diff --git a/ksp_plugin/plugin.cpp b/ksp_plugin/plugin.cpp
index f3722ac340..92a5c029d3 100644
--- a/ksp_plugin/plugin.cpp
+++ b/ksp_plugin/plugin.cpp
@@ -699,7 +699,7 @@ void Plugin::SetPredictionAdaptiveStepParameters(
 
 void Plugin::UpdatePrediction(GUID const& vessel_guid) const {
   CHECK(!initializing_);
-  FindOrDie(vessels_, vessel_guid)->UpdatePrediction(InfiniteFuture);
+  FindOrDie(vessels_, vessel_guid)->FlowPrediction(InfiniteFuture);
 }
 
 void Plugin::CreateFlightPlan(GUID const& vessel_guid,
diff --git a/ksp_plugin/renderer.cpp b/ksp_plugin/renderer.cpp
index db04016e26..743c7b5d67 100644
--- a/ksp_plugin/renderer.cpp
+++ b/ksp_plugin/renderer.cpp
@@ -76,7 +76,7 @@ Vessel const& Renderer::GetTargetVessel() const {
 DiscreteTrajectory<Barycentric> const& Renderer::GetTargetVesselPrediction(
     Instant const& time) const {
   CHECK(target_);
-  target_->vessel->UpdatePrediction(time);
+  target_->vessel->FlowPrediction(time);
   // The prediction may not have been prolonged to |time| if we are near a
   // singularity.
   CHECK_LE(time, target_->vessel->prediction().last().time());
@@ -109,7 +109,7 @@ Renderer::RenderBarycentricTrajectoryInPlotting(
   if (target_ && begin != end) {
     auto last = end;
     --last;
-    target_->vessel->UpdatePrediction(last.time());
+    target_->vessel->FlowPrediction(last.time());
   }
   for (auto it = begin; it != end; ++it) {
     Instant const& t = it.time();
diff --git a/ksp_plugin/vessel.cpp b/ksp_plugin/vessel.cpp
index 79bb1858e8..b6a62ebdbe 100644
--- a/ksp_plugin/vessel.cpp
+++ b/ksp_plugin/vessel.cpp
@@ -7,6 +7,7 @@
 #include <string>
 #include <vector>
 
+#include "astronomy/epoch.hpp"
 #include "ksp_plugin/integrators.hpp"
 #include "ksp_plugin/pile_up.hpp"
 #include "quantities/si.hpp"
@@ -16,6 +17,7 @@ namespace principia {
 namespace ksp_plugin {
 namespace internal_vessel {
 
+using astronomy::InfiniteFuture;
 using base::Contains;
 using base::FindOrDie;
 using base::make_not_null_unique;
@@ -36,9 +38,9 @@ Vessel::Vessel(GUID const& guid,
       prediction_adaptive_step_parameters_(prediction_adaptive_step_parameters),
       parent_(parent),
       ephemeris_(ephemeris),
-      history_(make_not_null_unique<DiscreteTrajectory<Barycentric>>()),
-      prediction_(make_not_null_unique<DiscreteTrajectory<Barycentric>>()) {
-  // Can't create the |psychohistory_| here because |history_| is empty;
+      history_(make_not_null_unique<DiscreteTrajectory<Barycentric>>()) {
+  // Can't create the |psychohistory_| and |prediction_| here because |history_|
+  // is empty;
 }
 
 Vessel::~Vessel() {
@@ -141,6 +143,7 @@ void Vessel::PrepareHistory(Instant const& t) {
     CHECK(psychohistory_ == nullptr);
     history_->Append(t, calculator.Get());
     psychohistory_ = history_->NewForkAtLast();
+    prediction_ = psychohistory_->NewForkAtLast();
   }
 }
 
@@ -192,6 +195,8 @@ void Vessel::AdvanceTime() {
   AppendToVesselTrajectory(&Part::psychohistory_begin,
                            &Part::psychohistory_end,
                            *psychohistory_);
+  prediction_ = psychohistory_->NewForkAtLast();
+
   for (auto const& pair : parts_) {
     Part& part = *pair.second;
     part.ClearHistory();
@@ -200,10 +205,9 @@ void Vessel::AdvanceTime() {
 
 void Vessel::ForgetBefore(Instant const& time) {
   // Make sure that the history keeps at least one (authoritative) point and
-  // don't change the psychohistory.  We cannot use the parts because they may
-  // have been moved to the future already.
+  // don't change the psychohistory or prediction.  We cannot use the parts
+  // because they may have been moved to the future already.
   history_->ForgetBefore(std::min(time, history_->last().time()));
-  prediction_->ForgetBefore(time);
   if (flight_plan_ != nullptr) {
     flight_plan_->ForgetBefore(time, [this]() { flight_plan_.reset(); });
   }
@@ -228,19 +232,30 @@ void Vessel::DeleteFlightPlan() {
   flight_plan_.reset();
 }
 
