the target for DART. While Didymos’ primary body is approximately 800 meters across, its secondary body (or “moonlet”) has a 150-meter size, which is more typical of the size of asteroids that could pose a more common hazard to Earth.

The DART spacecraft will achieve the kinetic impact by deliberately crashing itself into the moonlet at a speed of approximately 6 km/s, with the aid of an onboard camera and sophisticated autonomous navigation software. The collision will change the speed of the moonlet in its orbit around the main body by a fraction of one percent, enough to be measured using telescopes on Earth.

Wikipedia's Double Asteroid Redirection Test says that the launch mass is 500 kg, and the Lunar and Planetary Science XLVIII (2017) paper The Double Asteroid Redirection Test (DART) Element of the Asteroid Impact and Deflection Assessment (AIDA) Mission gives an impact mass of ~490 kg.

With a system mass $$M = m_1+m_2$$ of 5.28E+11 kg, a separation $$R$$ of 1180 meters, and the gravitational constant $$G$$ of 6.674E-11 m^3/kg s^2, the orbital period from (from here):

$$T^2 = \frac{4 \pi^2 R^3} {G(m1+m2)}$$

is about 42,900 seconds, and if the orbit were circular that corresponds to an orbital velocity of about 0.173 m/sec.

The momentum of the ~500 kg spacecraft at 6,000 m/s is 3E+06 kg m/s, that of the moonlet in the system's center of mass (assuming the moonlet has about 0.66 % of the system mass if you assume equal density) is about 6.01E+08 m/s, so the complete absorption of momentum could change the momentum of the moonlet by roughly half of one percent.

Schematic of the DART mission shows the impact on the moonlet of asteroid (65803) Didymos. Post-impact observations from Earth-based optical telescopes and planetary radar would, in turn, measure the change in the moonlet’s orbit about the parent body.

The near-Earth asteroid (185851) 2000 DP107 in many ways is an analog to Didymos. 2000 DP107 was the first binary asteroid ever imaged by radar. This animation is derived from planetary radar observations. In this example (2000 DP107), the primary and secondary are about 850 meters and 300 meters in diameter. They are separated by about 2.7 km. The primary rotates once every 2.77 hours while the tidally locked secondary orbits the primary about once every 42 hours.

• If the momentum of the moonlet is changed by roughly half of one percent, how much change of the orbit period of 11.92 hours? How long is the neccessary observation time to validate such a small change of the moonlet's orbital period? – Uwe Feb 8 at 15:19
• @Uwe I thought one of the links in my post addresses that but it doesn't. I guess a few months but there's no rush as far as I know. – uhoh Feb 8 at 15:26
• A few months for validation would be ok, but it should be done during the closest approach to Earth around October 2022. It should be done before the Didymos system is too far away again for precise observation of its moonlet. – Uwe Feb 8 at 15:33
• @Uwe I think the pair are an eclipsing binary seen from Earth (diameters of 300 and 800 meters) so a light curve is sufficient, they don't need to be spatially resolved. As long as they can be detected optically all that's necessary is a series of measurements of the eclipse and transit timings. I'm sure this has all been carefully thought out before planning the mission, let's see if we can find a reference to the observation plan. – uhoh Feb 8 at 20:31
• Just thinking, is it possible to impact an asteroid to change it course such that it hits the target asteroid which is in collision course to earth? Then I think we can change a trajectory a lot ! – Prakhar Feb 9 at 5:55

The phrase "2015 tools and knowledge" combines two very different things. If we are limited to today's tools, the best we can do is impact a body so the body absorbs the momentum. The easiest way to satisfy "meaningfully adjust the orbit of something" is to make it not hit (or hit) the earth. Rosetta is 2900 kg and my WAG for a closing speed, assuming you want it high even though everything goes around the sun, is $10\%$ of Mars orbital velocity or 2.4 km/s. That gives you the delta v you can impart (though you can't necessarily do it in any chosen direction) by dividing by the mass of the object. As far as I know, there are not any known objects heading this way. If we found a comet in an extremely eccentric orbit that would hit the earth next pass (or the one after that) a small nudge would prevent the disaster. Then you are into 2015 knowledge-how well can we measure the orbit.