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The Nikkey Asian Review article Probe nears asteroid to unlock secrets to life's origins; Hayabusa2 mission includes return of samples after 18-month study begins:

TOKYO -- A Japanese space probe is just weeks away from reaching an asteroid thought to contain primordial matter from the formation of the solar system, cruising toward the start of an 18-month observational orbit that may offer insight into the origins of life.

The Hayabusa2 vessel has come within roughly 260,000km of the asteroid, named Ryugu, with arrival expected between June 21 and July 5, Yuichi Tsuda, the mission's project manager at the Japan Aerospace Exploration Agency, said Thursday. The probe has enjoyed a smooth journey since its launch in 2014. (emphasis added)

The asteroid Ryugu will reach its ~0.96 AU perihelion in the first week of June, 2018.

Could JAXA's Hayabusa2 mission project manager really not know when Hayabusa2 will arrive at it's destination to better than a two week window? For comparison, see the BBC News article Apology after Japanese train departs 20 seconds early.

Is this an editorial oversight, or will the final arrival date be adjusted by propulsive maneuvers, or is the arrival time in fact not known for some other reason?

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From spaceflight101.com:

Hayabusa began firing three of its four ion engines again on January 10, 2018 marking the initiation of the far-field approach phase to take the spacecraft toward its destination with ion engine operation planned to last until early June when the final approach phase will be initiated from a distance of 2,500 Kilometers. [...]
Reaching the approach point in early June, Hayabusa 2 will rely on its three Optical Navigation Cameras to provide relative navigation data used by mission teams on Earth to plan the spacecraft’s final approach, first into a 20-Kilometer surveying orbit where the spacecraft is expected to arrive by July 5th. It will then be set for a step-wise descent first to five and then to one Kilometer from Ryugu’s surface to collect detailed remote-sensing data of the asteroid using a pair of infrared spectrometers tasked with studying the energy balance of the asteroid as well as its chemical composition.

I think one of two things might be at work here:

  1. The writer was oversimplifying the final approach phase and took the dates of the 2500km distance and 20km distance-arrival as 'between June 21 and July 5th.'
  2. Also it might refer to the following problem with the final approach phase:
    Ryugu is only a little chunk of rock of 400m across and, thus different from 67P, lacks any useful self-gravity for maneuvering at a distance of >2500km. The text I've cited above in bold is when the spacecraft starts flying 'in tandem' with the asteroid and has to be carefully maneuvered on sight.
    This is a risky procedure, and while the rough gravitational strength of Ryugu is known, it's asymmetric values are unknown, which already led Rosetta into trouble around 67P.
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  • $\begingroup$ Great answer, thanks! I remember Rosetta's approach was slow, cautious, and exploratory of 67P's gravity field, but I didn't know it was in "trouble" at any point. $\endgroup$ – uhoh Apr 23 '18 at 4:14
  • $\begingroup$ Wikipedia shows the estimate of about 800 m for the size, rather than 400 m. $\endgroup$ – uhoh Apr 23 '18 at 11:46
  • $\begingroup$ @uhoh: Well, estimates for asteroid sizes are based on the reflectivity of a body. That's the only measurment that is for certain, and then you can only find a size by assuming some albedo and a geometry, so it is pretty common for the estimated asteroid sizes from two different sources to differ by a factor of 2 or more. $\endgroup$ – AtmosphericPrisonEscape Apr 23 '18 at 23:07
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    $\begingroup$ Yep, indeed. I've only seen Wikipedia's 800m based on absolute magnitude 9.2 and albedo of 0.05 and Albedo, Size, and Surface Characteristics of Hayabusa-2 Sample-Return Target 162173 1999 JU3 from AKARI and Subaru Observations's 920 meters based on extensive photometry and an albedo of 0.063. Where does 400 meters and albedo of 0.2 come from? $\endgroup$ – uhoh Apr 23 '18 at 23:26
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@AtmosphericPrisonEscape's answer explains the situation nicely, I'll supplement with a few factoids.

Hyabusa 2 (2014-076A, 40319) left Earth 03-Dec-2014 while (162173) Ryugu was nearly on the opposite side of the Sun in a slightly slower a=1.19AU elliptical orbit. After traveling around the Sun in an orbit close to Earths, and in order to get on an intercept trajectory Hyabusa 2 executed an Earth flyby almost exactly one year later on 03-Dec-2015, gaining some orbital energy and slowing down so that it would intercept Ryugu's orbit in 2018.

I've downloaded some trajectories from JPL's Horizons and plotted them in a frame that rotates with Earth. The small loop shows Hyabusa 2's first year near Earth and swingby, then it's slow trajectory using propulsion from its ion engines in order to intercept Ryugu.

enter image description here

enter image description here

From astronomical measurements of Ryugu's apparent magnitude, it is assigned an absolute magnitude of 9.2 using the math which you can read about in this answer. With the assumption of an albedo of 0.05, Ryugu is estimated to have a size of about 800 meters in diameter. For more on that see Asteroid Size Estimator as well as Wikipedia's 162173 Ryugu.

The density of Ryugu is unknown, but if we assume that it's about 1.5 g/cm^3, (see for example Asteroid Density, Porosity, and Structure) then the mass would be about 4E+11 kg, and radius of the the Hill sphere around Ryugu at 1 AU from the Sun would be about 60 km.

Considering the considerable uncertainty, it seems JAXA will not attempt an orbit until much closer than that, or about 20 km.

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