-void Vessel::UpdatePrediction(Instant const& last_time) {
-  // TODO(phl): The prediction should probably be a fork of the psychohistory.
-  auto const psychohistory_last = psychohistory_->last();
-  auto const prediction_begin = prediction_->Begin();
-  if (prediction_->Empty() ||
-      prediction_begin.time() != psychohistory_last.time() ||
-      prediction_begin.degrees_of_freedom() !=
-          psychohistory_last.degrees_of_freedom()) {
-    prediction_ = make_not_null_unique<DiscreteTrajectory<Barycentric>>();
-    prediction_->Append(psychohistory_last.time(),
-                        psychohistory_last.degrees_of_freedom());
+void Vessel::FlowPrediction(Instant const& time) {
+  if (time > prediction_->last().time()) {
+    bool const finite_time = IsFinite(time - prediction_->last().time());
+    Instant const t = finite_time ? time : ephemeris_->t_max();
+    // This will not prolong the ephemeris if |time| is infinite (but it may do
+    // so if it is finite).
+    bool const reached_t = ephemeris_->FlowWithAdaptiveStep(
+        prediction_,
+        Ephemeris<Barycentric>::NoIntrinsicAcceleration,
+        t,
+        prediction_adaptive_step_parameters_,
+        FlightPlan::max_ephemeris_steps_per_frame,
+        /*last_point_only=*/false);
+    if (!finite_time && reached_t) {
+      // This will prolong the ephemeris by |max_ephemeris_steps_per_frame|.
+      ephemeris_->FlowWithAdaptiveStep(
+        prediction_,
+        Ephemeris<Barycentric>::NoIntrinsicAcceleration,
+        time,
+        prediction_adaptive_step_parameters_,
+        FlightPlan::max_ephemeris_steps_per_frame,
+        /*last_point_only=*/false);
+    }
   }
-  FlowPrediction(last_time);
 }
 
 DiscreteTrajectory<Barycentric> const& Vessel::psychohistory() const {
@@ -263,9 +278,7 @@ void Vessel::WriteToMessage(
     message->add_kept_parts(part_id);
   }
   history_->WriteToMessage(message->mutable_history(),
-                           /*forks=*/{psychohistory_});
-  prediction_->WriteToMessage(message->mutable_prediction(),
-                              /*forks=*/{});
+                           /*forks=*/{psychohistory_, prediction_});
   if (flight_plan_ != nullptr) {
     flight_plan_->WriteToMessage(message->mutable_flight_plan());
   }
@@ -277,6 +290,8 @@ not_null<std::unique_ptr<Vessel>> Vessel::ReadFromMessage(
     not_null<Ephemeris<Barycentric>*> const ephemeris,
     std::function<void(PartId)> const& deletion_callback) {
   bool const is_pre_cesàro = message.has_psychohistory_is_authoritative();
+  bool const is_pre_chasles = message.has_prediction();
+
   // NOTE(egg): for now we do not read the |MasslessBody| as it can contain no
   // information.
   auto vessel = make_not_null_unique<Vessel>(
@@ -319,15 +334,20 @@ not_null<std::unique_ptr<Vessel>> Vessel::ReadFromMessage(
     if (message.psychohistory_is_authoritative()) {
       vessel->psychohistory_ = vessel->history_->NewForkAtLast();
     }
-  } else {
+    vessel->prediction_ = vessel->psychohistory_->NewForkAtLast();
+    vessel->FlowPrediction(InfiniteFuture);
+  } else if (is_pre_chasles) {
     vessel->history_ = DiscreteTrajectory<Barycentric>::ReadFromMessage(
         message.history(),
         /*forks=*/{&vessel->psychohistory_});
+    vessel->prediction_ = vessel->psychohistory_->NewForkAtLast();
+    vessel->FlowPrediction(InfiniteFuture);
+  } else {
+    vessel->history_ = DiscreteTrajectory<Barycentric>::ReadFromMessage(
+        message.history(),
+        /*forks=*/{&vessel->psychohistory_, &vessel->prediction_});
   }
 
