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Bouncing radar off of Venus' surface has yielded a rich body of data about the planet. Based on Venus' aphelion and Earth's perihelion, we know then that the answer is at least 38 million kilometers with a round trip of over four minutes.

This was possible because Venus provides a very large radar cross-section.

But For longer distances, space exploration can active transponders on spacecraft in orbit around bodies or on their surface to gauge the distance to the body

Question: What is the farthest distance to a solar system object that's been determined by spacecraft transponder? If there is a transponder currently en route to a solar system body or is there but not yet activated, this will be interesting to know about as well.

fyi I've asked about reflections from the bodies themselves separately in Farthest distance to a solar system object that's been measured by radar?

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  • $\begingroup$ Would Oumuamua count? Distance at time of detection or distance currently? Would eccentric asteroids count? Just nitpicking, good question though. $\endgroup$ – Magic Octopus Urn Jul 30 '19 at 14:58
  • $\begingroup$ @MagicOctopusUrn the question is about a spacecraft transponder and radar. $\endgroup$ – uhoh Jul 30 '19 at 15:56
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Scientists routinely use the onboard transponders of deep space missions to return signals from the Deep Space Network in order to determine their distance from earth. This is briefly described in this Scientific American article.

Since the furthest object ever visited by a deep space mission is Ultima Thule by the New Horizon's spacecraft, it stands to reason that this is the " farthest distance to a solar system object that's been determined by spacecraft transponder". At flyby the distance from the Sun was around 6.6 billion kilometers, according to the above article, or 6.64 billion kilometers from the earth according to this article.

update:

The JPL Horizons web interface can be used to obtain state vectors for Ultima Thule.

The data returned contains the following information in the header:

Revised: May 21, 2019            486958 (2014 MU69)                    2486958
 *SPECIAL*

 This pre-computed trajectory is consistent with the New Horizons spacecraft
 Kuiper-Belt extended mission, with the reconstructed 3537.7 km flyby of 
 2014 MU69 on 2019-Jan-1 @ 05:34:31 TDB (05:33:22 UTC).

 Trajectories were provided by the New Horizons mission planning team at SWRI 
 and are fits to internal flight-project data that has not been made available 
 outside the flight project.

So it seems that Ultima Thule's orbit has been improved by using reconstructed flyby data from the New Horizons spacecraft, which itself would be constructed using range-rate data from the spacecraft's transponder.

Therefore to the question

Farthest distance to a solar system object that's been determined by spacecraft transponder?

The answer is 44.256 AU from Earth to Ultima Thule (2014 MU69) on 2019-Jan-1 @ 05:34:31 TDB (05:33:22 UTC):

Date__(UT)__HR:MN     R.A._(ICRF/J2000.0)_DEC  APmag  S-brt            delta      deldot    S-O-T /r    S-T-O
2019-Jan-01 05:33     19 08 36.78 -20 34 44.1   n.a.   n.a. 44.2560922822249   2.9001642   6.1051 /T   0.1431

 delta  deldot =

   Range ("delta") and range-rate ("delta-dot") of target center with respect
to the observer at the instant light seen by the observer at print-time would
have left the target center (print-time minus down-leg light-time); the
distance traveled by a light ray emanating from the center of the target and
recorded by the observer at print-time. "deldot" is a projection of the
velocity vector along this ray, the light-time-corrected line-of-sight from the
coordinate center, and indicates relative motion. A positive "deldot" means the
target center is moving away from the observer (coordinate center). A negative
"deldot" means the target center is moving toward the observer.

Units: AU and KM/S
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  • $\begingroup$ New Horizons didn't use radar for sensing Ultima Thule. Its sensors were purely passive, nothing that one could call a transponder as the OP asks for. $\endgroup$ – Camille Goudeseune Jul 29 '19 at 21:03
  • $\begingroup$ @CamilleGoudeseune Let's not rule out Ultima Thule yet. The series of beautiful, time-stamped photographs (1, 2, 3) of Ultima Thule during the straight-line flyby at a known velocity could be used to determine the asteroid's distance and direction from the spacecraft. Combining that with the transponder data and other trajectory information, an accurate distance determination is certainly possible. Let's wait to see if has been done. $\endgroup$ – uhoh Jul 29 '19 at 22:20
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    $\begingroup$ Fair enough. Transponder = transmitter + responder. And at least NH's trajectory was somewhat controlled (transmit) to position itself for good photos (respond). Or, what I think @uhoh meant, NH's own transponder (Earth communications, navigation) was crucial for extracting more data from the passively collected photos. $\endgroup$ – Camille Goudeseune Jul 29 '19 at 22:34
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    $\begingroup$ @CamilleGoudeseune my understanding of the OP was that "What is the farthest distance to a solar system object that's been determined by spacecraft transponder?" Since we figure out how far New Horizon's is by its transponder, I think that would apply. If we know how far the s/c is we know how far its flyby target is. We can quibble about error bars in that estimate, of course. $\endgroup$ – Carlos N Jul 31 '19 at 17:00
  • $\begingroup$ I had a "brain wave" and realized this might have been actually done and also noted in Horizons, and it turns out it was, and it was! ;-) $\endgroup$ – uhoh Aug 9 '19 at 11:02

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