-  vessel->prediction_ =
-      DiscreteTrajectory<Barycentric>::ReadFromMessage(message.prediction(),
-                                                       /*forks=*/{});
   if (message.has_flight_plan()) {
     vessel->flight_plan_ = FlightPlan::ReadFromMessage(message.flight_plan(),
                                                        ephemeris);
@@ -353,8 +373,7 @@ Vessel::Vessel()
       prediction_adaptive_step_parameters_(DefaultPredictionParameters()),
       parent_(testing_utilities::make_not_null<Celestial const*>()),
       ephemeris_(testing_utilities::make_not_null<Ephemeris<Barycentric>*>()),
-      history_(make_not_null_unique<DiscreteTrajectory<Barycentric>>()),
-      prediction_(make_not_null_unique<DiscreteTrajectory<Barycentric>>()) {}
+      history_(make_not_null_unique<DiscreteTrajectory<Barycentric>>()) {}
 
 void Vessel::AppendToVesselTrajectory(
     TrajectoryIterator const part_trajectory_begin,
@@ -404,32 +423,6 @@ void Vessel::AppendToVesselTrajectory(
   }
 }
 
-void Vessel::FlowPrediction(Instant const& time) {
-  if (time > prediction_->last().time()) {
-    bool const finite_time = IsFinite(time - prediction_->last().time());
-    Instant const t = finite_time ? time : ephemeris_->t_max();
-    // This will not prolong the ephemeris if |time| is infinite (but it may do
-    // so if it is finite).
-    bool const reached_t = ephemeris_->FlowWithAdaptiveStep(
-        prediction_.get(),
-        Ephemeris<Barycentric>::NoIntrinsicAcceleration,
-        t,
-        prediction_adaptive_step_parameters_,
-        FlightPlan::max_ephemeris_steps_per_frame,
-        /*last_point_only=*/false);
-    if (!finite_time && reached_t) {
-      // This will prolong the ephemeris by |max_ephemeris_steps_per_frame|.
-      ephemeris_->FlowWithAdaptiveStep(
-        prediction_.get(),
-        Ephemeris<Barycentric>::NoIntrinsicAcceleration,
-        time,
-        prediction_adaptive_step_parameters_,
-        FlightPlan::max_ephemeris_steps_per_frame,
-        /*last_point_only=*/false);
-    }
-  }
-}
-
 }  // namespace internal_vessel
 }  // namespace ksp_plugin
 }  // namespace principia
diff --git a/ksp_plugin/vessel.hpp b/ksp_plugin/vessel.hpp
index 3026b2e091..27105c7a96 100644
--- a/ksp_plugin/vessel.hpp
+++ b/ksp_plugin/vessel.hpp
@@ -138,7 +138,9 @@ class Vessel {
   // Deletes the |flight_plan_|.  Performs no action unless |has_flight_plan()|.
   virtual void DeleteFlightPlan();
 
-  virtual void UpdatePrediction(Instant const& last_time);
+  // Tries to extend the prediction up to and including |last_time|.  May not be
+  // able to do it next to a singularity.
+  virtual void FlowPrediction(Instant const& last_time);
 
   virtual DiscreteTrajectory<Barycentric> const& psychohistory() const;
 
@@ -168,8 +170,6 @@ class Vessel {
                                 TrajectoryIterator part_trajectory_end,
                                 DiscreteTrajectory<Barycentric>& trajectory);
 
-  void FlowPrediction(Instant const& time);
-
   GUID const guid_;
   std::string name_;
 
@@ -187,7 +187,8 @@ class Vessel {
   not_null<std::unique_ptr<DiscreteTrajectory<Barycentric>>> history_;
   DiscreteTrajectory<Barycentric>* psychohistory_ = nullptr;
 
-  not_null<std::unique_ptr<DiscreteTrajectory<Barycentric>>> prediction_;
+  // The |prediction_| is forked off the end of the |psychohistory_|.
+  DiscreteTrajectory<Barycentric>* prediction_ = nullptr;
 
   std::unique_ptr<FlightPlan> flight_plan_;
 };
diff --git a/ksp_plugin_test/mock_vessel.hpp b/ksp_plugin_test/mock_vessel.hpp
index 5d571c7d89..eb211de175 100644
--- a/ksp_plugin_test/mock_vessel.hpp
+++ b/ksp_plugin_test/mock_vessel.hpp
@@ -34,7 +34,7 @@ class MockVessel : public Vessel {
 
   MOCK_METHOD0(DeleteFlightPlan, void());
 
-  MOCK_METHOD1(UpdatePrediction, void(Instant const& last_time));
+  MOCK_METHOD1(FlowPrediction, void(Instant const& last_time));
 
   MOCK_CONST_METHOD0(psychohistory, DiscreteTrajectory<Barycentric> const&());
   MOCK_CONST_METHOD0(psychohistory_is_authoritative, bool());
diff --git a/ksp_plugin_test/vessel_test.cpp b/ksp_plugin_test/vessel_test.cpp
index a91dce4fdb..f4106d08b0 100644
--- a/ksp_plugin_test/vessel_test.cpp
+++ b/ksp_plugin_test/vessel_test.cpp
@@ -227,7 +227,7 @@ TEST_F(VesselTest, Prediction) {
                                                50.0 * Metre / Second,
                                                40.0 * Metre / Second}))),
                 Return(true)));
-  vessel_.UpdatePrediction(astronomy::J2000 + 1 * Second);
+  vessel_.FlowPrediction(astronomy::J2000 + 1 * Second);
 
   EXPECT_EQ(2, vessel_.prediction().Size());
   auto it = vessel_.prediction().Begin();
@@ -293,7 +293,7 @@ TEST_F(VesselTest, PredictBeyondTheInfinite) {
                                                60.0 * Metre / Second,
                                                50.0 * Metre / Second}))),
                 Return(true)));
-  vessel_.UpdatePrediction(astronomy::InfiniteFuture);
+  vessel_.FlowPrediction(astronomy::InfiniteFuture);
 
   EXPECT_EQ(3, vessel_.prediction().Size());
   auto it = vessel_.prediction().Begin();
diff --git a/physics/forkable_body.hpp b/physics/forkable_body.hpp
index 8bbea9f783..3e3b41c91e 100644
--- a/physics/forkable_body.hpp
+++ b/physics/forkable_body.hpp
@@ -296,8 +296,10 @@ std::int64_t Forkable<Tr4jectory, It3rator>::Size() const {
   // Go up the ancestry chain adding the sizes.
   Tr4jectory const* parent = ancestor->parent_;
   while (parent != nullptr) {
-    size += std::distance(parent->timeline_begin(),
-                          *ancestor->position_in_parent_timeline_) + 1;
+    if (!parent->timeline_empty()) {
+      size += std::distance(parent->timeline_begin(),
+                            *ancestor->position_in_parent_timeline_) + 1;
+    }
     ancestor = parent;
     parent = ancestor->parent_;
   }
@@ -307,7 +309,7 @@ std::int64_t Forkable<Tr4jectory, It3rator>::Size() const {
 
 template<typename Tr4jectory, typename It3rator>
 bool Forkable<Tr4jectory, It3rator>::Empty() const {
-  // If this object has an ancestor surely it is hook off of a point in some
+  // If this object has an ancestor surely it is hooked off of a point in some
   // timeline, so this object is not empty.
   return timeline_empty() && parent_ == nullptr;
 }
diff --git a/physics/forkable_test.cpp b/physics/forkable_test.cpp
index e4dbfe302a..f2cd820bd2 100644
--- a/physics/forkable_test.cpp
+++ b/physics/forkable_test.cpp
@@ -229,6 +229,18 @@ TEST_F(ForkableTest, ForkSuccess) {
   EXPECT_THAT(times, ElementsAre(t1_, t2_, t4_));
 }
 
+TEST_F(ForkableTest, Size) {
+  EXPECT_TRUE(trajectory_.Empty());
+  trajectory_.push_back(t1_);
+  EXPECT_EQ(1, trajectory_.Size());
+  not_null<FakeTrajectory*> const fork1 =
+      trajectory_.NewFork(trajectory_.timeline_find(t1_));
+  EXPECT_EQ(1, fork1->Size());
+  not_null<FakeTrajectory*> const fork2 = fork1->NewFork(fork1->timeline_end());
+  fork2->push_back(t2_);
+  EXPECT_EQ(2, fork2->Size());
+}
+
 TEST_F(ForkableTest, ForkAtLast) {
   trajectory_.push_back(t1_);
   trajectory_.push_back(t2_);
diff --git a/serialization/ksp_plugin.proto b/serialization/ksp_plugin.proto
index 45aa33a5f9..9238119db2 100644
--- a/serialization/ksp_plugin.proto
+++ b/serialization/ksp_plugin.proto
@@ -134,7 +134,7 @@ message Vessel {
   repeated fixed32 kept_parts = 15;
   required DiscreteTrajectory history = 16;
   optional bool psychohistory_is_authoritative = 17;  // Pre-Cesàro.
-  required DiscreteTrajectory prediction = 18;
+  optional DiscreteTrajectory prediction = 18;  // Pre-Chasles.
   optional FlightPlan flight_plan = 4;
 
   // Pre-Буняковский.